ARTICLE Determinants of Genital Human Papillomavirus Infection in Young Women
Carcinoma of the cervix has several well-established epidemiologic risk factors, including multiple sexual partners and early age at first intercourse. Human papillomavirus (HPV) infection appears to have an etiologic role in the development of cervical neoplasia, but evidence linking HPV infection to known risk factors for cervical cancer has been inconsistent The lack of expected correlations may be due to the inaccuracy of HPV assays previously used. A polymerase chain reaction DNA amplification method for the detection of HPV was used to investigate the determinants of genital HPV infection in a cross-sectional sample of 467 women attending a university health service. In contrast to studies using less accurate detection methods, the risk factors for HPV infection found here were consistent with those for cervical neoplasia. The risk of HPV infection was strongly and independently associated with increasing numbers of sexual partners in a lifetime, use of oral contraceptives, younger age, and black race. Age at first intercourse, smoking, and history of a prior sexually transmitted disease were correlated with, but not independently predictive of, HPV infection. These results demonstrate that the key risk factors for cervical carcinoma are strongly associated with genital HPV infection. This correlation suggests that HPV has an etiologic role in cervical neoplasia and reaffirms the sexual route of HPV transmission. [J Natl Cancer Inst 83:997-1003,1991]
pathogenesis of cervical cancer, there is, as yet, no conclusive proof of a specific causal agent. Increasingly, however, lines of evidence point to human papillomavirus (HPV) as having a central etiologic role in cervical cancer. [See (13) for review.] HPV DNA can be found in most cervical neoplastic lesions, from minimal cervical intraepithelial neoplasia to invasive cancer. Specific types of HPV, particularly HPV16 and HPV18, have been detected in the majority of squamous cell carcinomas of the cervix, in higher grades of cervical intraepithelial neoplasia, and in cervical cancer cell lines. In vitro, the HPV gene products E6 and E7 have transforming properties, and related viruses, such as cottontail rabbit papillomavirus, induce cancers in their animal hosts. While the molecular and biochemical studies supporting a link between HPV and cervical cancer are persuasive, epidemiologic evidence linking HPV infection to recognized risk factors for cervical cancer has been inconsistent or negative (14-21). One reason for this lack of association may be the inaccuracy of the assays used to detect infection with HPV and resultant misclassification. [See (22) for review.] Until recently, investigators have lacked a specific and sensitive method for detecting HPV in large numbers of clinical specimens. Filter in situ hybridization, which had been used in many large studies because of its practicality, is now thought to be nonspecific and insensitive (2324). The most frequently used dot-blot hybridization assay (ViraPap/Vira Type; Digene Diagnostics, Inc., Silver
Among women, carcinoma of the cervix is the second most common form of cancer worldwide and the most common cancer in many developing countries (/). Cervical cancer has several well-established epidemiologic risk factors, such as multiple sexual partners, early age at first intercourse, and black race (in the United States) (2-5). Other possible risk factors include smoking (6), diet (7), multiparity (8), long-term use of oral contraceptives (5,9-11), and non-use of barrier contraceptives or spermicides (12). While these findings suggest that a sexually transmitted agent plays an important role in the
Received February 22, 1991;revisedApril 5, 1991; accepted April 8, 1991. C. Ley, A. Reingold (Epidemiology Program), J. C. Chambers, C. J. Tashiro (University Health Service), University of California, Berkeley. H. M. Bauer, M. M. Manos, Department of Infectious Diseases, Cetus Corporation, Emeryville, Calif. M. H. Schiffman, Environmental Epidemiology Branch, Division of Cancer Etiology, National Cancer Institute, Bethesda, Md. We thank Drs. John Sninsky and Ira Friedman for their support; the members of the EnvironmentaJ Studies Section of the National Cancer Institute, particularly, Drs. Rolando Herrero and Allan Hildesheim, for critical discussions; and Robin Kurka for preparation of this manuscript. 'Correspondence to: M. Michele Manos, Ph.D., Department of Infectious Diseases, Cetus Corporation, 1400 Fifty-third St., Emeryville, CA 94608.
Vol. 83, No. U.July 17, 1991
ARTICLE
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Catherine Ley, Heidi M. Bauer, Arthur Reingold, Mark H. Schiffman, Janet C. Chambers, Cathy J. Tashiro, M. Michele Manos*
997
Spring, Md.) detects only a subset of the HPV types that apparently are prevalent in the general population (25). Even Southern blot DNA hybridization has demonstrated poor reproducibility of results (26) and has a variable sensitivity that is highly dependent on the adequacy of the cervical cell samples
(27).
