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Chapter 3: Cofactors in Human Papillomavirus Carcinogenesis—Role of Parity, Oral Contraceptives, and Tobacco Smoking

Chapter 3: Cofactors in Human Papillomavirus Carcinogenesis—Role of Parity, Oral Contraceptives,... Abstract It is now well established that infection with oncogenic human papillomavirus (HPV) types is the necessary cause of cervical cancer (CC) and its immediate precursor cervical intraepithelial neoplasia 3. However, HPV infection alone may not be sufficient to cause CC, and other exogenous and endogenous factors may exist that, in conjunction with HPV, influence the risk of progression from cervical HPV infection to CC. In this chapter, we review the evidence for the role of parity, oral contraceptive (OC) use, and tobacco smoking in CC. We also discuss limitations and methodologic problems encountered in assessing available data and outline recommendations for future research. Based on key studies on high-grade squamous intraepithelial lesions (HSILs) and CC conducted among HPV-positive women, it can be concluded that high parity, smoking, and less consistently long-term OC use are cofactors that may modulate the risk of progression from HPV infection to HSIL/CC. From a public health point of view, parity seems to be the behavioral cofactor explaining the highest proportion of CC cases among HPV-infected women. Smoking and long-term OC use may have a similar impact in populations that are heavily exposed to HPV and to these cofactors. Large prospective and retrospective cohort studies of HSIL and CC among middle-aged women in which several markers of HPV exposure are used and HPV persistence is documented would be valuable to study the role of these and other cofactors in HPV carcinogenesis. If confirmed, our conclusions may imply that multiparous women, women who are smokers, and women on long-term OC use may need closer surveillance for cytologic abnormalities and HPV infections than women in the general population. It is now well established that infection with oncogenic human papillomavirus (HPV) types is the necessary cause of cervical cancer (CC) and its immediate precursor cervical intraepithelial neoplasia (CIN) 3 (CIN3). However, HPV infection alone may not be sufficient to cause CC, and other exogenous or endogenous factors might exist that, in conjunction with HPV, influence the risk of progression from cervical HPV infection to CC. Candidate cofactors may be classified into three groups: 1) environmental or exogenous cofactors, including use of oral contraceptives (OCs), tobacco smoking, diet, cervical trauma, and coinfection with human immunodeficiency virus (HIV) and other sexually transmitted agents; 2) viral cofactors, such as infection by specific types, coinfection with other types, HPV variants, viral load, and viral integration; and 3) host cofactors, including endogenous hormones, genetic factors such as human leukocyte antigen, and other host factors related to the host’s immune response. The purpose of this chapter is to review, summarize, and discuss the evidence of the role of the more established cofactors, including parity, OC use, and tobacco smoking. Cofactors involved in the natural history of HPV infection are reviewed in Chapter 2. In this chapter, the focus is on cofactors affecting progression from HPV infection to high-grade squamous intraepithelial lesions (HSILs) and CC. Ideally, if we accept the premise that all CCs are caused by oncogenic HPVs, a strict assessment of cofactors requires a study group known to be exposed to HPV. From that HPV-exposed group, retrospectively or ideally prospectively, the added risk attributable to other factors can be estimated. Although this approach is admittedly controversial (see the “Discussion” section), we believe that, in the absence of repeated HPV measures, restriction to HPV DNA-positive case patients and control subjects conveys the strictest approach to adjustment for HPV. This review will thus focus on selected key studies that, using reliable DNA-detection methods, report associations between cofactors and HSIL/CC within a well-defined HPV-positive group. These studies do exist and the main characteristics of the most important ones are summarized in Table 1 [(1–9); Plummer M, Herrero R, Franceschi S, Meijer CJ, Snijders P, Bosch FX, et al.: unpublished data]. Studies among HPV-positive women that included low-grade squamous intraepithelial lesions (LSILs) (10) or the three grades of CIN (11) are not considered in detail in this chapter. Evidence for a Role of OC use in HPV Carcinogenesis Use of OCs has been found to be associated with CC in many, but not in all, epidemiologic studies that adjusted for HPV status. In studies restricted to HPV-positive women, however, the evidence for an association is in general weaker (Table 2). Of the six studies reporting results restricted to HPV DNA-positive subjects, three found positive statistically significant associations, but these were for a particular histologic type or for a subgroup of women. The Eastern U.S. study (3) reported an odds ratio (OR) of 17.1 (95% confidence interval [CI] = 1.5 to 188.2) for current versus never OC use, but this was only for adenocarcinoma in situ; although the risk increased with longer duration of use, the trend was of borderline statistical significance. The study in Costa Rica (6) found a 3.1-fold increased risk for users of 5 or more years as compared with never users, but this association was observed only among women who had two or fewer pregnancies, and the HSIL/cancer group included only 30 cases. The strongest evidence for a role of OC use in HPV carcinogenesis derives from the large pooled analysis of the International Agency for Research on Cancer (IARC) studies (9). Even though ever use of OCs was moderately associated with cancer risk (OR = 1.4), there was a strong dose–response relationship with increasing years of use (Table 2). No increase in the risk of cervical neoplasia was found for the duration of OC use for up to 4 years. However, use of OCs for longer than 5 years was significantly associated with cervical neoplasia (OR = 3.4; 95% CI = 2.1 to 5.5). The risk of OC use for longer than 5 years was increased fourfold for invasive CC (OR = 4.0; 95% CI = 2.0 to 8.0) and threefold for carcinoma in situ (CIS) (OR = 3.4; 95% CI = 2.1 to 5.5). Special attention should be given to the lack of association reported in the only prospective study of CIN3 and CC (7). As the same authors discuss, first, only one measurement of OC use was obtained at enrollment, thus not accounting for the possible discontinuation or initiation of use of OCs during the course of the study. Second, OC users had shorter follow-up times than nonusers, which may have resulted in censoring bias among the users. Third, among the control subjects, OC users were more likely to be diagnosed with CIN1 and CIN2 during follow-up than nonusers, which may have resulted in an increased detection and treatment of women in the control group whose disease might have otherwise progressed to CIN3 or CC. Finally, no information regarding duration was collected, thus potentially missing an association with long-term use. Not much data are available concerning the mechanisms by which hormonal influences may modulate the risk of progression to HSIL/CC among HPV-infected women. Hormone-related mechanisms may influence the progression from premalignant to malignant cervical lesions by promoting integration of HPV DNA into the host genome, which results in deregulation of E6 and E7 expression (12). An experimental study (13) has shown that estradiol may stimulate the transcription of HPV type 16 (HPV16) E6 and E7 in cell lines that contain integrated HPV16. Since the E6 and E7 open reading frames have been associated with the oncogenic potential of HPV16, the effect of estrogen on the transcription of these viral genes may be of biologic relevance in the malignant transformation of HPV16-infected cervical cells. Data from experimental studies (14,15) demonstrate a synergistic mechanism between long-term estrogen exposure and HPV16 oncogenes that modulates squamous carcinogenesis in the female reproductive tract of transgenic HPV16-expressing mice. Alternatively, OCs might facilitate HPV reactivation or persistence, although indirect evidence from several studies (9, see review in 16) does not find an association between OC use and HPV positivity among control women. Evidence for a Role of Parity in HPV Carcinogenesis High parity has consistently been found in most case–control studies to be associated with both CC and CIS. Most of the major studies restricting the analysis to HPV-positive women also report an increased risk of HSIL/CC with an increasing number of pregnancies (Table 3). In the IARC-pooled analysis, the OR for CC in women with seven or more full-term pregnancies was fourfold higher than that in nulliparous women, and the risk increased linearly with an increasing number of full-term pregnancies (8). Risk of HSIL/CC significantly increased with an increasing number of live births in the Costa Rica study (6). A borderline association with CIN3 was found in the Manchester study (5). The study in Denmark (1) and the U.S. prospective study (7) did not find an association with the risk of HSIL and CIN3/CC, respectively. However, this result could be explained by the low parity of the study populations. In addition, in the U.S. cohort, information on parity was recruited only at enrollment. Thus, pregnancies occurring during the 10-year follow-up period of the study might have been more relevant to the prospective risk of CIN3/CC than those occurring before enrollment. In addition to the studies restricted to HPV-positive women, further evidence of a hormonal effect on CC comes from the analysis of the age-distribution curve based on cohort effects in relation to mortality without the distortion potentially introduced by screening and secular changes. The analysis shows that CC mortality rates increase very sharply up to age 50 years (i.e., around menopause) and flatten thereafter. Nutritional, hormonal, traumatic, and immunologic mechanisms have been hypothesized as biologically plausible explanations for the association between parity and HSIL/CC among infected women; however, because of the concordance of effects with OC use, hormonal influences can be considered to be one of the most promising candidates in the search for HPV cofactors. High parity may likely increase the risk of CC because it maintains the transformation zone on the exocervix for many years (17), facilitating the direct exposure to HPV and, possibly, to other cofactors. Hormonal changes induced by pregnancy (increased levels of estrogen and progesterone) may also modulate the immune response to HPV and influence risk of persistence or progression (8,18). Evidence for a Role of Tobacco Smoking in HPV Carcinogenesis The effects of smoking have been well studied in many case–control studies, and they show a moderate and statistically significant association with CC, even after adjusting for the strong effects of HPV. These findings are strikingly consistent with those obtained in studies restricted to HPV-positive women. As shown in Table 4, all such studies report some evidence that tobacco smoking increases the risk of developing HSIL and CC. The ORs for ever smoking among HPV-positive women are in the range of 2 to 5. Furthermore, most studies reporting risk estimates according to intensity, duration, or pack-years show an increased risk of CC with increasing exposure to tobacco smoking. Because of the prospective nature of the study, the positive association found with smoking status and smoking intensity in the U.S. prospective study is particularly relevant (7). Despite the consistency of these findings, the possibility remains that smoking or smoking duration is a proxy for time since HPV exposure, because long-duration smokers may also have had an HPV infection for a long time. Thus, residual confounding by time since HPV infection cannot be ruled out as a possible explanation for the observed effects of smoking. Almost 60 years ago, Rous and Friedwald (19) reported the carcinogenic effect of tar on virus-induced rabbit papillomas. More recently, malignant transformation of HPV16-immortalized human endocervical cells by cigarette smoke condensate has been proven (20). The fact that nicotine and tobacco-specific carcinogens have been detected in the cervical mucus of smokers (21) further strengthens the hypothesis of a synergistic action between cigarette smoking and HPV for the development of HSIL/CC. Chemical tobacco-related carcinogens may exert a direct mitogenic effect causing DNA damage. Some authors (22) hypothesize that exposure to tobacco may affect the ability of the host to mount an effective local immune response against viral infections, since it has been shown that smoking may reduce the number of Langerhans’ cells and other markers of immune function. A recent prospective study (23) presents convincing evidence that smokers maintain cervical HPV infections significantly longer and have a lower probability of clearing an oncogenic infection than women who never smoked. The significant association found between the extent of smoking reduction and the reduction in lesion size in an intervention study of smoking cessation among women with minor-grade lesions further strengthens the plausible role of tobacco smoking in HPV carcinogenesis (24). Evidence From Studies Comparing LSIL, HSIL, and CC Cofactor Profiles Finally, it is worth mentioning that few cofactors have been identified to distinguish invasive cancer from intraepithelial lesions or HSIL from LSIL. The IARC Spain–Colombia study (25) considered a large number of risk factors and found that both the CIN3 and CC patients had very similar profiles of risk factors. A recent study (26) conducted in Thailand found that, after controlling for HPV type, the risk of developing CC, as compared with the risk of developing intraepithelial lesions, was not related to any of the cofactors considered, except for two indices of socioeconomic status. In contrast, a study comparing HPV-positive women with CIN3 with HPV-positive women with CIN1 (27) found that cigarette smoking was significantly associated with CIN3, suggesting that HPV-infected cells may relate to tobacco-containing carcinogens for neoplastic progression. Discussion Methodologic Issues in the Study of Cofactors in HPV Carcinogenesis Definition of case patients and control subjects. HPV cofactors in CC may act in at least three ways: 1) by influencing the acquisition of HPV infection, 2) by increasing the risk of HPV persistence, and 3) by increasing the risk of progression from HPV infection to HSIL and cancer. Studies aimed at identifying factors for HPV acquisition and persistence were considered in Chapter 2. In these studies, case patients and control subjects can be defined on the basis of HPV/LSIL status. Here, we focused on factors that, acting once HPV infection has been established, modulate the risk of progression from HPV infection to HSIL/CC. These cofactors can be identified by cohort or case–control studies of HPV-positive women in which case patients include those diagnosed with HSIL, CIN3, CIS, or CC but not those diagnosed with LSIL or CIN1. Accepting that HPV is a necessary cause of CC, we believe that proper control subjects for these studies ought to be HPV-positive women. Which assay or combination of assays is best suited to select HPV-exposed or HPV-positive women still remains an important research issue for which more data are needed. Because many investigators consider LSIL an early manifestation of HPV infection, some have also included women with LSIL in the HPV-positive control group (6). Studies (11) including the whole spectrum of precancerous lesions as cases are difficult to interpret because the case group includes a mixture of exposed and diseased women, because some have LSIL, a marker of HPV exposure more than a disease outcome, and some have HSIL/CC, clearly an HPV-related disease outcome. The nested case–control studies carried out in Denmark (1) and in the United Kingdom (5) suggest that the pattern of risk factors for HPV infection or LSIL are different from the patterns for CIN3 and HSIL. Again, under the premise that HPV is a necessary cause of CC, a more refined case definition could be made on the basis of both HPV and disease statuses. In relation to the control subjects and the selection of HPV-exposed women, the key issue is whether one-point HPV positivity can be considered to be a sufficient criterion to define the proper control group. In other words, can we assume that cross-sectionally identified HPV-positive control women are carrying a persistent or chronic infection? How long should persistent infection persist to increase the risk of CC: many months or just a few months as suggested recently (28)? If persistent infections are associated with CC risk, the definition of HPV-positive control women based on a single measure (used in most studies) might not be sufficient. However, if most HPV infections in middle-aged women tend to be persistent, the use of a single HPV test may be more of a problem in studies of HSIL than in studies of invasive CC. Furthermore, the interpretation of results from studies including young women (most of the HSIL studies in Table 1) is further limited by the fact that measures of the putative environmental cofactors are strongly age dependent. Younger women are less likely to have had a high number of pregnancies, to have smoked, or to have used OCs for a long time. HPV adjustment strategies. Using the IARC series of case–control studies, we were able to estimate the impact of different strategies of HPV adjustment on associations between environmental cofactors and CC risk. For each of the three cofactors of interest, we fitted three different models: 1) one model including all case patients and all control subjects but ignoring adjustment for HPV, 2) a second including all subjects but adjusting for HPV DNA status, and 3) a third restricting the analysis to case patients and control subjects who tested positive for HPV DNA. As shown in Table 5, of the three strategies, models restricted to HPV DNA-positive subjects yielded higher associations that were between 1.1- and 2.0-fold higher than those derived from HPV-adjusted models. For parity and smoking, the magnitudes of the ORs were lowest for the crude models, were intermediate for the HPV-adjusted models, and were highest for the HPV-restricted models. For OC use, the HPV-adjusted models yielded lower ORs than the crude and HPV-restricted models. If we believe that HPV restriction is the strictest approach to HPV adjustment, one could then conclude that the studies adjusting for HPV are likely to underestimate the magnitude of the association but that this underestimation is not greater than twofold. It remains to be explained why we see stronger rather than weaker effects when analyses are restricted to HPV-positive case patients and control subjects. If it is claimed that this method of adjustment is better than the other strategies in getting rid of residual confounding, then we should expect lower rather than higher ORs. Perhaps negative confounding between cofactors and HPV status or selection bias introduced by restricting the analysis to HPV-positive subjects (i.e., the case–control matching is totally ignored after restriction) might explain the increase in the ORs. Beyond the discussion of which adjustment method is more suitable in the assessment of cofactor, it is reassuring to observe that, regardless of the strategy used, the same conclusions concerning the direction and statistical significance of the associations are reached. Since the goal in the assessment of cofactors is to explore associations among HPV-exposed women, an alternative approach is to explore the use, alone or in combination with HPV DNA detection, of serologic markers of HPV infection. Use of HPV DNA detection alone may overlook past infections that being clinically relevant at the time were cleared from the cervix. In contrast, while HPV serology is highly specific, its low sensitivity (between 50% and 70%, depending on the assay and on the number of types included) might pose problems of selection bias (i.e., women who did not trigger a detectable seroconversion after a relevant HPV infection would not be selected by the serologic assay). Estimation of attributable fractions of CC explained by the various cofactors. To assess the overall impact of cofactors on CC burden, we found that it is useful to estimate the percentage of disease that can be attributed to each cofactor. However, the “necessary-cause” model of HPV carcinogenesis poses a methodologic challenge, since the use of standard methods to estimate attributable fractions (AFs) may be inadequate under a model that assumes that the exposures for which AFs are to be estimated are only relevant once the individual is or has been exposed to a necessary etiologic factor. Despite these limitations, we (Table 6) and others (6) have estimated AFs for cofactors by using as parameters the percentages of women exposed to the cofactor of interest among HPV-positive women and the OR for CC derived from analyses restricted to infected women. As shown in Table 6, among HPV-positive women, AFs are comparatively higher for parity than for the other cofactors. Thus, given the high percentage of HPV-positive women with two or more full-term pregnancies and the relatively strong association for CC at this parity level, this analysis suggests that multiparous HPV-positive women constitute the group at the highest risk of CC. However, these conclusions should be taken only as a crude approximation to the issue because the overall disease burden attributable to the cofactor depends not only on the percentage of HPV-positive women exposed to the cofactor but also on the overall HPV prevalence in the population; the estimates reported in Table 6 or in the Costa Rica study do not take the latter parameter into account. We also assessed the impact of different combinations of cofactors on CC burden. Table 6 shows that the most prevalent combination of cofactors among case patients is that including high parity, low OC use, and never smoking (55.8%). We also show that only 5.4% of the case patients were exposed to none of the risk categories of the cofactors considered, suggesting that the fraction of the case patients in whom these cofactors may not play a role is probably very low. Recommendations for Future Research On parity and hormonal factors. 1) Taking into account the second peak in the prevalence of HPV DNA observed in perimenopausal and postmenopausal women in some populations (29–31), it would be of interest to conduct studies in these women and their male partners to try to distinguish if this second peak is the result of reactivation of latent infections or new infections. In addition, these studies will be able to explore the interaction between endogenous and exogenous hormones and HPV infection in middle-aged women. Are the endogenous hormone levels and/or hormone replacement therapy (HRT) associated with HPV DNA detection and progression to cancer in postmenopausal women? 2) Little is known about the influence of hormones on the mechanisms of HPV carcinogenesis. Epidemiologists should encourage their laboratory colleagues to carry out experimental studies on this issue, since such studies will be valuable for bolstering (or not) the biologic plausibility of the epidemiologic associations. 3) Further studies on OC use and HRT are needed to clarify their role in relation to type of hormones, concentrations, duration, recency, and latency and to determine at which stages of carcinogenesis they may act. These issues will be better studied in cohort studies of premenopausal and postmenopausal women than in case–control studies. 