Methods The study design and viral detection methods have been described by Bauer et al. (25). Briefly, the study included 467 women attending the University Health Service at the University of California, Berkeley, for a routine annual pelvic examination; participation rate was 97%. At the time of their visit, women completed a self-administered questionnaire regarding lifetime number of male sexual partners, age at first intercourse, history of pregnancy and sexually transmitted diseases, contraceptive methods used, smoking, and other variables. A licensed nurse practitioner performed the pelvic examination and collected both vulvar (inner aspects of the labia minora and posterior fourchette) and cervical swabs for the detection of HPV. Vulvar and cervical samples were analyzed by both the ViraPap assay according to the manufacturer's instructions and the PCR method using a consensus primer amplification system, a generic HPV probe, and type-specific oligonucleotide probes (25). Women found to be positive for HPV infection by the PCR method at either site for any HPV type were classified as positive for HPV in the following analyses unless otherwise specified. Pap smears were reviewed by a pathologist without knowledge of clinical or laboratory data. Crude odds ratios were calculated to compare grouped demographic and behavioral characteristics between women positive and negative for HPV infection; P values for trend were also calculated when appropriate. Factors that were significantly related to HPV infection in univariate analyses were further analyzed using logistic regression models to permit the statistical control of potential confounding variables (30). A number of multivariate models were constructed that included those variables thought a priori to be related, e.g., number of partners, age at first intercourse, and age. Factors that remained significant despite adjustment for all related variables were included in a final model. As this final model presupposed vaginal intercourse with a man at some point, all virgins (n = 15) were excluded. Three racial groups were large enough for analysis (white, 998
Results A total of 467 women enrolled in the study; most were young (mean age = 22.9 years; median age = 22 years; range = 17-50 years), white (72%), non-Hispanic (87%), and single (94%). Samples from the vulva were obtained from all 467 women, and samples from the cervix from 459 women, although 12 samples from the cervix and five from the vulva were inadequate for analysis. Altogether, 213 (46%) of 467 women were identified as positive for HPV by the PCR DNA amplification method at either or both sites. According to the results of the ViraPap assay, only 51 (11%) of the women were found to be infected with HPV and all of those were positive by the PCR method. [See (25) for details.] Of 454 women from whom Pap smears were evaluated, 33 (7%) had abnormal cytologic diagnoses including dysplasia or atypia; 85% were positive for HPV. The prevalence of HPV was 63% among the additional 34 women who reported a prior abnormal Pap result. Of the 41 women who reported a history of genital warts, 66% were positive for HPV. We obtained no data on history of treatment for cervical disease or genital warts. Five women had genital warts on examination at enrollment; 80% were positive for HPV, and none of the five reported a history of genital warts. Except where indicated, statistical analyses presented below include the women with past or current abnormal Pap smears and past or current genital warts. Analyses in which they were excluded led to similar findings (see below). In univariate analyses, the presence of HPV infection was associated with a number of demographic factors (Table 1). Women 22-25 years of age had the highest prevalence of HPV infection, while younger and older women were less likely to be
Table 1. Prevalence of HPV infection with crude odds ratios and 95% confidence intervals, by demographic factors No. of women
% positive for HPV
Crude OR (95% CI)
Age.y 17-19 20-21 22-23 24-25 26-29 30-50
79 135 93 58 77 25
48.1 43.7 54.8 51.7 35.1 32.0
1.0 0.8(0.5-1.5) 1.3(0.7-2.4) 1.2(0.6-2.3) 0.6(0.3-1.1) 0.5(0.2-1.3) /'trend = .13
Race White Asian Black Other
338 84 33 12
47.0 34.5 60.6 41.7
1.0 0.6(0.4-1.0) 1.7(0.8-3.6) 0.8 (0.3-2.5)
Ethnicity Non-Hispanic Hispanic Unreported
407 51 9
46.0 45.1 33.3
1.0 1.0(0.5-1.7)
Marital status Single Married
437 30
47.4 20.0
1.0 0.3(0.1-0.7)
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Polymerase chain reaction (PCR) DNA amplification, which is the most sensitive technique currently available for the detection of specific DNA sequences, has not yet been used extensively in population studies of HPV infection. Previously, we (28) described a PCR-based method that reliably identifies a broad spectrum of HPV types (29), and we (25) recently reported its use to determine the prevalence of cervical and vulvar HPV infection in a university population. In the study reported here, we investigated whether genital HPV infection, as detected by the PCR method in this university population of mostly young women, is correlated with known risk factors for cervical cancer. This work is part of an ongoing project (25) examining the epidemiology of genital HPV infection in this population.
Asian, and black); all others (n = 12) were excluded from the final multivariate model. In addition, women using oral contraceptives for an unknown duration (n = 11) were excluded.
Table 2. Prevalence of HPV infection with crude odds ratios and 95% confidence intervals, by reproductive and sexual factors No. of women
% positive for HPV
Lifetime No. of male sexual partners 15 0 90 1 101 2-3 93 4-5 66 6-9 102 10+
20.0 21.1 32.7 54.8 56.1 68.6
Age at menarche, y 85 £11 138 12 132 13 71 14 39 15+ 2 Unreported
41.2 39.9 47.7 50.7 56.4 100.0
Crude OR (95% CI)
1.0 1.8(0.9-3.5) 4 5 (2.4-8.7) 4.8 (2.4-9.6) 8.2(4.2-15.8) P trend < .01 1.0 1.0(0.6-1.6) 1.3(0.8-2.3) 1.5(0.8-2.8) 1.9(0.9-4.0) P trend = .03
Age at first intercourse, y 20+ 18-19 16-17 14-15 12-13 Never
84 167 137 56 8 15
28.6 43.1 55.5 58.9 62.5 20.0
1.0 1.9(1.1-3.3) 3.1 (1.7-5.6) 3.6(1.8-7.3) 4.2(0.9-18.8) P trend < .01
History of pregnancy Never pregnant 380 Prior pregnancy 83 Unreported 4
45.0 48.2 50.0
1.0 1.1 (0.7-1.8)
Sexual preference Heterosexual Other Unreported
45.6 47.8 33.3
1.0 1.1 (0.5-2.5)
43.4 53.7 20.0
1.0 1-5 (1.0-2.3)
441 23 3
Known intercourse with an uncircumcised man No 318 Yes 134 Unreported 15
Vol. 83, No. 14, July 17, 1991
% HPV Positive
•0
21 or less
1 partner 2-3 partners
D H
4-5 partners
a
10+partners
6-9 partners
22-23
Age Group Fig. 1. HPV prevalence in three age groups by lifetime number of sexual partners.
these associations were explained by lifetime number of sexual partners. No women reported being pregnant at the time of the study; prevalence of HPV did not differ by prior pregnancy status. HPV infection also did not correlate with current sexual preference. Women who reported intercourse with an uncircumcised man were more likely to be positive for HPV. Women who reported a known prior exposure to a partner with genital warts also appeared to be at increased risk of infection; 71 % of these women had a history of warts. Seventy women (15%) reported a history of one or more non-HPV-related sexually transmitted diseases, including chlamydia, herpes simplex virus 2, syphilis, or gonorrhea. Among those who reported at least one prior sexually transmitted disease, the rate of HPV was 60%, as compared with 43% among those without a prior sexually transmitted disease (data not shown). In particular, a history of chlamydia infection was significantly associated with HPV infection (67% versus 44% positive for HPV). However, these associations of HPV infection with other sexually transmitted diseases and with male partner characteristics disappeared after adjustment for lifetime number of male sexual partners. Current and former smokers were more likely to be infected with HPV than women who had never smoked. Current smokers were most likely to be infected (Table 3). Smoking was, however, highly associated with increasing lifetime numbers of male sexual partners, which explained the crude association with HPV (data not shown). HPV infection was strongly associated with both current and former use of oral contraceptives and increased somewhat with duration of oral contraceptive use (Table 3). This association was not explained by sexual activity. Among women who had ever used oral contraceptives, the use of spermicidal barrier methods of birth control appeared to decrease the risk of HPV infection. However, compared with women using only condoms, the rhythm method, or no contraception, women using only spermicidal barrier methods were not at decreased risk of infection with HPV (data not shown). Only nine (2%) women had never used contraception. Variables significantly related to HPV infection in univariate analyses were included in various multivariate models to deterARTICLE 999
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infected. HPV infection was most common among black women and least common among Asian women. Hispanic ethnicity was not associated with HPV infection. Single women were more likely to be positive for HPV, although the sample of currently married women available for comparison was small (n = 30). With regard to sexual factors (Table 2), the HPV prevalence increased with increasing lifetime number of sexual partners. The reported lifetime number of male sexual partners ranged from 0 to at least 99 (median = 4). Of the 15 (3%) individuals in this study who had never had vaginal intercourse with a man, three (20%) were positive for HPV at the vulva; none of the 12 from whom cervical specimens were available were positive at the cervix. Lifetime number of sexual partners increased with age and with decreasing age at first intercourse. However, the increase in HPV prevalence with increasing lifetime partners persisted within each age group (Fig. 1) and within each level of age at first intercourse, as well as among all racial groups (data not shown). Both increasing age at menarche and decreasing age at first intercourse were positively associated with HPV. However,
Table 3. Prevalence of HPV infection with crude odds ratios and 95% confidence intervals, by smoking and contraceptive method No. of women
% positive for HPV
Crude OR (95% CI)
History of smoking Never 374 Past 65 Current 28
43.6 49.2 64.3
1.0 1.3(0.7-2.1) 2.3(1.1-5.2)
History of oral contraceptive use Never 142 134 Past Current 191
26.8 52.2 55.0
1.0 3.0(1.8-5.0) 3.3 (2.1-5.3)
26.8 50.5 52.8 62.5 45.8 72.7
1.0 2.8(1.6-4.7) 3.1 (1.8-5.1) 4.6 (2.4-8.8) 2.3(1.0-5.6) P trend < .01
History of spermicidal barrier method use* Never 207 Past 183 Current 77
Adjustedt (95% CI)
1 1.8(0.8,3-5) 4.5 (2.4,8.7) 4.8 (2.4,9.6) 8.2(4.2, 15.8)
1 1.5(0.7,3.1) 4.5 (2.2,9.2) 5.0(2.3, 11.2) 11.2(4.9,25.4)
Age.y 17-19 20-21 22-23 24-25 26+
1 0.8(0.5,1.5) 1.1 (0.7,2.4) 1.2(0.6,2.3) 0.6(0.3,1.0)
1 0.6(0.3,1.3) 0.9(0.4, 1.9) 0.5(0.2, 1.2) 0.2(0.1,0.4)
Race White Asian Black
1 0.6(0.4, 1.0) 1.7(0.8,3.6)
1 0.9 (0.5, 1.6) 3.5(1.3,9.7)
1 2.8(1.6,4.7) 3.1 (1.8,5.1) 3.7(2.1,6.6)
1 2.2(1.2,4.0) 2.1 (1.2,3.9) 2.5(1.2,5.1)
Lifetime No. of male sexual partners 1 2-3 4-5 6-9 10+
Years of oral contraceptive use 47.3 47.0 37.7
1.0 1.0 (0.7-1.5) 0.7(0.4-1.2)
•Includes spermicidal foam, diaphragm, cervical cap, and contraceptive sponge.