4) Also, it would be important to undertake studies making measurements of markers of local immunity and inflammation and correlating them with other cofactors, such as pregnancy, age, and intake of exogenous hormones. Correlation analyses of time trends. It would be valuable to study the influence of declining birthrates on declining CC rates, especially in those countries where screening programs do not exist or have had little impact. Similarly, it would be interesting to study the relationship between time trends of CC rates and those of OC use and smoking. Choice of proper control subjects. The selection of proper control subjects depends on the basic assumption that most HPV infections in older women are persistent. However, unpublished results from a follow-up study of control women included in our case–control studies in Spain and Colombia suggest that this may not be the case. Thus, prospective studies of middle-aged HPV-positive women are needed to assess this assumption and to better understand the natural history of HPV infections in older women. Alternatively, the conduct of large case–control studies, including as control subjects women with at least two consecutive HPV-positive smears over time to confirm their chronic carrier state, would circumvent some of the limitations currently encountered in the interpretation of results from previous studies. A quick approximation to this approach for recently completed studies would be to recontact HPV-positive control women, retest them for cervical HPV DNA, and analyze the new data stratifying by HPV-persistence status. This strategy would, in addition, provide an assessment of the potential bias introduced in studies that restricted analyses to cross-sectionally detected HPV-positive control subjects. One should, however, realize that the use of persistently HPV-positive control subjects, while allowing for the assessment of risk factors for disease progression, would limit the ability of evaluating factors potentially associated with viral persistence itself. Finally, it is evident that more epidemiologic research is needed to understand the value of serologic markers of HPV exposure in studies on cofactors. The first pending issue is to improve the sensitivity of current serologic assays of HPV infection, probably by increasing multiple serotypes. Second, studies using both HPV DNA- and HPV antibody-detection methods are needed to contrast our current DNA-based risk estimates linked to the different cofactors and to assess whether the combination of both assays could improve unbiased selection of exposed women. Studies comparing the natural history of HPV infections with the dynamics and patterns of seroconversion are needed to better understand the meaning of these markers and to comprehend why some women experience seroconversion and some do not. Other studies. Except for the U.S. cohort study (7), no prospective data on cofactors are yet available from the large cohort studies being conducted in Costa Rica, Brazil, and Colombia. Assessment of unpublished results from these cohort studies would be informative in the planning of further research strategies on this issue. These studies will be of great value in establishing if the effects of cofactors that we are detecting in case–control studies are real or, on the contrary, merely due to residual confounding. These cohort studies will be able to adjust for the number of HPV infections and also be useful to determine at which stage of the carcinogenic process these cofactors act. As mentioned before, the value of HPV serology as a marker of cumulative lifetime exposure needs further study. As more refined serologic assays are being developed and validated, they could be introduced alone or in combination with HPV DNA in retrospective cohort studies that stored sera and cells and prospectively monitored relevant cofactors. No published data are available on the analysis of cofactors by main HPV types (16 and 18) or their phylogenetic groups. Unpublished data from the IARC series of case–control studies showed that associations between cofactors and CCs are not modified by specific genotypes, but more data are needed to confirm this. Finally, further methodologic work is needed to estimate AF to assess the global impact of these cofactors on CC occurrence, taking into account the HPV necessary-cause model. Conclusions Based on key studies on HSIL, CIN3, and CC conducted in HPV-positive women as determined by accurate polymerase chain reaction-based HPV DNA-detection methods, we conclude that high parity, smoking, and long-term OC use are cofactors that may modulate the risk of progression from HPV infection or LSIL to HSIL and CC. The evidence seems to be more consistent for parity and smoking than for OC use. The association with OC use was detected only for in situ adenocarcinoma in a U.S. study (3) and in the IARC studies for both squamous cell carcinoma and adenocarcinoma (9), but no associations were found in the U.K. nested case–control study of CIN3 (5) or in the U.S. prospective cohort study (7). From a public health point of view, parity seems to be the behavioral cofactor explaining the highest proportion of CC cases among HPV-infected women. However, smoking and long-term OC use may have a similar impact in populations heavily exposed to HPV and to these cofactors. The studies reviewed herein indicate that smoking and parity may act both at the HSIL and the CC stages. Ongoing prospective studies will shed more light into the role of these and other cofactors in HPV carcinogenesis; however, if confirmed, these conclusions imply that multiparous women, women who are smokers, and women on long-term OC use may need closer surveillance for cytologic abnormalities and HPV infections than women in the general population. Large prospective and retrospective cohort studies of HSIL and CC among middle-aged women in which several markers of HPV exposure are used and HPV persistence or chronicity is documented are still needed to understand the role and impact of cofactors in cervical carcinogenesis. Table 1. Characteristics of studies assessing the role of parity, OC use, and tobacco smoking in cervical carcinogenesis among HPV DNA-positive women*   Study  Study characteristics  Denmark  Norway  United States, Eastern  Manchester, U.K.  Costa Rica  United States Portland, OR  IARC,† international  *CC = cervical cancer; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; OC = oral contraceptive; PCR = polymerase chain reaction.  †Includes studies in Spain, Colombia, Brazil, Thailand, The Philippines, Morocco, Peru, and Paraguay.  ‡HPV16 only.  §All case subjects were included regardless of HPV status.  ||High-risk genotypes only.  ¶Cofactors in bold type indicate that a statistically significant association was found for that cofactor.  #For squamous cell.  **For adenocarcinoma in situ.  Authors, y, (reference No.)  Kruger-Kjaer et al., 1998 (1)  Olsen et al., 1998 (2)  Lacey et al., 1999 (3), 2001 (4)  Deacon et al., 2000 (5)  Hildesheim et al., 2001 (6)  Castle et al., 2002 (7)  Muñoz et al., 2002 (8); Moreno et al., 2002 (9); Plummer et al., unpublished data, 2002  Design  Case–control within cohort  Case–control  Case–control  Case–control within cohort  Case–control within cohort  Prospective cohort  Case–control, pooled analysis  Source of subjects  Population  Population  Population  Population  Population  Health plan  Clinic and population  Outcome  HSIL  CIN2 and CIN3  CIS and CC  CIN3  HSIL and CC  CIN3 and CC  CIS and CC  HPV-positive case subjects/ total tested  71/79  60/90‡  263§  199  146/168||  Cohort of 1812 women positive for oncogenic HPV DNA  1676/1853  HPV-positive control subjects/total tested  155/994  14/216‡  49/307  181  843||    255/1916  HPV DNA detection  GP5+/6+  L1 PCR  MY09/MY11  MY09/MY11  PCR  Hybrid Capture 2  MY09/MY11 and GP5+/6+  Relevant cofactors assessed¶  Parity, OC use, smoking  Smoking  Smoking,# OC use**  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking    Study  Study characteristics  Denmark  Norway  United States, Eastern  Manchester, U.K.  Costa Rica  United States Portland, OR  IARC,† international  *CC = cervical cancer; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; OC = oral contraceptive; PCR = polymerase chain reaction.  †Includes studies in Spain, Colombia, Brazil, Thailand, The Philippines, Morocco, Peru, and Paraguay.  ‡HPV16 only.  §All case subjects were included regardless of HPV status.  ||High-risk genotypes only.  ¶Cofactors in bold type indicate that a statistically significant association was found for that cofactor.  #For squamous cell.  **For adenocarcinoma in situ.  Authors, y, (reference No.)  Kruger-Kjaer et al., 1998 (1)  Olsen et al., 1998 (2)  Lacey et al., 1999 (3), 2001 (4)  Deacon et al., 2000 (5)  Hildesheim et al., 2001 (6)  Castle et al., 2002 (7)  Muñoz et al., 2002 (8); Moreno et al., 2002 (9); Plummer et al., unpublished data, 2002  Design  Case–control within cohort  Case–control  Case–control  Case–control within cohort  Case–control within cohort  Prospective cohort  Case–control, pooled analysis  Source of subjects  Population  Population  Population  Population  Population  Health plan  Clinic and population  Outcome  HSIL  CIN2 and CIN3  CIS and CC  CIN3  HSIL and CC  CIN3 and CC  CIS and CC  HPV-positive case subjects/ total tested  71/79  60/90‡  263§  199  146/168||  Cohort of 1812 women positive for oncogenic HPV DNA  1676/1853  HPV-positive control subjects/total tested  155/994  14/216‡  49/307  181  843||    255/1916  HPV DNA detection  GP5+/6+  L1 PCR  MY09/MY11  MY09/MY11  PCR  Hybrid Capture 2  MY09/MY11 and GP5+/6+  Relevant cofactors assessed¶  Parity, OC use, smoking  Smoking  Smoking,# OC use**  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking  View Large Table 2. Summary results of studies assessing the role of OC use as a cofactor in cervical carcinogenesis among HPV DNA-positive women*   Study (study outcome)  Exposure measures  Denmark (HSIL)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; OC = oral contraceptive; OR = odds ratio. Bold numbers denote statistical significance.  OC use status, OR  (95% CI)      Ever vs. never  NR  5.4 (0.7 to 43.4)  NR  NR  NR  1.4 (1.0 to 2.0)      Former vs. never  NR  3.1 (0.4 to 27.5)  1.2 (0.6 to 2.1)  0.9 (0.6 to 1.6)  NR  NR      Current vs. never  NR  17.1 (1.5 to 188.2)  1.3 (0.7 to 2.5)  1.5 (0.8 to 2.8)  0.8 (0.5 to 1.5)  NR  OC use duration      [Years] OR (95% CI) vs. never  Decreasing risk with increasing duration  [−2] 4.0 (0.4 to 44.3)  [−3] 1.2 (0.6 to 2.4)  [−4] 1.8 (0.6 to 4.9)  NR  [1] 0.7 (0.4 to 1.1)      [−6] 4.8 (0.4 to 51.9)  [−8] 0.8 (0.4 to 1.5)  [⩾5] 3.1 (1.1 to 9.1)    [−4] 0.8 (0.5 to 1.2)      [⩾7] 6.2 (0.7 to 52.7)  [>8] 1.5 (0.8 to 2.9)      [−9] 2.8 (1.5 to 5.4)              [⩾10] 4.0 (2.1 to 7.8)          P for trend  NR  .12  NR  NR  NR  <.001  Comments  ORs not reported  For adeno- carcinoma in situ    Duration estimates computed among women with <3 pregnancies  Relative risk refers to current vs. not current OC use at enrollment  For both squamous cell carcinoma and adenocarcinoma    Study (study outcome)  Exposure measures  Denmark (HSIL)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; OC = oral contraceptive; OR = odds ratio. Bold numbers denote statistical significance.  OC use status, OR  (95% CI)      Ever vs. never  NR  5.4 (0.7 to 43.4)  NR  NR  NR  1.4 (1.0 to 2.0)      Former vs. never  NR  3.1 (0.4 to 27.5)  1.2 (0.6 to 2.1)  0.9 (0.6 to 1.6)  NR  NR      Current vs. never  NR  17.1 (1.5 to 188.2)  1.3 (0.7 to 2.5)  1.5 (0.8 to 2.8)  0.8 (0.5 to 1.5)  NR  OC use duration      [Years] OR (95% CI) vs. never  Decreasing risk with increasing duration  [−2] 4.0 (0.4 to 44.3)  [−3] 1.2 (0.6 to 2.4)  [−4] 1.8 (0.6 to 4.9)  NR  [1] 0.7 (0.4 to 1.1)      [−6] 4.8 (0.4 to 51.9)  [−8] 0.8 (0.4 to 1.5)  [⩾5] 3.1 (1.1 to 9.1)    [−4] 0.8 (0.5 to 1.2)      [⩾7] 6.2 (0.7 to 52.7)  [>8] 1.5 (0.8 to 2.9)      [−9] 2.8 (1.5 to 5.4)              [⩾10] 4.0 (2.1 to 7.8)          P for trend  NR  .12  NR  NR  NR  <.001  Comments  ORs not reported  For adeno- carcinoma in situ    Duration estimates computed among women with <3 pregnancies  Relative risk refers to current vs. not current OC use at enrollment  For both squamous cell carcinoma and adenocarcinoma  View Large Table 3. Summary results of studies assessing the role of parity/pregnancy in cervical carcinogenesis among HPV DNA-positive women*   Study (study outcome)  Exposure measures  Denmark (HSIL)  Manchester, U.K. (CIN3)  Costa Rica, (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratio (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk. CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  †OR for ever versus never pregnant.  ‡Ever pregnant but 0 live births.  §Reference includes women with 0 or 1 live birth.  Ever vs. never, OR (95% CI)  NR  NR  4.6 (1.1 to 20)†  NR  NR  No. of live births or pregnancies      [No.] OR (95% CI) vs. never  [0]‡ 0.8 (0.4 to 1.7)  [1] 1.6 (0.9 to 2.8)  [2] 1.0 (0.5 to 2.2)§  [1–2] 1.1 (0.6 to 1.7)  [1–2] 1.8 (1.0 to 3.5)    [⩾1] 1.8 (0.3 to 2.3)  [2] 1.1 (0.6 to 2.0)  [3] 1.5 (0.7 to 3.2)§  [⩾3] 0.7 (0.3 to 1.6)  [3–4] 2.6 (1.3 to 4.9)      [⩾3] 1.9 (0.9 to 3.8)  [4–5] 3.5 (1.7 to 7.2)§    [5–6] 2.9 (1.4 to 5.6)        [6–8] 2.2 (1.0 to 5.0)§    [⩾7] 3.9 (1.9 to 7.9)        [⩾9] 1.4 (0.6 to 3.4)§              P for trend  NR  NS  .04  NR  <.0001  Comments    Unadjusted      For both CC and CIS    Study (study outcome)  Exposure measures  Denmark (HSIL)  Manchester, U.K. (CIN3)  Costa Rica, (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratio (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk. CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  †OR for ever versus never pregnant.  ‡Ever pregnant but 0 live births.  §Reference includes women with 0 or 1 live birth.  Ever vs. never, OR (95% CI)  NR  NR  4.6 (1.1 to 20)†  NR  NR  No. of live births or pregnancies      [No.] OR (95% CI) vs. never  [0]‡ 0.8 (0.4 to 1.7)  [1] 1.6 (0.9 to 2.8)  [2] 1.0 (0.5 to 2.2)§  [1–2] 1.1 (0.6 to 1.7)  [1–2] 1.8 (1.0 to 3.5)    [⩾1] 1.8 (0.3 to 2.3)  [2] 1.1 (0.6 to 2.0)  [3] 1.5 (0.7 to 3.2)§  [⩾3] 0.7 (0.3 to 1.6)  [3–4] 2.6 (1.3 to 4.9)      [⩾3] 1.9 (0.9 to 3.8)  [4–5] 3.5 (1.7 to 7.2)§    [5–6] 2.9 (1.4 to 5.6)        [6–8] 2.2 (1.0 to 5.0)§    [⩾7] 3.9 (1.9 to 7.9)        [⩾9] 1.4 (0.6 to 3.4)§              P for trend  NR  NS  .04  NR  <.0001  Comments    Unadjusted      For both CC and CIS  View Large Table 4. Summary results of studies assessing the role of cigarette smoking in cervical carcinogenesis among HPV DNA-positive women*   Study (study outcome)  Exposure measures  Denmark (HSIL)  Norway (CIN2 and CIN3)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  Smoking status, OR (95% CI)      Ever vs. never  NR  4.6 (0.9 to 22.9)  1.5 (0.7 to 3.0)  2.2 (1.4 to 3.4)  NR  NR  2.2 (1.5 to 3.2)      Former vs. never  3.2 (0.9 to 11.4)  4.2 (0.5 to 37.9)  1.2 (0.5 to 3.1)  1.7 (0.8 to 3.7)  1.7 (0.8 to 4.0)  2.1 (1.1 to 3.9)  1.8 (0.9 to 3.4)      Current vs. never  1.9 (1.0 to 3.8)  NR  1.6 (0.7 to 3.5)  NR  2.3 (1.2 to 4.3)  NR  2.3 (1.3 to 4.0)  Smoking amount      [cigarettes/day] OR (95% CI) vs. never  NR  [−10] 3.3 (0.5 to 21)  [−19] 1.6 (0.6 to 3.9)  [−10] 1.4 (0.7 to 2.5)  [−5] 1.8 (1.0 to 3.3)  [−19] 2.2 (1.2 to 4.2)  [−5] 1.9 (1.1 to 3.4)      [⩾11] 5.9 (1.0 to 36)  [⩾20] 1.3 (0.6 to 3.0)  [−16] 2.2 (1.2 to 3.9)  [⩾6] 3.1 (1.2 to 7.9)  [⩾20] 2.9 (1.5 to 5.6)  [⩾6] 2.2 (1.2 to 4.2)          [⩾17] 3.1 (1.8 to 5.3)                P for trend  NR  NR  .49  <.0001  .003  NR  NS  Smoking duration      [years] OR (95% CI) vs. never  NR  [−9] 2.1 (0.3 to 12.3)  [−10] 1.2 (0.5 to 3.0)  [−9] 1.8 (0.9 to 3.6)  [−9] 2.2 (1.0 to 4.8)  NR  [−19] 2.6 (1.4 to 4.7)      [⩾10] 7.5 (1.2 to 46)  [−20] 3.2 (1.0 to 9.7)  [−19] 2.0 (1.2 to 3.3)  [⩾10] 2.0 (1.0 to 3.8)    [⩾20] 1.9 (1.0 to 3.5)        [>20] 0.8 (0.3 to 2.7)  [⩾20] 3.1 (1.6 to 6.2)                P for trend  NR  NR  .57  <.0005  NR    NS  Comments    Stronger asso- ciations found among HPV16- seropositive subjects  For squamous cell carcinoma  Univariate OR. Adjusted OR also statistically significant          Study (study outcome)  Exposure measures  Denmark (HSIL)  Norway (CIN2 and CIN3)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  Smoking status, OR (95% CI)      Ever vs. never  NR  4.6 (0.9 to 22.9)  1.5 (0.7 to 3.0)  2.2 (1.4 to 3.4)  NR  NR  2.2 (1.5 to 3.2)      Former vs. never  3.2 (0.9 to 11.4)  4.2 (0.5 to 37.9)  1.2 (0.5 to 3.1)  1.7 (0.8 to 3.7)  1.7 (0.8 to 4.0)  2.1 (1.1 to 3.9)  1.8 (0.9 to 3.4)      Current vs. never  1.9 (1.0 to 3.8)  NR  1.6 (0.7 to 3.5)  NR  2.3 (1.2 to 4.3)  NR  2.3 (1.3 to 4.0)  Smoking amount      [cigarettes/day] OR (95% CI) vs. never  NR  [−10] 3.3 (0.5 to 21)  [−19] 1.6 (0.6 to 3.9)  [−10] 1.4 (0.7 to 2.5)  [−5] 1.8 (1.0 to 3.3)  [−19] 2.2 (1.2 to 4.2)  [−5] 1.9 (1.1 to 3.4)      [⩾11] 5.9 (1.0 to 36)  [⩾20] 1.3 (0.6 to 3.0)  [−16] 2.2 (1.2 to 3.9)  [⩾6] 3.1 (1.2 to 7.9)  [⩾20] 2.9 (1.5 to 5.6)  [⩾6] 2.2 (1.2 to 4.2)          [⩾17] 3.1 (1.8 to 5.3)                P for trend  NR  NR  .49  <.0001  .003  NR  NS  Smoking duration      [years] OR (95% CI) vs. never  NR  [−9] 2.1 (0.3 to 12.3)  [−10] 1.2 (0.5 to 3.0)  [−9] 1.8 (0.9 to 3.6)  [−9] 2.2 (1.0 to 4.8)  NR  [−19] 2.6 (1.4 to 4.7)      [⩾10] 7.5 (1.2 to 46)  [−20] 3.2 (1.0 to 9.7)  [−19] 2.0 (1.2 to 3.3)  [⩾10] 2.0 (1.0 to 3.8)    [⩾20] 1.9 (1.0 to 3.5)        [>20] 0.8 (0.3 to 2.7)  [⩾20] 3.1 (1.6 to 6.2)                P for trend  NR  NR  .57  <.0005  NR    NS  Comments    Stronger asso- ciations found among HPV16- seropositive subjects  For squamous cell carcinoma  Univariate OR. Adjusted OR also statistically significant        View Large Table 5. Impact of different strategies of HPV adjustment on associations between cofactors and risk of CC (from the IARC case–control studies)*   All women  HPV-positive women  Cofactor  Cases/controls  Not HPV adjusted, OR (95% CI)  HPV adjusted, OR (95% CI)  Case/control  OR (95% CI)  *ORs adjusted for center, age (<37, 37–45, 46–55, or ⩾56 years), educational level (none, primary, secondary, or higher), smoking amount (never, 1–5 cigarettes/day, or ⩾6 cigarettes/day), age at first sexual intercourse (<17, 17–18, 19–22, or ⩾23 years), lifetime number of sexual partners (1, 2–3, or ⩾4), OC use (never, 1–4 years, 5–9 years, or ⩾10 years), lifetime number of Pap smears (0, 1–5, or ⩾6), and parity (0, 1–2, 3–4, 5–6, or ⩾7). CC = cervical cancer; CI = confidence interval; HPV = human papillomavirus; IARC = International Agency for Research on Cancer; OC = oral contraceptive; OR = odds ratio.  †Referent.  Full-term pregnancies (status and No.)      Never  95/164  1†  1†  57/24  1†      Ever  2183/2209  1.08  1.32  1616/229  2.45      (0.80 to 1.47)  (0.88 to 1.98)    (1.33 to 4.51)      1–2  444/747  0.82  0.99  279/59  1.79      (0.60 to 1.12)  (0.65 to 1.50)    (0.94 to 3.40)      3–4  644/677  1.33  1.68  450/70  2.61      (0.97 to 1.83)  (1.09 to 2.57)    (1.37 to 5.00)      ⩾5  1095/785  1.73  2.03  887/100  3.88      (1.24 to 2.41)  (1.30 to 3.16)    (1.99 to 7.55)  OC use (status and years)      Never  1419/1508  1†  1†  1071/163  1†      Ever  864/886  1.09  0.97  605/92  1.13      (0.94 to 1.27)  (0.79 to 1.19)    (0.80 to 1.59)      1–4 y  351/445  0.90  0.75  274/64  0.66      (0.75 to 1.09)  (0.58 to 0.97)    (0.45 to 0.98)      ⩾5 y  510/427  1.33  1.17  331/28  2.35      (1.11 to 1.59)  (0.92 to 1.49)    (1.44 to 3.85)  Smoking (status and amount)      Never  1645/1905  1†  1†  1265/218  1†      Ever  636/488  1.30  1.68  409/36  1.99      (1.11 to 1.52)  (1.36 to 2.08)    (1.29 to 3.07)      1–5 cigarettes/day  251/200  1.21  1.46  181/17  1.72      (0.98 to 1.51)  (1.09 to 1.97)    (0.98 to 3.01)      ⩾6 cigarettes/day  350/216  1.79  2.07  211/18  2.16      (1.45 to 2.22)  (1.56 to 2.75)    (1.18 to 3.97)    All women  HPV-positive women  Cofactor  Cases/controls  Not HPV adjusted, OR (95% CI)  HPV adjusted, OR (95% CI)  Case/control  OR (95% CI)  *ORs adjusted for center, age (<37, 37–45, 46–55, or ⩾56 years), educational level (none, primary, secondary, or higher), smoking amount (never, 1–5 cigarettes/day, or ⩾6 cigarettes/day), age at first sexual intercourse (<17, 17–18, 19–22, or ⩾23 years), lifetime number of sexual partners (1, 2–3, or ⩾4), OC use (never, 1–4 years, 5–9 years, or ⩾10 years), lifetime number of Pap smears (0, 1–5, or ⩾6), and parity (0, 1–2, 3–4, 5–6, or ⩾7). CC = cervical cancer; CI = confidence interval; HPV = human papillomavirus; IARC = International Agency for Research on Cancer; OC = oral contraceptive; OR = odds ratio.  †Referent.  Full-term pregnancies (status and No.)      Never  95/164  1†  1†  57/24  1†      Ever  2183/2209  1.08  1.32  1616/229  2.45      (0.80 to 1.47)  (0.88 to 1.98)    (1.33 to 4.51)      1–2  444/747  0.82  0.99  279/59  1.79      (0.60 to 1.12)  (0.65 to 1.50)    (0.94 to 3.40)      3–4  644/677  1.33  1.68  450/70  2.61      (0.97 to 1.83)  (1.09 to 2.57)    (1.37 to 5.00)      ⩾5  1095/785  1.73  2.03  887/100  3.88      (1.24 to 2.41)  (1.30 to 3.16)    (1.99 to 7.55)  OC use (status and years)      Never  1419/1508  1†  1†  1071/163  1†      Ever  864/886  1.09  0.97  605/92  1.13      (0.94 to 1.27)  (0.79 to 1.19)    (0.80 to 1.59)      1–4 y  351/445  0.90  0.75  274/64  0.66      (0.75 to 1.09)  (0.58 to 0.97)    (0.45 to 0.98)      ⩾5 y  510/427  1.33  1.17  331/28  2.35      (1.11 to 1.59)  (0.92 to 1.49)    (1.44 to 3.85)  Smoking (status and amount)      Never  1645/1905  1†  1†  1265/218  1†      Ever  636/488  1.30  1.68  409/36  1.99      (1.11 to 1.52)  (1.36 to 2.08)    (1.29 to 3.07)      1–5 cigarettes/day  251/200  1.21  1.46  181/17  1.72      (0.98 to 1.51)  (1.09 to 1.97)    (0.98 to 3.01)      ⩾6 cigarettes/day  350/216  1.79  2.07  211/18  2.16      (1.45 to 2.22)  (1.56 to 2.75)    (1.18 to 3.97)  View Large Table 6. Proportion of cervical cancer cases attributable to various HPV cofactors, separately or in combination* Separately          HPV cofactor  Level of exposure  % among control subjects  % among case subjects  OR†  AF, %  Full-term pregnancies  Ever vs. never  90.5  96.6  2.51  58    ⩾2 vs. 0–1  83.0  89.8  1.63  34    ⩾5 vs. 0–1  39.5  53.0  2.27  33  OC use  Ever vs. never  36.1  36.1  1.13  4    ⩾5 vs. 0–4  11.0  19.7  2.70  16  Smoking  Ever vs. never  14.2  24.4  1.98  12    ⩾6 cigarettes/day vs. 0 cigarettes/day  7.1  12.7  2.08  7  Separately          HPV cofactor  Level of exposure  % among control subjects  % among case subjects  OR†  AF, %  Full-term pregnancies  Ever vs. never  90.5  96.6  2.51  58    ⩾2 vs. 0–1  83.0  89.8  1.63  34    ⩾5 vs. 0–1  39.5  53.0  2.27  33  OC use  Ever vs. never  36.1  36.1  1.13  4    ⩾5 vs. 0–4  11.0  19.7  2.70  16  Smoking  Ever vs. never  14.2  24.4  1.98  12    ⩾6 cigarettes/day vs. 0 cigarettes/day  7.1  12.7  2.08  7  Combination of cofactors          Parity  OC use, y  Smoking  % among control subjects  % among case subjects  OR†  AF, %  *AF = attributable fraction; HPV = human papillomavirus; OC = oral contraceptive; OR = odds ratio. 1 = referent category.  †Adjusted for center, age, educational level, smoking amount, age at first sexual intercourse, lifetime number of sexual partners, OC use (years), lifetime number of Pap smears, and parity.  0–1  0–4  Never  12.4  5.4  1  —  0–1  0–4  Ever  3.6  3.5  2.12  4  0–1  ⩾5  Never  0.8  0.6  2.51  1  0–1  ⩾5  Ever  0.4  0.7  3.60  1  ⩾2  0–4  Never  63.7  55.8  1.61  28  ⩾2  0–4  Ever  9.2  15.5  2.87  15  ⩾2  ⩾5  Never  8.8  13.7  4.16  22  ⩾2  ⩾5  Ever  1.2  4.8  11.02  11  Combination of cofactors          Parity  OC use, y  Smoking  % among control subjects  % among case subjects  OR†  AF, %  *AF = attributable fraction; HPV = human papillomavirus; OC = oral contraceptive; OR = odds ratio. 1 = referent category.  †Adjusted for center, age, educational level, smoking amount, age at first sexual intercourse, lifetime number of sexual partners, OC use (years), lifetime number of Pap smears, and parity.  0–1  0–4  Never  12.4  5.4  1  —  0–1  0–4  Ever  3.6  3.5  2.12  4  0–1  ⩾5  Never  0.8  0.6  2.51  1  0–1  ⩾5  Ever  0.4  0.7  3.60  1  ⩾2  0–4  Never  63.7  55.8  1.61  28  ⩾2  0–4  Ever  9.2  15.5  2.87  15  ⩾2  ⩾5  Never  8.8  13.7  4.16  22  ⩾2  ⩾5  Ever  1.2  4.8  11.02  11  View Large Supported in part by the International Agency for Research on Cancer, Lyon, France; by grants 01/1237, 01/1236, and BAE 01/5013 from the Fondo de Investigaciones Sanitarias, Madrid, Spain; and by a Yamagiwa-Yoshida Memorial International Cancer study grant from the International Union Against Cancer. We thank Gina Albero (Institut Català d’Oncologia) for her data handling and statistical analyses. References 1 Kruger-Kjaer S, van den Brule AJ, Svare EI, Engholm G, Sherman ME, Poll PA, et al. Different risk factor patterns for high-grade and low-grade intraepithelial lesions on the cervix among HPV-positive and HPV-negative young women. Int J Cancer  1998; 76: 613–9. Google Scholar 2 Olsen AO, Dillner J, Skrondal A, Magnus P. Combined effect of smoking and human papillomavirus type 16 infection in cervical carcinogenesis. Epidemiology  1998; 9: 346–9. Google Scholar 3 Lacey JV Jr, Brinton LA, Abbas FM, Barnes WA, Gravitt PE, Greenberg MD, et al. Oral contraceptives as risk factors for cervical adenocarcinomas and squamous cell carcinomas. Cancer Epidemiol Biomarkers Prev  1999; 8: 1079–85. Google Scholar 4 Lacey JV Jr, Frisch M, Brinton LA, Abbas FM, Barnes WA, Gravitt PE, et al. 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IARC monographs on the evaluation of carcinogenic risks to humans. Vol. 64. Human papillomaviruses. Lyon (France): IARC; 1995. Google Scholar 13 Mitrani-Rosenbaum S, Tsvieli R, Tur-Kaspa R. Oestrogen stimulates differential transcription of human papillomavirus type 16 in SiHa cervical carcinoma cells. J Gen Virol  1989; 70(Pt 8): 2227–32. Google Scholar 14 Arbeit JM, Howley PM, Hanahan D. Chronic estrogen-induced cervical and vaginal squamous carcinogenesis in human papillomavirus type 16 transgenic mice. Proc Natl Acad Sci U S A  1996; 93: 2930–5. Google Scholar 15 Elson DA, Riley RR, Lacey A, Thordarson G, Talamantes FJ, Arbeit JM. Sensitivity of the cervical transformation zone to estrogen-induced squamous carcinogenesis. Cancer Res  2000; 60: 1267–75. Google Scholar 16 International Agency for Research on Cancer (IARC). IARC monographs on the evaluation of the carcinogenic risk to humans. Vol 72. Hormonal contraception and post-menopausal hormonal therapy. 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J Natl Cancer Inst  1997; 89: 868–73. Google Scholar 22 Poppe WA, Ide PS, Drijkoningen MP, Lauweryns JM, Van Assche FA. Tobacco smoking impairs the local immunosurveillance in the uterine cervix. An immunohistochemical study. Gynecol Obstet Invest  1995; 39: 34–8. Google Scholar 23 Giulian AR, Sedjo RL, Roe DJ, Harri R, Baldwi S, Papenfuss MR, et al. Clearance of oncogenic human papillomavirus (HPV) infection: effect of smoking (United States). Cancer Causes Control  2002; 13: 839–46. Google Scholar 24 Szarewski A, Jarvis MJ, Sasieni P, Anderson M, Edwards R, Steele SJ, et al. Effect of smoking cessation on cervical lesion size. Lancet  1996; 347: 941–3. Google Scholar 25 Moreno V, Munoz N, Bosch FX, de Sanjose S, Gonzalez LC, Tafur L, et al. Risk factors for progression of cervical intraepithelial neoplasm grade III to invasive cervical cancer. Cancer Epidemiol Biomarkers Prev  1995; 4: 459–67. Google Scholar 26 Thomas DB, Qin Q, Kuypers J, Kiviat N, Ashley RL, Koetsawang A, et al. Human papillomaviruses and cervical cancer in Bangkok. II. Risk factors for in situ and invasive squamous cell cervical carcinomas. Am J Epidemiol  2001; 153: 732–9. Google Scholar 27 Ho GY, Kadish AS, Burk RD, Basu J, Palan PR, Mikhail M, et al. HPV 16 and cigarette smoking as risk factors for high-grade cervical intra-epithelial neoplasia. Int J Cancer  1998; 78: 281–5. Google Scholar 28 Woodman CB, Collins S, Winter H, Bailey A, Ellis J, Prior P, et al. Natural history of cervical human papillomavirus infection in young women: a longitudinal cohort study. Lancet  2001; 357: 1831–6. Google Scholar 29 Herrero R, Hildesheim A, Bratti C, Sherman ME, Hutchinson M, Morales J, et al. Population-based study of human papillomavirus infection and cervical neoplasia in rural Costa Rica. J Natl Cancer Inst  2000; 92: 464–74. Google Scholar 30 Lazcano-Ponce E, Herrero R, Munoz N, Cruz A, Shah KV, Alonso P, et al. Epidemiology of HPV infection among Mexican women with normal cervical cytology. Int J Cancer  2001; 91: 412–20. Google Scholar 31 Sellors JW, Mahony JB, Kaczorowski J, Lytwyn A, Bangura H, Chong S, et al. Prevalence and predictors of human papillomavirus infection in women in Ontario, Canada. Survey of HPV in Ontario Women (SHOW) Group. CMAJ  2000; 163: 503–8. Google Scholar © Oxford University Press http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png JNCI Monographs Oxford University Press