mine independent risk factors. The strongest independent predictor of HPV infection was lifetime number of male sexual partners (Table 4). This variable alone explained the apparent association of infection with age at menarche, age at first intercourse, known intercourse with an uncircumcised man, a history of other sexually transmitted diseases, and a history of smoking. In the final analysis, the risk of HPV infection appeared to decrease steadily with age, despite adjustment for age-related covariates, such as lifetime number of partners, oral contraceptive use, and marital status. Black race persisted as an independent risk factor for HPV infection; however, the association of Asian race with decreased infection rate was no longer significant after adjustment for other factors. Oral contraceptive use also remained a risk factor, but duration of use had little effect. Single marital status remained a weak, although not significant, risk factor for HPV infection, unexplained by lifetime number of partners, oral contraceptive use, or age. Exclusion of women with abnormal cytologic diagnoses or current genital warts had little effect on the observed associations. In addition, when women were classified according to HPV status based only on the findings at the cervix or at the vulva, the models produced virtually identical results (data not shown). Similar results were found when women infected only at the vulva (n = 50) were compared with uninfected women (data not shown). The same independent risk factors were found when women (n = 108) infected with known HPV types (6, 11, 16, 18, 31, 33, 39, 45, 51, or 52) were compared with all uninfected women and when those (n = 96) infected only with unidentified HPV types, clinical isolates and "not typed" (25), were compared with uninfected women. However, risk relationships were stronger, in general, for known types. In particular, 1000
OR Crude (95% CI)
o1 2-3 4+
•Excludes virgins (n = 15), other races (n = 12), and unknown duration of oral contraceptive use (n = 11). tFor all variables presented in this table.
when women (n = 55) infected with the most common cancerassociated HPV types (16 or 18) were compared with uninfected women, the relationships of HPV infection with lifetime number of partners and oral contraceptive use were markedly strengthened (odds ratio for 10+ partners versus one partner = 60.0, with 95% confidence interval = 6.8-531.9; odds ratio for 4+ years' oral contraceptive use versus no use = 7.1, with 95% confidence interval = 1.8-27.8). When HPV infection status was based on ViraPap assay results, only lifetime number of partners and black race were identified as significant predictors of HPV infection.
Discussion There is growing evidence that HPV infection is the sexually transmitted cause of cervical neoplasia, as predicted by several decades of epidemiologic research. If this suspicion is correct, then the risk of HPV infection should correlate with the same sexual variables that have been firmly linked to the risk of cervical cancer. In fact, HPV infection should explain the risk of neoplasia associated with these sexual variables. Accordingly, a number of investigations have attempted to correlate genital HPV infection with sexual variables, such as lifetime number of partners and age at first intercourse, and with other risk factors for cervical neoplasia, such as race, smoking, and oral contraceptive use. Clinically manifest HPV infection, i.e., genital condylomata, has been shown to be more common among women with multiple sexual partners and among women using oral contraceptives for extended periods (31\32). However, studies examining the correlates of subclinical genital HPV inJournal of the National Cancer Institute
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Total No. of years of oral contraceptive use 0 142 1 109 2-3 125 4-5 56 6+ 24 Unre ported 11
Table 4. Crude and adjusted odds ratios for HPV infection and 95% confidence intervals, for independent risk factors*
In our data, the use of oral contraceptives was strongly associated with an increased likelihood of detectable HPV infection, independent of correlated variables such as sexual activity. Vol. 83, No. 14, July 17, 1991
This finding corroborates three recent reports {21J7J8). If oral contraceptive use increases the biological activity of HPV, this result may explain the association of oral contraceptive use and risk of cervical cancer seen in many {5,9-11), but not all {39), previous investigations. The mechanism whereby oral contraceptive use might increase the risk of detectable HPV infection remains speculative, although hormonal perturbation of the immune response or steroid-responsive regulation of HPV activity {40,41) has been suggested. Residual confounding by unmeasured sexual behaviors correlated with both oral contraceptive use and HPV infection cannot be excluded as an explanation for this finding. We found no evidence to suggest that barrier contraception or spermicide use was associated with a decreased risk of HPV infection, in contrast with the protective effect of each method on risk of cervical cancer reported in some studies {12,42,43). However, the small sample of women using neither spermicidal barrier methods nor oral contraceptives, a necessary control group, limited our analysis; small protective effects of spermicidal barrier methods may have been missed. Smoking was correlated with HPV infection in univariate analyses, but this association was explained by taking into account sexual behavior. The strength of association between smoking and risk of cervical neoplasia is reduced by adjustment for sexual variables {6,44), although a residual independent role for smoking has been seen in most investigations, especially those that focused on high-grade cervical intraepithelial neoplasia and invasive cervical cancer. Our data suggest that the effect of smoking on the risk of cervical neoplasia is not mediated by the risk of HPV infection. Smoking may promote the development or progression of cervical neoplasia among HPVpositive women {45), or it may act independently of HPV. HPV infection was less prevalent in women who were currently married. This difference, although not statistically significant, persisted even after we considered sexual behavior, oral contraceptive use, and age. While this result may be due to chance, being married may reflect a predictor variable that we did not measure. In the past, marital status was used as an indicator of sexual activity; this may require re-evaluation in light of our data. An age trend in HPV prevalence was observed in this study. Women 22-25 years of age had the highest prevalence of HPV infection, while younger and older women were less likely to be infected. This peak prevalence corresponds well to the average age of women with condylomatous atypia of the cervix, the earliest cytopathologic change thought to be caused by HPV {46). After adjustment for other variables, the prevalence of HPV infection decreased markedly with increasing age. This trend has been observed in other studies in diverse geographical areas [{14,16); Schiffman M, Bauer H, Manos M: unpublished observations]. Increasing age was strongly and positively associated with increasing lifetime numbers of sexual partners, the most powerful predictor of HPV positivity in our data. Therefore, the decrease of HPV prevalence with age may suggest that many HPV infections are transient. This transience might parallel the spontaneous regression of many cases of condylomatous atypia. Whether the disappearance of detectable virus with age reflects ARTICLE 1001