Chapter 3: Cofactors in Human Papillomavirus Carcinogenesis—Role of Parity, Oral Contraceptives, and Tobacco Smoking

Castellsagué, Xavier; Muñoz, Nubia
JNCI Monographs , Volume 2003 (31) – Jun 1, 2003
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Oxford University Press
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© Oxford University Press
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1052-6773
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10.1093/oxfordjournals.jncimonographs.a003477
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Abstract

Abstract It is now well established that infection with oncogenic human papillomavirus (HPV) types is the necessary cause of cervical cancer (CC) and its immediate precursor cervical intraepithelial neoplasia 3. However, HPV infection alone may not be sufficient to cause CC, and other exogenous and endogenous factors may exist that, in conjunction with HPV, influence the risk of progression from cervical HPV infection to CC. In this chapter, we review the evidence for the role of parity, oral contraceptive (OC) use, and tobacco smoking in CC. We also discuss limitations and methodologic problems encountered in assessing available data and outline recommendations for future research. Based on key studies on high-grade squamous intraepithelial lesions (HSILs) and CC conducted among HPV-positive women, it can be concluded that high parity, smoking, and less consistently long-term OC use are cofactors that may modulate the risk of progression from HPV infection to HSIL/CC. From a public health point of view, parity seems to be the behavioral cofactor explaining the highest proportion of CC cases among HPV-infected women. Smoking and long-term OC use may have a similar impact in populations that are heavily exposed to HPV and to these cofactors. Large prospective and retrospective cohort studies of HSIL and CC among middle-aged women in which several markers of HPV exposure are used and HPV persistence is documented would be valuable to study the role of these and other cofactors in HPV carcinogenesis. If confirmed, our conclusions may imply that multiparous women, women who are smokers, and women on long-term OC use may need closer surveillance for cytologic abnormalities and HPV infections than women in the general population. It is now well established that infection with oncogenic human papillomavirus (HPV) types is the necessary cause of cervical cancer (CC) and its immediate precursor cervical intraepithelial neoplasia (CIN) 3 (CIN3). However, HPV infection alone may not be sufficient to cause CC, and other exogenous or endogenous factors might exist that, in conjunction with HPV, influence the risk of progression from cervical HPV infection to CC. Candidate cofactors may be classified into three groups: 1) environmental or exogenous cofactors, including use of oral contraceptives (OCs), tobacco smoking, diet, cervical trauma, and coinfection with human immunodeficiency virus (HIV) and other sexually transmitted agents; 2) viral cofactors, such as infection by specific types, coinfection with other types, HPV variants, viral load, and viral integration; and 3) host cofactors, including endogenous hormones, genetic factors such as human leukocyte antigen, and other host factors related to the host’s immune response. The purpose of this chapter is to review, summarize, and discuss the evidence of the role of the more established cofactors, including parity, OC use, and tobacco smoking. Cofactors involved in the natural history of HPV infection are reviewed in Chapter 2. In this chapter, the focus is on cofactors affecting progression from HPV infection to high-grade squamous intraepithelial lesions (HSILs) and CC. Ideally, if we accept the premise that all CCs are caused by oncogenic HPVs, a strict assessment of cofactors requires a study group known to be exposed to HPV. From that HPV-exposed group, retrospectively or ideally prospectively, the added risk attributable to other factors can be estimated. Although this approach is admittedly controversial (see the “Discussion” section), we believe that, in the absence of repeated HPV measures, restriction to HPV DNA-positive case patients and control subjects conveys the strictest approach to adjustment for HPV. This review will thus focus on selected key studies that, using reliable DNA-detection methods, report associations between cofactors and HSIL/CC within a well-defined HPV-positive group. These studies do exist and the main characteristics of the most important ones are summarized in Table 1 [(1–9); Plummer M, Herrero R, Franceschi S, Meijer CJ, Snijders P, Bosch FX, et al.: unpublished data]. Studies among HPV-positive women that included low-grade squamous intraepithelial lesions (LSILs) (10) or the three grades of CIN (11) are not considered in detail in this chapter. Evidence for a Role of OC use in HPV Carcinogenesis Use of OCs has been found to be associated with CC in many, but not in all, epidemiologic studies that adjusted for HPV status. In studies restricted to HPV-positive women, however, the evidence for an association is in general weaker (Table 2). Of the six studies reporting results restricted to HPV DNA-positive subjects, three found positive statistically significant associations, but these were for a particular histologic type or for a subgroup of women. The Eastern U.S. study (3) reported an odds ratio (OR) of 17.1 (95% confidence interval [CI] = 1.5 to 188.2) for current versus never OC use, but this was only for adenocarcinoma in situ; although the risk increased with longer duration of use, the trend was of borderline statistical significance. The study in Costa Rica (6) found a 3.1-fold increased risk for users of 5 or more years as compared with never users, but this association was observed only among women who had two or fewer pregnancies, and the HSIL/cancer group included only 30 cases. The strongest evidence for a role of OC use in HPV carcinogenesis derives from the large pooled analysis of the International Agency for Research on Cancer (IARC) studies (9). Even though ever use of OCs was moderately associated with cancer risk (OR = 1.4), there was a strong dose–response relationship with increasing years of use (Table 2). No increase in the risk of cervical neoplasia was found for the duration of OC use for up to 4 years. However, use of OCs for longer than 5 years was significantly associated with cervical neoplasia (OR = 3.4; 95% CI = 2.1 to 5.5). The risk of OC use for longer than 5 years was increased fourfold for invasive CC (OR = 4.0; 95% CI = 2.0 to 8.0) and threefold for carcinoma in situ (CIS) (OR = 3.4; 95% CI = 2.1 to 5.5). Special attention should be given to the lack of association reported in the only prospective study of CIN3 and CC (7). As the same authors discuss, first, only one measurement of OC use was obtained at enrollment, thus not accounting for the possible discontinuation or initiation of use of OCs during the course of the study. Second, OC users had shorter follow-up times than nonusers, which may have resulted in censoring bias among the users. Third, among the control subjects, OC users were more likely to be diagnosed with CIN1 and CIN2 during follow-up than nonusers, which may have resulted in an increased detection and treatment of women in the control group whose disease might have otherwise progressed to CIN3 or CC. Finally, no information regarding duration was collected, thus potentially missing an association with long-term use. Not much data are available concerning the mechanisms by which hormonal influences may modulate the risk of progression to HSIL/CC among HPV-infected women. Hormone-related mechanisms may influence the progression from premalignant to malignant cervical lesions by promoting integration of HPV DNA into the host genome, which results in deregulation of E6 and E7 expression (12). An experimental study (13) has shown that estradiol may stimulate the transcription of HPV type 16 (HPV16) E6 and E7 in cell lines that contain integrated HPV16. Since the E6 and E7 open reading frames have been associated with the oncogenic potential of HPV16, the effect of estrogen on the transcription of these viral genes may be of biologic relevance in the malignant transformation of HPV16-infected cervical cells. Data from experimental studies (14,15) demonstrate a synergistic mechanism between long-term estrogen exposure and HPV16 oncogenes that modulates squamous carcinogenesis in the female reproductive tract of transgenic HPV16-expressing mice. Alternatively, OCs might facilitate HPV reactivation or persistence, although indirect evidence from several studies (9, see review in 16) does not find an association between OC use and HPV positivity among control women. Evidence for a Role of Parity in HPV Carcinogenesis High parity has consistently been found in most case–control studies to be associated with both CC and CIS. Most of the major studies restricting the analysis to HPV-positive women also report an increased risk of HSIL/CC with an increasing number of pregnancies (Table 3). In the IARC-pooled analysis, the OR for CC in women with seven or more full-term pregnancies was fourfold higher than that in nulliparous women, and the risk increased linearly with an increasing number of full-term pregnancies (8). Risk of HSIL/CC significantly increased with an increasing number of live births in the Costa Rica study (6). A borderline association with CIN3 was found in the Manchester study (5). The study in Denmark (1) and the U.S. prospective study (7) did not find an association with the risk of HSIL and CIN3/CC, respectively. However, this result could be explained by the low parity of the study populations. In addition, in the U.S. cohort, information on parity was recruited only at enrollment. Thus, pregnancies occurring during the 10-year follow-up period of the study might have been more relevant to the prospective risk of CIN3/CC than those occurring before enrollment. In addition to the studies restricted to HPV-positive women, further evidence of a hormonal effect on CC comes from the analysis of the age-distribution curve based on cohort effects in relation to mortality without the distortion potentially introduced by screening and secular changes. The analysis shows that CC mortality rates increase very sharply up to age 50 years (i.e., around menopause) and flatten thereafter. Nutritional, hormonal, traumatic, and immunologic mechanisms have been hypothesized as biologically plausible explanations for the association between parity and HSIL/CC among infected women; however, because of the concordance of effects with OC use, hormonal influences can be considered to be one of the most promising candidates in the search for HPV cofactors. High parity may likely increase the risk of CC because it maintains the transformation zone on the exocervix for many years (17), facilitating the direct exposure to HPV and, possibly, to other cofactors. Hormonal changes induced by pregnancy (increased levels of estrogen and progesterone) may also modulate the immune response to HPV and influence risk of persistence or progression (8,18). Evidence for a Role of