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fection as measured by viral DNA detection have produced conflicting or even negative results. Several studies comparing HPV-positive and HPV-negative women have found no difference in age at first intercourse, in lifetime number of male sexual partners, or in use of oral contraceptives {17,18\20J3). For example, a recent study of adolescents in New York (55) showed that the lifetime number of sexual partners did not differ between HPV-positive and HPV-negative individuals. Recent studies in Latin America {17,18) did not show a statistically significant association between HPV infection and number of lifetime sexual partners. One study (20) surprisingly found that HPV-positive women had fewer lifetime sexual partners than HPV-negative women. In contrast to previous investigations, the findings in our study showed the expected strong correlation between lifetime number of male sexual partners and prevalence of HPV infection. Women with 10 or more sexual partners were 11 times more likely to be infected with HPV than monogamous women, and the trend of increasing prevalence with increasing numbers of sexual partners was seen at all ages and in all races. As a strong confounding variable, lifetime number of sexual partners explained the apparent correlation of HPV infection with age at first intercourse. Given the strength of association between number of sexual partners and HPV prevalence, we conclude that genital HPV infection is primarily transmitted in adulthood by sexual intercourse. Data from those previous studies (using filter in situ hybridization) that failed to find such an association were probably limited by misclassification of HPV infection status due to the nonspecificity and insensitivity of the method {17,1820). Although less sensitive than the PCR DNA amplification, the ViraPap assay results identified number of sexual partners and race to be risk factors for infection, but not oral contraceptive use or age. A recent study {34), which used ViraPap/ViraType HPV detection, found the lifetime number of sexual partners to be the only risk factor for infection. It appears that the full complement of risk factors for a given population may be revealed only with a specific and sensitive, broadspectrum HPV detection method. In this study, black women were more likely to be positive for HPV, even after we accounted for lifetime number of sexual partners, oral contraceptive use, and age. This finding suggests that these women were more likely either to encounter an HPVpositive partner or, given exposure, to acquire the infection. Confounding by socioeconomic status could explain this result; despite the fact that all women in this study were university students, racial groups may have differed in their socioeconomic backgrounds. This information, however, was not available. The finding of increased HPV infection in blacks appears inconsistent with two studies {35J6) in which genital warts, known to be caused by HPV types 6 and 11, were shown to be more frequent in whites than in blacks. Our finding is accordant, however, with the observation that black women are at greater risk of developing cervical cancer {4); a higher prevalence of genital HPV infection may play a role.
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(36) BECKER TM, STONE KM, ALEXANDER ER: Genital human papillomavirus
uteri and contraception: A possible adverse effect of the pill. Lancet 2:930934,1983
infection: A growing concern. Obstet Gynecol Clin North Am 14:389—396, 1987
(10) WORLD HEALTH ORGANIZATION- WHO collaborative study of neoplasia and
(37) HILDESHEIM A, REEVES WC, BRINTON LA, ET AL: Association of oral con-
steroid contraceptives: Invasive cervical cancer and combined oral contraceptives. BrMedJ 290:961-965, 1985
traceptive use and human papillomaviruses in invasive cervical cancers. Int J Cancer 45:860-864, 1990
(//)
1002
(13) KOUTSKY LA, GALLOWAY DA, HOLMES KK: Epidemiology of genital
BRINTON LA, HUOGINS GR, LEHMAN HF, ET AL: Long-term use of oral con-
(38) NEGRTNI BP, SCHIFFMAN MH, KURMAN RJ, ET AL: Oral contraceptive use,
traceptives and risk of invasive cervical cancer. Int J Cancer 38:339-344, 1986
human papillomavirus infection, and risk of early cytologic abnormalities of the cervix. Cancer Res 50:4670-4675,1990
(12) SLATTERY ML, OVERALL JC JR, ABBOTT TM, ET AL: Sexual activity, con-
(39) IRWTN KL, ROSERO-BDCBY L, OBERLE MW, ET AL. Oral contraceptives and
traception, genital infections, and cervical cancer Support for a sexually transmitted disease hypothesis. Am J Epidemiol 130:248-258, 1989
cervical cancer risk in Costa Rica: Detection bias or causal association? JAMA 259:59-64, 1988
Journal of the National Cancer Institute
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the immunologic elimination of HPV from the genital tract or merely the suppression of HPV to levels escaping detection, even by a technique as sensitive as the PCR method, cannot be determined from our data. Alternatively, the decrease in HPV prevalence with increasing age seen in this study and in other studies may reflect a cohort effect, in other words, that HPV prevalence is increasing. The associations of HPV infection with age, race, oral contraceptive use, and sexual behavior were strengthened by excluding unidentified HPV types from the analysis. Although such results might occur by chance, several other explanations are possible. Partial misclassification (nongenital HPV detection or false positives) within the "unidentified" HPV group could explain this finding. Alternatively, this group may contain genital HPVs that are not sexually transmitted. The results with unidentified types, as well as the dramatic increase in strength of sexual behavior associations with HPV16/HPV18 infection, may reflect true biological differences between virus types. Such differences could affect viral colonization or shedding in, or transmission by, both the female and male host and, thus, explain the observed variation in risk factors for infection. Further analyses are ongoing to investigate HPV type-specific risk factors. This study demonstrates that the determinants of genital HPV infection are consistent with those associated with an increased risk of cervical cancer, supporting a role for HPV in the etiology of cervical cancer. If our findings are replicated in other populations, the final epidemiologic test of the etiologic role of HPV will be to incorporate HPV measurements into natural history studies of cervical neoplasia. If the classical sexual risk factors for cervical neoplasia can be shown to influence the probability of disease via their effects on risk of acquiring HPV infection, then a central etiologic role for HPV will have been confirmed.