Tobacco Smoking in HPV Carcinogenesis The effects of smoking have been well studied in many case–control studies, and they show a moderate and statistically significant association with CC, even after adjusting for the strong effects of HPV. These findings are strikingly consistent with those obtained in studies restricted to HPV-positive women. As shown in Table 4, all such studies report some evidence that tobacco smoking increases the risk of developing HSIL and CC. The ORs for ever smoking among HPV-positive women are in the range of 2 to 5. Furthermore, most studies reporting risk estimates according to intensity, duration, or pack-years show an increased risk of CC with increasing exposure to tobacco smoking. Because of the prospective nature of the study, the positive association found with smoking status and smoking intensity in the U.S. prospective study is particularly relevant (7). Despite the consistency of these findings, the possibility remains that smoking or smoking duration is a proxy for time since HPV exposure, because long-duration smokers may also have had an HPV infection for a long time. Thus, residual confounding by time since HPV infection cannot be ruled out as a possible explanation for the observed effects of smoking. Almost 60 years ago, Rous and Friedwald (19) reported the carcinogenic effect of tar on virus-induced rabbit papillomas. More recently, malignant transformation of HPV16-immortalized human endocervical cells by cigarette smoke condensate has been proven (20). The fact that nicotine and tobacco-specific carcinogens have been detected in the cervical mucus of smokers (21) further strengthens the hypothesis of a synergistic action between cigarette smoking and HPV for the development of HSIL/CC. Chemical tobacco-related carcinogens may exert a direct mitogenic effect causing DNA damage. Some authors (22) hypothesize that exposure to tobacco may affect the ability of the host to mount an effective local immune response against viral infections, since it has been shown that smoking may reduce the number of Langerhans’ cells and other markers of immune function. A recent prospective study (23) presents convincing evidence that smokers maintain cervical HPV infections significantly longer and have a lower probability of clearing an oncogenic infection than women who never smoked. The significant association found between the extent of smoking reduction and the reduction in lesion size in an intervention study of smoking cessation among women with minor-grade lesions further strengthens the plausible role of tobacco smoking in HPV carcinogenesis (24). Evidence From Studies Comparing LSIL, HSIL, and CC Cofactor Profiles Finally, it is worth mentioning that few cofactors have been identified to distinguish invasive cancer from intraepithelial lesions or HSIL from LSIL. The IARC Spain–Colombia study (25) considered a large number of risk factors and found that both the CIN3 and CC patients had very similar profiles of risk factors. A recent study (26) conducted in Thailand found that, after controlling for HPV type, the risk of developing CC, as compared with the risk of developing intraepithelial lesions, was not related to any of the cofactors considered, except for two indices of socioeconomic status. In contrast, a study comparing HPV-positive women with CIN3 with HPV-positive women with CIN1 (27) found that cigarette smoking was significantly associated with CIN3, suggesting that HPV-infected cells may relate to tobacco-containing carcinogens for neoplastic progression. Discussion Methodologic Issues in the Study of Cofactors in HPV Carcinogenesis Definition of case patients and control subjects. HPV cofactors in CC may act in at least three ways: 1) by influencing the acquisition of HPV infection, 2) by increasing the risk of HPV persistence, and 3) by increasing the risk of progression from HPV infection to HSIL and cancer. Studies aimed at identifying factors for HPV acquisition and persistence were considered in Chapter 2. In these studies, case patients and control subjects can be defined on the basis of HPV/LSIL status. Here, we focused on factors that, acting once HPV infection has been established, modulate the risk of progression from HPV infection to HSIL/CC. These cofactors can be identified by cohort or case–control studies of HPV-positive women in which case patients include those diagnosed with HSIL, CIN3, CIS, or CC but not those diagnosed with LSIL or CIN1. Accepting that HPV is a necessary cause of CC, we believe that proper control subjects for these studies ought to be HPV-positive women. Which assay or combination of assays is best suited to select HPV-exposed or HPV-positive women still remains an important research issue for which more data are needed. Because many investigators consider LSIL an early manifestation of HPV infection, some have also included women with LSIL in the HPV-positive control group (6). Studies (11) including the whole spectrum of precancerous lesions as cases are difficult to interpret because the case group includes a mixture of exposed and diseased women, because some have LSIL, a marker of HPV exposure more than a disease outcome, and some have HSIL/CC, clearly an HPV-related disease outcome. The nested case–control studies carried out in Denmark (1) and in the United Kingdom (5) suggest that the pattern of risk factors for HPV infection or LSIL are different from the patterns for CIN3 and HSIL. Again, under the premise that HPV is a necessary cause of CC, a more refined case definition could be made on the basis of both HPV and disease statuses. In relation to the control subjects and the selection of HPV-exposed women, the key issue is whether one-point HPV positivity can be considered to be a sufficient criterion to define the proper control group. In other words, can we assume that cross-sectionally identified HPV-positive control women are carrying a persistent or chronic infection? How long should persistent infection persist to increase the risk of CC: many months or just a few months as suggested recently (28)? If persistent infections are associated with CC risk, the definition of HPV-positive control women based on a single measure (used in most studies) might not be sufficient. However, if most HPV infections in middle-aged women tend to be persistent, the use of a single HPV test may be more of a problem in studies of HSIL than in studies of invasive CC. Furthermore, the interpretation of results from studies including young women (most of the HSIL studies in Table 1) is further limited by the fact that measures of the putative environmental cofactors are strongly age dependent. Younger women are less likely to have had a high number of pregnancies, to have smoked, or to have used OCs for a long time. HPV adjustment strategies. Using the IARC series of case–control studies, we were able to estimate the impact of different strategies of HPV adjustment on associations between environmental cofactors and CC risk. For each of the three cofactors of interest, we fitted three different models: 1) one model including all case patients and all control subjects but ignoring adjustment for HPV, 2) a second including all subjects but adjusting for HPV DNA status, and 3) a third restricting the analysis to case patients and control subjects who tested positive for HPV DNA. As shown in Table 5, of the three strategies, models restricted to HPV DNA-positive subjects yielded higher associations that were between 1.1- and 2.0-fold higher than those derived from HPV-adjusted models. For parity and smoking, the magnitudes of the ORs were lowest for the crude models, were intermediate for the HPV-adjusted models, and were highest for the HPV-restricted models. For OC use, the HPV-adjusted models yielded lower ORs than the crude and HPV-restricted models. If we believe that HPV restriction is the strictest approach to HPV adjustment, one could then conclude that the studies adjusting for HPV are likely to underestimate the magnitude of the association but that this underestimation is not greater than twofold. It remains to be explained why we see stronger rather than weaker effects when analyses are restricted to HPV-positive case patients and control subjects. If it is claimed that this method of adjustment is better than the other strategies in getting rid of residual confounding, then we should expect lower rather than higher ORs. Perhaps negative confounding between cofactors and HPV status or selection bias introduced by restricting the analysis to HPV-positive subjects (i.e., the case–control matching is totally ignored after restriction) might explain the increase in the ORs. Beyond the discussion of which adjustment method is more suitable in the assessment of cofactor, it is reassuring to observe that, regardless of the strategy used, the same conclusions concerning the direction and statistical significance of the associations are reached. Since the goal in the assessment of cofactors is to explore associations among HPV-exposed women, an alternative approach is to explore the use, alone or in combination with HPV DNA detection, of serologic markers of HPV infection. Use of HPV DNA detection alone may overlook past infections that being clinically relevant at the time were cleared from the cervix. In contrast, while HPV serology is highly specific, its low sensitivity (between 50% and 70%, depending on the assay and on the number of types included) might pose problems of selection bias (i.e., women who did not trigger a detectable seroconversion after a relevant HPV infection would not be selected by the serologic assay). Estimation of attributable fractions of CC explained by the various cofactors. To assess the overall impact of cofactors on CC burden, we found that it is useful to estimate the percentage of disease that can be attributed to each cofactor. However, the “necessary-cause” model of HPV carcinogenesis poses a methodologic challenge, since the use of standard methods to estimate attributable fractions (AFs) may be inadequate under a model that assumes that the exposures for which AFs are to be estimated are only relevant once the individual is or has been exposed to a necessary etiologic factor. Despite these limitations, we (Table 6) and others (6) have estimated AFs for cofactors by using as parameters the percentages of women exposed to the cofactor of interest among HPV-positive women and the OR for CC derived from analyses restricted to infected women. As shown in Table 6, among HPV-positive women, AFs are comparatively higher for parity than for the other cofactors. Thus, given the high percentage of HPV-positive women with two or more full-term pregnancies and the relatively strong association for CC at this parity level, this analysis suggests that multiparous HPV-positive women constitute the group at the highest risk of CC. However, these conclusions should be taken only as a crude approximation to the issue because the overall disease burden attributable to the cofactor depends not only on the percentage of HPV-positive women exposed to the cofactor but also on the overall HPV prevalence in the population; the estimates reported in Table 6 or in the Costa Rica study do not take the latter parameter into account. We also assessed the impact of different combinations of cofactors on CC burden. Table 6 shows that the most prevalent combination of cofactors among case patients is that including high parity, low OC use, and never smoking (55.8%). We also show that only 5.4% of the case patients were exposed to none of the risk categories of the cofactors considered, suggesting that the fraction of the case patients in whom these cofactors may not play a role is probably very low. Recommendations for Future Research On parity and hormonal factors. 1) Taking into account the second peak in the prevalence of HPV DNA observed in perimenopausal and postmenopausal women in some populations (29–31), it would be of interest to conduct studies in these women and their male partners to try to distinguish if this second peak is the result of reactivation of latent infections or new infections. In addition, these studies will be able to explore the interaction between endogenous and exogenous hormones and HPV infection in middle-aged women. Are the endogenous hormone levels and/or hormone replacement therapy (HRT) associated with HPV DNA detection and progression to cancer in postmenopausal women? 2) Little is known about the influence of hormones on the mechanisms of HPV carcinogenesis. Epidemiologists should encourage their laboratory colleagues to carry out experimental studies on this issue, since such studies will be valuable for bolstering (or not) the biologic plausibility of the epidemiologic associations. 