(40) PATER MM, HUGHES GA, HYSLOP DE, ET AL: Glucocorticoid-dependent
(43) CELENTANO DD, KLASSEN AC, WEISMAN CS. ET AL: The role of contracep-
oncogenic transformation by type 16 but not type 11 human papillomavirus DNA. Nature 335:832-835, 1988
tive use in cervical cancer: The Maryland Cervical Cancer Case-Control Study. Am J Epidemiol 126:592-604,1987
(41) CHAN WK, KLOCK G, BERNARD HU: Progesterone and glucocorticoid
(44) BRINTON LA, SCHAIRER C, HAENSZEL W, ET AL: Cigarette smoking and in-
response elements occur in the long control regions of several human papillomaviruses involved in anogenital neoplasia. J Virol 63:3261-3269, 1989
(45) HERRERO R, BRINTON LA, REEVES WC, ET AL: Invasive cervical cancer and
(42) WRIGHT NH, VESSEY MP, KENWARD B, ET AL: Neoplasia and dysplasia of
(46) BRISSON J, ROY M, FORTIER M, ET AU Condyloma and intraepithelial
the cervix uteri and contraception: A possible protective effect of the diaphragm. Br J Cancer 38:273-279, 1978
vasive cervical cancer. JAMA 255:3265-3269, 1986 smoking in Latin America. J Natl Cancer Inst 81:205-211. 1989 neoplasia of the uterine cervix: A case-control study. Am J Epidemiol 128:337-342,1988
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Vol. 83, No. 14, July 17, 1991
ARTICLE 1003
Carcinoma of the cervix has several well-established epidemiologic risk factors, including multiple sexual partners and early age at first intercourse. Human papillomavirus (HPV) infection appears to have an etiologic role in the development of cervical neoplasia, but evidence linking HPV infection to known risk factors for cervical cancer has been inconsistent The lack of expected correlations may be due to the inaccuracy of HPV assays previously used. A polymerase chain reaction DNA amplification method for the detection of HPV was used to investigate the determinants of genital HPV infection in a cross-sectional sample of 467 women attending a university health service. In contrast to studies using less accurate detection methods, the risk factors for HPV infection found here were consistent with those for cervical neoplasia. The risk of HPV infection was strongly and independently associated with increasing numbers of sexual partners in a lifetime, use of oral contraceptives, younger age, and black race. Age at first intercourse, smoking, and history of a prior sexually transmitted disease were correlated with, but not independently predictive of, HPV infection. These results demonstrate that the key risk factors for cervical carcinoma are strongly associated with genital HPV infection. This correlation suggests that HPV has an etiologic role in cervical neoplasia and reaffirms the sexual route of HPV transmission. [J Natl Cancer Inst 83:997-1003,1991]
pathogenesis of cervical cancer, there is, as yet, no conclusive proof of a specific causal agent. Increasingly, however, lines of evidence point to human papillomavirus (HPV) as having a central etiologic role in cervical cancer. [See (13) for review.] HPV DNA can be found in most cervical neoplastic lesions, from minimal cervical intraepithelial neoplasia to invasive cancer. Specific types of HPV, particularly HPV16 and HPV18, have been detected in the majority of squamous cell carcinomas of the cervix, in higher grades of cervical intraepithelial neoplasia, and in cervical cancer cell lines. In vitro, the HPV gene products E6 and E7 have transforming properties, and related viruses, such as cottontail rabbit papillomavirus, induce cancers in their animal hosts. While the molecular and biochemical studies supporting a link between HPV and cervical cancer are persuasive, epidemiologic evidence linking HPV infection to recognized risk factors for cervical cancer has been inconsistent or negative (14-21). One reason for this lack of association may be the inaccuracy of the assays used to detect infection with HPV and resultant misclassification. [See (22) for review.] Until recently, investigators have lacked a specific and sensitive method for detecting HPV in large numbers of clinical specimens. Filter in situ hybridization, which had been used in many large studies because of its practicality, is now thought to be nonspecific and insensitive (2324). The most frequently used dot-blot hybridization assay (ViraPap/Vira Type; Digene Diagnostics, Inc., Silver
Among women, carcinoma of the cervix is the second most common form of cancer worldwide and the most common cancer in many developing countries (/). Cervical cancer has several well-established epidemiologic risk factors, such as multiple sexual partners, early age at first intercourse, and black race (in the United States) (2-5). Other possible risk factors include smoking (6), diet (7), multiparity (8), long-term use of oral contraceptives (5,9-11), and non-use of barrier contraceptives or spermicides (12). While these findings suggest that a sexually transmitted agent plays an important role in the
Received February 22, 1991;revisedApril 5, 1991; accepted April 8, 1991. C. Ley, A. Reingold (Epidemiology Program), J. C. Chambers, C. J. Tashiro (University Health Service), University of California, Berkeley. H. M. Bauer, M. M. Manos, Department of Infectious Diseases, Cetus Corporation, Emeryville, Calif. M. H. Schiffman, Environmental Epidemiology Branch, Division of Cancer Etiology, National Cancer Institute, Bethesda, Md. We thank Drs. John Sninsky and Ira Friedman for their support; the members of the EnvironmentaJ Studies Section of the National Cancer Institute, particularly, Drs. Rolando Herrero and Allan Hildesheim, for critical discussions; and Robin Kurka for preparation of this manuscript. 'Correspondence to: M. Michele Manos, Ph.D., Department of Infectious Diseases, Cetus Corporation, 1400 Fifty-third St., Emeryville, CA 94608.
Vol. 83, No. U.July 17, 1991
ARTICLE
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Catherine Ley, Heidi M. Bauer, Arthur Reingold, Mark H. Schiffman, Janet C. Chambers, Cathy J. Tashiro, M. Michele Manos*
997
Spring, Md.) detects only a subset of the HPV types that apparently are prevalent in the general population (25). Even Southern blot DNA hybridization has demonstrated poor reproducibility of results (26) and has a variable sensitivity that is highly dependent on the adequacy of the cervical cell samples
(27).
Methods The study design and viral detection methods have been described by Bauer et al. (25). Briefly, the study included 467 women attending the University Health Service at the University of California, Berkeley, for a routine annual pelvic examination; participation rate was 97%. At the time of their visit, women completed a self-administered questionnaire regarding lifetime number of male sexual partners, age at first intercourse, history of pregnancy and sexually transmitted diseases, contraceptive methods used, smoking, and other variables. A licensed nurse practitioner performed the pelvic examination and collected both vulvar (inner aspects of the labia minora and posterior fourchette) and cervical swabs for the detection of HPV. Vulvar and cervical samples were analyzed by both the ViraPap assay according to the manufacturer's instructions and the PCR method using a consensus primer amplification system, a generic HPV probe, and type-specific oligonucleotide probes (25). Women found to be positive for HPV infection by the PCR method at either site for any HPV type were classified as positive for HPV in the following analyses unless otherwise specified. Pap smears were reviewed by a pathologist without knowledge of clinical or laboratory data. Crude odds ratios were calculated to compare grouped demographic and behavioral characteristics between women positive and negative for HPV infection; P values for trend were also calculated when appropriate. Factors that were significantly related to HPV infection in univariate analyses were further analyzed using logistic regression models to permit the statistical control of potential confounding variables (30). A number of multivariate models were constructed that included those variables thought a priori to be related, e.g., number of partners, age at first intercourse, and age. Factors that remained significant despite adjustment for all related variables were included in a final model. As this final model presupposed vaginal intercourse with a man at some point, all virgins (n = 15) were excluded. Three racial groups were large enough for analysis (white, 998
Results A total of 467 women enrolled in the study; most were young (mean age = 22.9 years; median age = 22 years; range = 17-50 years), white (72%), non-Hispanic (87%), and single (94%). Samples from the vulva were obtained from all 467 women, and samples from the cervix from 459 women, although 12 samples from the cervix and five from the vulva were inadequate for analysis. Altogether, 213 (46%) of 467 women were identified as positive for HPV by the PCR DNA amplification method at either or both sites. According to the results of the ViraPap assay, only 51 (11%) of the women were found to be infected with HPV and all of those were positive by the PCR method. [See (25) for details.] Of 454 women from whom Pap smears were evaluated, 33 (7%) had abnormal cytologic diagnoses including dysplasia or atypia; 85% were positive for HPV. The prevalence of HPV was 63% among the additional 34 women who reported a prior abnormal Pap result. Of the 41 women who reported a history of genital warts, 66% were positive for HPV. We obtained no data on history of treatment for cervical disease or genital warts. Five women had genital warts on examination at enrollment; 80% were positive for HPV, and none of the five reported a history of genital warts. Except where indicated, statistical analyses presented below include the women with past or current abnormal Pap smears and past or current genital warts. Analyses in which they were excluded led to similar findings (see below). In univariate analyses, the presence of HPV infection was associated with a number of demographic factors (Table 1). Women 22-25 years of age had the highest prevalence of HPV infection, while younger and older women were less likely to be
Table 1. Prevalence of HPV infection with crude odds ratios and 95% confidence intervals, by demographic factors No. of women
% positive for HPV
Crude OR (95% CI)
Age.y 17-19 20-21 22-23 24-25 26-29 30-50
79 135 93 58 77 25
48.1 43.7 54.8 51.7 35.1 32.0
1.0 0.8(0.5-1.5) 1.3(0.7-2.4) 1.2(0.6-2.3) 0.6(0.3-1.1) 0.5(0.2-1.3) /'trend = .13
Race White Asian Black Other
338 84 33 12
47.0 34.5 60.6 41.7
1.0 0.6(0.4-1.0) 1.7(0.8-3.6) 0.8 (0.3-2.5)
Ethnicity Non-Hispanic Hispanic Unreported
407 51 9
46.0 45.1 33.3
1.0 1.0(0.5-1.7)
Marital status Single Married
437 30
47.4 20.0
1.0 0.3(0.1-0.7)
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Polymerase chain reaction (PCR) DNA amplification, which is the most sensitive technique currently available for the detection of specific DNA sequences, has not yet been used extensively in population studies of HPV infection. Previously, we (28) described a PCR-based method that reliably identifies a broad spectrum of HPV types (29), and we (25) recently reported its use to determine the prevalence of cervical and vulvar HPV infection in a university population. In the study reported here, we investigated whether genital HPV infection, as detected by the PCR method in this university population of mostly young women, is correlated with known risk factors for cervical cancer. This work is part of an ongoing project (25) examining the epidemiology of genital HPV infection in this population.