3) Further studies on OC use and HRT are needed to clarify their role in relation to type of hormones, concentrations, duration, recency, and latency and to determine at which stages of carcinogenesis they may act. These issues will be better studied in cohort studies of premenopausal and postmenopausal women than in case–control studies. 4) Also, it would be important to undertake studies making measurements of markers of local immunity and inflammation and correlating them with other cofactors, such as pregnancy, age, and intake of exogenous hormones. Correlation analyses of time trends. It would be valuable to study the influence of declining birthrates on declining CC rates, especially in those countries where screening programs do not exist or have had little impact. Similarly, it would be interesting to study the relationship between time trends of CC rates and those of OC use and smoking. Choice of proper control subjects. The selection of proper control subjects depends on the basic assumption that most HPV infections in older women are persistent. However, unpublished results from a follow-up study of control women included in our case–control studies in Spain and Colombia suggest that this may not be the case. Thus, prospective studies of middle-aged HPV-positive women are needed to assess this assumption and to better understand the natural history of HPV infections in older women. Alternatively, the conduct of large case–control studies, including as control subjects women with at least two consecutive HPV-positive smears over time to confirm their chronic carrier state, would circumvent some of the limitations currently encountered in the interpretation of results from previous studies. A quick approximation to this approach for recently completed studies would be to recontact HPV-positive control women, retest them for cervical HPV DNA, and analyze the new data stratifying by HPV-persistence status. This strategy would, in addition, provide an assessment of the potential bias introduced in studies that restricted analyses to cross-sectionally detected HPV-positive control subjects. One should, however, realize that the use of persistently HPV-positive control subjects, while allowing for the assessment of risk factors for disease progression, would limit the ability of evaluating factors potentially associated with viral persistence itself. Finally, it is evident that more epidemiologic research is needed to understand the value of serologic markers of HPV exposure in studies on cofactors. The first pending issue is to improve the sensitivity of current serologic assays of HPV infection, probably by increasing multiple serotypes. Second, studies using both HPV DNA- and HPV antibody-detection methods are needed to contrast our current DNA-based risk estimates linked to the different cofactors and to assess whether the combination of both assays could improve unbiased selection of exposed women. Studies comparing the natural history of HPV infections with the dynamics and patterns of seroconversion are needed to better understand the meaning of these markers and to comprehend why some women experience seroconversion and some do not. Other studies. Except for the U.S. cohort study (7), no prospective data on cofactors are yet available from the large cohort studies being conducted in Costa Rica, Brazil, and Colombia. Assessment of unpublished results from these cohort studies would be informative in the planning of further research strategies on this issue. These studies will be of great value in establishing if the effects of cofactors that we are detecting in case–control studies are real or, on the contrary, merely due to residual confounding. These cohort studies will be able to adjust for the number of HPV infections and also be useful to determine at which stage of the carcinogenic process these cofactors act. As mentioned before, the value of HPV serology as a marker of cumulative lifetime exposure needs further study. As more refined serologic assays are being developed and validated, they could be introduced alone or in combination with HPV DNA in retrospective cohort studies that stored sera and cells and prospectively monitored relevant cofactors. No published data are available on the analysis of cofactors by main HPV types (16 and 18) or their phylogenetic groups. Unpublished data from the IARC series of case–control studies showed that associations between cofactors and CCs are not modified by specific genotypes, but more data are needed to confirm this. Finally, further methodologic work is needed to estimate AF to assess the global impact of these cofactors on CC occurrence, taking into account the HPV necessary-cause model. Conclusions Based on key studies on HSIL, CIN3, and CC conducted in HPV-positive women as determined by accurate polymerase chain reaction-based HPV DNA-detection methods, we conclude that high parity, smoking, and long-term OC use are cofactors that may modulate the risk of progression from HPV infection or LSIL to HSIL and CC. The evidence seems to be more consistent for parity and smoking than for OC use. The association with OC use was detected only for in situ adenocarcinoma in a U.S. study (3) and in the IARC studies for both squamous cell carcinoma and adenocarcinoma (9), but no associations were found in the U.K. nested case–control study of CIN3 (5) or in the U.S. prospective cohort study (7). From a public health point of view, parity seems to be the behavioral cofactor explaining the highest proportion of CC cases among HPV-infected women. However, smoking and long-term OC use may have a similar impact in populations heavily exposed to HPV and to these cofactors. The studies reviewed herein indicate that smoking and parity may act both at the HSIL and the CC stages. Ongoing prospective studies will shed more light into the role of these and other cofactors in HPV carcinogenesis; however, if confirmed, these conclusions imply that multiparous women, women who are smokers, and women on long-term OC use may need closer surveillance for cytologic abnormalities and HPV infections than women in the general population. Large prospective and retrospective cohort studies of HSIL and CC among middle-aged women in which several markers of HPV exposure are used and HPV persistence or chronicity is documented are still needed to understand the role and impact of cofactors in cervical carcinogenesis. Table 1. Characteristics of studies assessing the role of parity, OC use, and tobacco smoking in cervical carcinogenesis among HPV DNA-positive women*   Study  Study characteristics  Denmark  Norway  United States, Eastern  Manchester, U.K.  Costa Rica  United States Portland, OR  IARC,† international  *CC = cervical cancer; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; OC = oral contraceptive; PCR = polymerase chain reaction.  †Includes studies in Spain, Colombia, Brazil, Thailand, The Philippines, Morocco, Peru, and Paraguay.  ‡HPV16 only.  §All case subjects were included regardless of HPV status.  ||High-risk genotypes only.  ¶Cofactors in bold type indicate that a statistically significant association was found for that cofactor.  #For squamous cell.  **For adenocarcinoma in situ.  Authors, y, (reference No.)  Kruger-Kjaer et al., 1998 (1)  Olsen et al., 1998 (2)  Lacey et al., 1999 (3), 2001 (4)  Deacon et al., 2000 (5)  Hildesheim et al., 2001 (6)  Castle et al., 2002 (7)  Muñoz et al., 2002 (8); Moreno et al., 2002 (9); Plummer et al., unpublished data, 2002  Design  Case–control within cohort  Case–control  Case–control  Case–control within cohort  Case–control within cohort  Prospective cohort  Case–control, pooled analysis  Source of subjects  Population  Population  Population  Population  Population  Health plan  Clinic and population  Outcome  HSIL  CIN2 and CIN3  CIS and CC  CIN3  HSIL and CC  CIN3 and CC  CIS and CC  HPV-positive case subjects/ total tested  71/79  60/90‡  263§  199  146/168||  Cohort of 1812 women positive for oncogenic HPV DNA  1676/1853  HPV-positive control subjects/total tested  155/994  14/216‡  49/307  181  843||    255/1916  HPV DNA detection  GP5+/6+  L1 PCR  MY09/MY11  MY09/MY11  PCR  Hybrid Capture 2  MY09/MY11 and GP5+/6+  Relevant cofactors assessed¶  Parity, OC use, smoking  Smoking  Smoking,# OC use**  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking    Study  Study characteristics  Denmark  Norway  United States, Eastern  Manchester, U.K.  Costa Rica  United States Portland, OR  IARC,† international  *CC = cervical cancer; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; OC = oral contraceptive; PCR = polymerase chain reaction.  †Includes studies in Spain, Colombia, Brazil, Thailand, The Philippines, Morocco, Peru, and Paraguay.  ‡HPV16 only.  §All case subjects were included regardless of HPV status.  ||High-risk genotypes only.  ¶Cofactors in bold type indicate that a statistically significant association was found for that cofactor.  #For squamous cell.  **For adenocarcinoma in situ.  Authors, y, (reference No.)  Kruger-Kjaer et al., 1998 (1)  Olsen et al., 1998 (2)  Lacey et al., 1999 (3), 2001 (4)  Deacon et al., 2000 (5)  Hildesheim et al., 2001 (6)  Castle et al., 2002 (7)  Muñoz et al., 2002 (8); Moreno et al., 2002 (9); Plummer et al., unpublished data, 2002  Design  Case–control within cohort  Case–control  Case–control  Case–control within cohort  Case–control within cohort  Prospective cohort  Case–control, pooled analysis  Source of subjects  Population  Population  Population  Population  Population  Health plan  Clinic and population  Outcome  HSIL  CIN2 and CIN3  CIS and CC  CIN3  HSIL and CC  CIN3 and CC  CIS and CC  HPV-positive case subjects/ total tested  71/79  60/90‡  263§  199  146/168||  Cohort of 1812 women positive for oncogenic HPV DNA  1676/1853  HPV-positive control subjects/total tested  155/994  14/216‡  49/307  181  843||    255/1916  HPV DNA detection  GP5+/6+  L1 PCR  MY09/MY11  MY09/MY11  PCR  Hybrid Capture 2  MY09/MY11 and GP5+/6+  Relevant cofactors assessed¶  Parity, OC use, smoking  Smoking  Smoking,# OC use**  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking  Parity, OC use, smoking  View Large Table 2. Summary results of studies assessing the role of OC use as a cofactor in cervical carcinogenesis among HPV DNA-positive women*   Study (study outcome)  Exposure measures  Denmark (HSIL)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; OC = oral contraceptive; OR = odds ratio. Bold numbers denote statistical significance.  OC use status, OR  (95% CI)      Ever vs. never  NR  5.4 (0.7 to 43.4)  NR  NR  NR  1.4 (1.0 to 2.0)      Former vs. never  NR  3.1 (0.4 to 27.5)  1.2 (0.6 to 2.1)  0.9 (0.6 to 1.6)  NR  NR      Current vs. never  NR  17.1 (1.5 to 188.2)  1.3 (0.7 to 2.5)  1.5 (0.8 to 2.8)  0.8 (0.5 to 1.5)  NR  OC use duration      [Years] OR (95% CI) vs. never  Decreasing risk with increasing duration  [−2] 4.0 (0.4 to 44.3)  [−3] 1.2 (0.6 to 2.4)  [−4] 1.8 (0.6 to 4.9)  NR  [1] 0.7 (0.4 to 1.1)      [−6] 4.8 (0.4 to 51.9)  [−8] 0.8 (0.4 to 1.5)  [⩾5] 3.1 (1.1 to 9.1)    [−4] 0.8 (0.5 to 1.2)      [⩾7] 6.2 (0.7 to 52.7)  [>8] 1.5 (0.8 to 2.9)      [−9] 2.8 (1.5 to 5.4)              [⩾10] 4.0 (2.1 to 7.8)          P for trend  NR  .12  NR  NR  NR  <.001  Comments  ORs not reported  For adeno- carcinoma in situ    Duration estimates computed among women with <3 pregnancies  Relative risk refers to current vs. not current OC use at enrollment  For both squamous cell carcinoma and adenocarcinoma    Study (study outcome)  Exposure measures  Denmark (HSIL)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; OC = oral contraceptive; OR = odds ratio. Bold numbers denote statistical significance.  OC use status, OR  (95% CI)      Ever vs. never  NR  5.4 (0.7 to 43.4)  NR  NR  NR  1.4 (1.0 to 2.0)      Former vs. never  NR  3.1 (0.4 to 27.5)  1.2 (0.6 to 2.1)  0.9 (0.6 to 1.6)  NR  NR      Current vs. never  NR  17.1 (1.5 to 188.2)  1.3 (0.7 to 2.5)  1.5 (0.8 to 2.8)  0.8 (0.5 to 1.5)  NR  OC use duration      [Years] OR (95% CI) vs. never  Decreasing risk with increasing duration  [−2] 4.0 (0.4 to 44.3)  [−3] 1.2 (0.6 to 2.4)  [−4] 1.8 (0.6 to 4.9)  NR  [1] 0.7 (0.4 to 1.1)      [−6] 4.8 (0.4 to 51.9)  [−8] 0.8 (0.4 to 1.5)  [⩾5] 3.1 (1.1 to 9.1)    [−4] 0.8 (0.5 to 1.2)      [⩾7] 6.2 (0.7 to 52.7)  [>8] 1.5 (0.8 to 2.9)      [−9] 2.8 (1.5 to 5.4)              [⩾10] 4.0 (2.1 to 7.8)          P for trend  NR  .12  NR  NR  NR  <.001  Comments  ORs not reported  For adeno- carcinoma in situ    Duration estimates computed among women with <3 pregnancies  Relative risk refers to current vs. not current OC use at enrollment  For both squamous cell carcinoma and adenocarcinoma  View Large Table 3. Summary results of studies assessing the role of parity/pregnancy in cervical carcinogenesis among HPV DNA-positive women*   Study (study outcome)  Exposure measures  Denmark (HSIL)  Manchester, U.K. (CIN3)  Costa Rica, (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratio (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk. CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  †OR for ever versus never pregnant.  ‡Ever pregnant but 0 live births.  §Reference includes women with 0 or 1 live birth.  Ever vs. never, OR (95% CI)  NR  NR  4.6 (1.1 to 20)†  NR  NR  No. of live births or pregnancies      [No.] OR (95% CI) vs. never  [0]‡ 0.8 (0.4 to 1.7)  [1] 1.6 (0.9 to 2.8)  [2] 1.0 (0.5 to 2.2)§  [1–2] 1.1 (0.6 to 1.7)  [1–2] 1.8 (1.0 to 3.5)    [⩾1] 1.8 (0.3 to 2.3)  [2] 1.1 (0.6 to 2.0)  [3] 1.5 (0.7 to 3.2)§  [⩾3] 0.7 (0.3 to 1.6)  [3–4] 2.6 (1.3 to 4.9)      [⩾3] 1.9 (0.9 to 3.8)  [4–5] 3.5 (1.7 to 7.2)§    [5–6] 2.9 (1.4 to 5.6)        [6–8] 2.2 (1.0 to 5.0)§    [⩾7] 3.9 (1.9 to 7.9)        [⩾9] 1.4 (0.6 to 3.4)§              P for trend  NR  NS  .04  NR  <.0001  Comments    Unadjusted      For both CC and CIS    Study (study outcome)  Exposure measures  Denmark (HSIL)  Manchester, U.K. (CIN3)  Costa Rica, (HSIL and CC)  United States, Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratio (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk. CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  †OR for ever versus never pregnant.  ‡Ever pregnant but 0 live births.  §Reference includes women with 0 or 1 live birth.  Ever vs. never, OR (95% CI)  NR  NR  4.6 (1.1 to 20)†  NR  NR  No. of live births or pregnancies      [No.] OR (95% CI) vs. never  [0]‡ 0.8 (0.4 to 1.7)  [1] 1.6 (0.9 to 2.8)  [2] 1.0 (0.5 to 2.2)§  [1–2] 1.1 (0.6 to 1.7)  [1–2] 1.8 (1.0 to 3.5)    [⩾1] 1.8 (0.3 to 2.3)  [2] 1.1 (0.6 to 2.0)  [3] 1.5 (0.7 to 3.2)§  [⩾3] 0.7 (0.3 to 1.6)  [3–4] 2.6 (1.3 to 4.9)      [⩾3] 1.9 (0.9 to 3.8)  [4–5] 3.5 (1.7 to 7.2)§    [5–6] 2.9 (1.4 to 5.6)        [6–8] 2.2 (1.0 to 5.0)§    [⩾7] 3.9 (1.9 to 7.9)        [⩾9] 1.4 (0.6 to 3.4)§              P for trend  NR  NS  .04  NR  <.0001  Comments    Unadjusted      For both CC and CIS  View Large Table 4. Summary results of studies assessing the role of cigarette smoking in cervical carcinogenesis among HPV DNA-positive women*   Study (study outcome)  Exposure measures  Denmark (HSIL)  Norway (CIN2 and CIN3)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  Smoking status, OR (95% CI)      Ever vs. never  NR  4.6 (0.9 to 22.9)  1.5 (0.7 to 3.0)  2.2 (1.4 to 3.4)  NR  NR  2.2 (1.5 to 3.2)      Former vs. never  3.2 (0.9 to 11.4)  4.2 (0.5 to 37.9)  1.2 (0.5 to 3.1)  1.7 (0.8 to 3.7)  1.7 (0.8 to 4.0)  2.1 (1.1 to 3.9)  1.8 (0.9 to 3.4)      Current vs. never  1.9 (1.0 to 3.8)  NR  1.6 (0.7 to 3.5)  NR  2.3 (1.2 to 4.3)  NR  2.3 (1.3 to 4.0)  Smoking amount      [cigarettes/day] OR (95% CI) vs. never  NR  [−10] 3.3 (0.5 to 21)  [−19] 1.6 (0.6 to 3.9)  [−10] 1.4 (0.7 to 2.5)  [−5] 1.8 (1.0 to 3.3)  [−19] 2.2 (1.2 to 4.2)  [−5] 1.9 (1.1 to 3.4)      [⩾11] 5.9 (1.0 to 36)  [⩾20] 1.3 (0.6 to 3.0)  [−16] 2.2 (1.2 to 3.9)  [⩾6] 3.1 (1.2 to 7.9)  [⩾20] 2.9 (1.5 to 5.6)  [⩾6] 2.2 (1.2 to 4.2)          [⩾17] 3.1 (1.8 to 5.3)                P for trend  NR  NR  .49  <.0001  .003  NR  NS  Smoking duration      [years] OR (95% CI) vs. never  NR  [−9] 2.1 (0.3 to 12.3)  [−10] 1.2 (0.5 to 3.0)  [−9] 1.8 (0.9 to 3.6)  [−9] 2.2 (1.0 to 4.8)  NR  [−19] 2.6 (1.4 to 4.7)      [⩾10] 7.5 (1.2 to 46)  [−20] 3.2 (1.0 to 9.7)  [−19] 2.0 (1.2 to 3.3)  [⩾10] 2.0 (1.0 to 3.8)    [⩾20] 1.9 (1.0 to 3.5)        [>20] 0.8 (0.3 to 2.7)  [⩾20] 3.1 (1.6 to 6.2)                P for trend  NR  NR  .57  <.0005  NR    NS  Comments    Stronger asso- ciations found among HPV16- seropositive subjects  For squamous cell carcinoma  Univariate OR. Adjusted OR also statistically significant          Study (study outcome)  Exposure measures  Denmark (HSIL)  Norway (CIN2 and CIN3)  United States, Eastern (CIS and CC)  Manchester, U.K. (CIN3)  Costa Rica (HSIL and CC)  United States Portland, OR (CIN3 and CC)  IARC (CIS and CC)  *Numbers are odds ratios (or relative risks) and 95% confidence intervals for the association between the corresponding exposure measure and HSIL/CC risk.  CC = cervical cancer; CI = confidence interval; CIN = cervical intraepithelial neoplasia; CIS = carcinoma in situ; HPV = human papillomavirus; HSIL = high-grade squamous intraepithelial lesion; IARC = International Agency for Research on Cancer; NR = not reported; NS = not significant; OR = odds ratio. Bold numbers denote statistical significance.  Smoking status, OR (95% CI)      Ever vs. never  NR  4.6 (0.9 to 22.9)  1.5 (0.7 to 3.0)  2.2 (1.4 to 3.4)  NR  NR  2.2 (1.5 to 3.2)      Former vs. never  3.2 (0.9 to 11.4)  4.2 (0.5 to 37.9)  1.2 (0.5 to 3.1)  1.7 (0.8 to 3.7)  1.7 (0.8 to 4.0)  2.1 (1.1 to 3.9)  1.8 (0.9 to 3.4)      Current vs. never  1.9 (1.0 to 3.8)  NR  1.6 (0.7 to 3.5)  NR  2.3 (1.2 to 4.3)  NR  2.3 (1.3 to 4.0)  Smoking amount      [cigarettes/day] OR (95% CI) vs. never  NR  [−10] 3.3 (0.5 to 21)  [−19] 1.6 (0.6 to 3.9)  [−10] 1.4 (0.7 to 2.5)  [−5] 1.8 (1.0 to 3.3)  [−19] 2.2 (1.2 to 4.2)  [−5] 1.9 (1.1 to 3.4)      [⩾11] 5.9 (1.0 to 36)  [⩾20] 1.3 (0.6 to 3.0)  [−16] 2.2 (1.2 to 3.9)  [⩾6] 3.1 (1.2 to 7.9)  [⩾20] 2.9 (1.5 to 5.6)  [⩾6] 2.2 (1.2 to 4.2)          [⩾17] 3.1 (1.8 to 5.3)                P for trend  NR  NR  .49  <.0001  .003  NR  NS  Smoking duration      [years] OR (95% CI) vs. never  NR  [−9] 2.1 (0.3 to 12.3)  [−10] 1.2 (0.5 to 3.0)  [−9] 1.8 (0.9 to 3.6)  [−9] 2.2 (1.0 to 4.8)  NR  [−19] 2.6 (1.4 to 4.7)      [⩾10] 7.5 (1.2 to 46)  [−20] 3.2 (1.0 to 9.7)  [−19] 2.0 (1.2 to 3.3)  [⩾10] 2.0 (1.0 to 3.8)    [⩾20] 1.9 (1.0 to 3.5)        [>20] 0.8 (0.3 to 2.7)  [⩾20] 3.1 (1.6 to 6.2)                P for trend  NR  NR  .57  <.0005  NR    NS  Comments    Stronger asso- ciations found among HPV16- seropositive subjects  For squamous cell carcinoma  Univariate OR. Adjusted OR also statistically significant        View Large Table 5. Impact of different strategies of HPV adjustment on associations between cofactors and risk of CC (from the IARC case–control studies)*   All women  HPV-positive women  Cofactor  Cases/controls  Not HPV adjusted, OR (95% CI)  HPV adjusted, OR (95% CI)  Case/control  OR (95% CI)  *ORs adjusted for center, age (<37, 37–45, 46–55, or ⩾56 years), educational level (none, primary, secondary, or higher), smoking amount (never, 1–5 cigarettes/day, or ⩾6 cigarettes/day), age at first sexual intercourse (<17, 17–18, 19–22, or ⩾23 years), lifetime number of sexual partners (1, 2–3, or ⩾4), OC use (never, 1–4 years, 5–9 years, or ⩾10 years), lifetime number of Pap smears (0, 1–5, or ⩾6), and parity (0, 1–2, 3–4, 5–6, or ⩾7). CC = cervical cancer; CI = confidence interval; HPV = human papillomavirus; IARC = International Agency for Research on Cancer; OC = oral contraceptive; OR = odds ratio.  †Referent.  Full-term pregnancies (status and No.)      Never  95/164  1†  1†  57/24  1†      Ever  2183/2209  1.08  1.32  1616/229  2.45      (0.80 to 1.47)  (0.88 to 1.98)    (1.33 to 4.51)      1–2  444/747  0.82  0.99  279/59  1.79      (0.60 to 1.12)  (0.65 to 1.50)    (0.94 to 3.40)      3–4  644/677  1.33  1.68  450/70  2.61      (0.97 to 1.83)  (1.09 to 2.57)    (1.37 to 5.00)      ⩾5  1095/785  1.73  2.03  887/100  3.88      (1.24 to 2.41)  (1.30 to 3.16)    (1.99 to 7.55)  OC use (status and years)      Never  1419/1508  1†  1†  1071/163  1†      Ever  864/886  1.09  0.97  605/92  1.13      (0.94 to 1.27)  (0.79 to 1.19)    (0.80 to 1.59)      1–4 y  351/445  0.90  0.75  274/64  0.66      (0.75 to 1.09)  (0.58 to 0.97)    (0.45 to 0.98)      ⩾5 y  510/427  1.33  1.17  331/28  2.35      (1.11 to 1.59)  (0.92 to 1.49)    (1.44 to 3.85)  Smoking (status and amount)      Never  1645/1905  1†  1†  1265/218  1†      Ever  636/488  1.30  1.68  409/36  1.99      (1.11 to 1.52)  (1.36 to 2.08)    (1.29 to 3.07)      1–5 cigarettes/day  251/200  1.21  1.46  181/17  1.72      (0.98 to 1.51)  (1.09 to 1.97)    (0.98 to 3.01)      ⩾6 cigarettes/day  350/216  1.79  2.07  211/18  2.16      (1.45 to 2.22)  (1.56 to 2.75)    (1.18 to 3.97)    All women  HPV-positive women  Cofactor  Cases/controls  Not HPV adjusted, OR (95% CI)  HPV adjusted, OR (95% CI)  Case/control  OR (95% CI)  *ORs adjusted for center, age (<37, 37–45, 46–55, or ⩾56 years), educational level (none, primary, secondary, or higher), smoking amount (never, 1–5 cigarettes/day, or ⩾6 cigarettes/day), age at first sexual intercourse (<17, 17–18, 19–22, or ⩾23 years), lifetime number of sexual partners (1, 2–3, or ⩾4), OC use (never, 1–4 years, 5–9 years, or ⩾10 years), lifetime number of Pap smears (0, 1–5, or ⩾6), and parity (0, 1–2, 3–4, 5–6, or ⩾7). CC = cervical cancer; CI = confidence interval; HPV = human papillomavirus; IARC = International Agency for Research on Cancer; OC = oral contraceptive; OR = odds ratio.  †Referent.  Full-term pregnancies (status and No.)      Never  95/164  1†  1†  57/24  1†      Ever  2183/2209  1.08  1.32  1616/229  2.45      (0.80 to 1.47)  (0.88 to 1.98)    (1.33 to 4.51)      1–2  444/747  0.82  0.99  279/59  1.79      (0.60 to 1.12)  (0.65 to 1.50)    (0.94 to 3.40)      3–4  644/677  1.33  1.68  450/70  2.61      (0.97 to 1.83)  (1.09 to 2.57)    (1.37 to 5.00)      ⩾5  1095/785  1.73  2.03  887/100  3.88      (1.24 to 2.41)  (1.30 to 3.16)    (1.99 to 7.55)  OC use (status and years)      Never  1419/1508  1†  1†  1071/163  1†      Ever  864/886  1.09  0.97  605/92  1.13      (0.94 to 1.27)  (0.79 to 1.19)    (0.80 to 1.59)      1–4 y  351/445  0.90  0.75  274/64  0.66      (0.75 to 1.09)  (0.58 to 0.97)    (0.45 to 0.98)      ⩾5 y  510/427  1.33  1.17  331/28  2.35      (1.11 to 1.59)  (0.92 to 1.49)    (1.44 to 3.85)  Smoking (status and amount)      Never  1645/1905  1†  1†  1265/218  1†      Ever  636/488  1.30  1.68  409/36  1.99      (1.11 to 1.52)  (1.36 to 2.08)    (1.29 to 3.07)      1–5 cigarettes/day  251/200  1.21  1.46  181/17  1.72      (0.98 to 1.51)  (1.09 to 1.97)    (0.98 to 3.01)      ⩾6 cigarettes/day  350/216  1.79  2.07  211/18  2.16      (1.45 to 2.22)  (1.56 to 2.75)    (1.18 to 3.97)  View Large Table 6. Proportion of cervical cancer cases attributable to various HPV cofactors, separately or in combination* Separately          HPV cofactor  Level of exposure  % among control subjects  % among case subjects  OR†  AF, %  Full-term pregnancies  Ever vs. never  90.5  96.6  2.51  58    ⩾2 vs. 0–1  83.0  89.8  1.63  34    ⩾5 vs. 0–1  39.5  53.0  2.27  33  OC use  Ever vs. never  36.1  36.1  1.13  4    ⩾5 vs. 0–4  11.0  19.7  2.70  16  Smoking  Ever vs. never  14.2  24.4  1.98  12    ⩾6 cigarettes/day vs. 0 cigarettes/day  7.1  12.7  2.08  7  Separately          HPV cofactor  Level of exposure  % among control subjects  % among case subjects  OR†  AF, %  Full-term pregnancies  Ever vs. never  90.5  96.6  2.51  58    ⩾2 vs. 0–1  83.0  89.8  1.63  34    ⩾5 vs. 0–1  39.5  53.0  2.27  33  OC use  Ever vs. never  36.1  36.1  1.13  4    ⩾5 vs. 0–4  11.0  19.7  2.70  16  Smoking  Ever vs. never  14.2  24.4  1.98  12    ⩾6 cigarettes/day vs. 0 cigarettes/day  7.1  12.7  2.08  7  Combination of cofactors          Parity  OC use, y  Smoking  % among control subjects  % among case subjects  OR†  AF, %  *AF = attributable fraction; HPV = human papillomavirus; OC = oral contraceptive; OR = odds ratio. 1 = referent category.  †Adjusted for center, age, educational level, smoking amount, age at first sexual intercourse, lifetime number of sexual partners, OC use (years), lifetime number of Pap smears, and parity.  0–1  0–4  Never  12.4  5.4  1  —  0–1  0–4  Ever  3.6  3.5  2.12  4  0–1  ⩾5  Never  0.8  0.6  2.51  1  0–1  ⩾5  Ever  0.4  0.7  3.60  1  ⩾2  0–4  Never  63.7  55.8  1.61  28  ⩾2  0–4  Ever  9.2  15.5  2.87  15  ⩾2  ⩾5  Never  8.8  13.7  4.16  22  ⩾2  ⩾5  Ever  1.2  4.8  11.02  11  Combination of cofactors          Parity  OC use, y  Smoking  % among control subjects  % among case subjects  OR†  AF, %  *AF = attributable fraction; HPV = human papillomavirus; OC = oral contraceptive; OR = odds ratio. 1 = referent category.  †Adjusted for center, age, educational level, smoking amount, age at first sexual intercourse, lifetime number of sexual partners, OC use (years), lifetime number of Pap smears, and parity.  0–1  0–4  Never  12.4  5.4  1  —  0–1  0–4  Ever  3.6  3.5  2.12  4  0–1  ⩾5  Never  0.8  0.6  2.51  1  0–1  ⩾5  Ever  0.4  0.7  3.60  1  ⩾2  0–4  Never  63.7  55.8  1.61  28  ⩾2  0–4  Ever  9.2  15.5  2.87  15  ⩾2  ⩾5  Never  8.8  13.7  4.16  22  ⩾2  ⩾5  Ever  1.2  4.8  11.02  11  View Large Supported in part by the International Agency for Research on Cancer, Lyon, France; by grants 01/1237, 01/1236, and BAE 01/5013 from the Fondo de Investigaciones Sanitarias, Madrid, Spain; and by a Yamagiwa-Yoshida Memorial International Cancer study grant from the International Union Against Cancer. 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Published: Jun 1, 2003

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