Asian, and black); all others (n = 12) were excluded from the final multivariate model. In addition, women using oral contraceptives for an unknown duration (n = 11) were excluded.
Table 2. Prevalence of HPV infection with crude odds ratios and 95% confidence intervals, by reproductive and sexual factors No. of women
% positive for HPV
Lifetime No. of male sexual partners 15 0 90 1 101 2-3 93 4-5 66 6-9 102 10+
20.0 21.1 32.7 54.8 56.1 68.6
Age at menarche, y 85 £11 138 12 132 13 71 14 39 15+ 2 Unreported
41.2 39.9 47.7 50.7 56.4 100.0
Crude OR (95% CI)
1.0 1.8(0.9-3.5) 4 5 (2.4-8.7) 4.8 (2.4-9.6) 8.2(4.2-15.8) P trend < .01 1.0 1.0(0.6-1.6) 1.3(0.8-2.3) 1.5(0.8-2.8) 1.9(0.9-4.0) P trend = .03
Age at first intercourse, y 20+ 18-19 16-17 14-15 12-13 Never
84 167 137 56 8 15
28.6 43.1 55.5 58.9 62.5 20.0
1.0 1.9(1.1-3.3) 3.1 (1.7-5.6) 3.6(1.8-7.3) 4.2(0.9-18.8) P trend < .01
History of pregnancy Never pregnant 380 Prior pregnancy 83 Unreported 4
45.0 48.2 50.0
1.0 1.1 (0.7-1.8)
Sexual preference Heterosexual Other Unreported
45.6 47.8 33.3
1.0 1.1 (0.5-2.5)
43.4 53.7 20.0
1.0 1-5 (1.0-2.3)
441 23 3
Known intercourse with an uncircumcised man No 318 Yes 134 Unreported 15
Vol. 83, No. 14, July 17, 1991
% HPV Positive
•0
21 or less
1 partner 2-3 partners
D H
4-5 partners
a
10+partners
6-9 partners
22-23
Age Group Fig. 1. HPV prevalence in three age groups by lifetime number of sexual partners.
these associations were explained by lifetime number of sexual partners. No women reported being pregnant at the time of the study; prevalence of HPV did not differ by prior pregnancy status. HPV infection also did not correlate with current sexual preference. Women who reported intercourse with an uncircumcised man were more likely to be positive for HPV. Women who reported a known prior exposure to a partner with genital warts also appeared to be at increased risk of infection; 71 % of these women had a history of warts. Seventy women (15%) reported a history of one or more non-HPV-related sexually transmitted diseases, including chlamydia, herpes simplex virus 2, syphilis, or gonorrhea. Among those who reported at least one prior sexually transmitted disease, the rate of HPV was 60%, as compared with 43% among those without a prior sexually transmitted disease (data not shown). In particular, a history of chlamydia infection was significantly associated with HPV infection (67% versus 44% positive for HPV). However, these associations of HPV infection with other sexually transmitted diseases and with male partner characteristics disappeared after adjustment for lifetime number of male sexual partners. Current and former smokers were more likely to be infected with HPV than women who had never smoked. Current smokers were most likely to be infected (Table 3). Smoking was, however, highly associated with increasing lifetime numbers of male sexual partners, which explained the crude association with HPV (data not shown). HPV infection was strongly associated with both current and former use of oral contraceptives and increased somewhat with duration of oral contraceptive use (Table 3). This association was not explained by sexual activity. Among women who had ever used oral contraceptives, the use of spermicidal barrier methods of birth control appeared to decrease the risk of HPV infection. However, compared with women using only condoms, the rhythm method, or no contraception, women using only spermicidal barrier methods were not at decreased risk of infection with HPV (data not shown). Only nine (2%) women had never used contraception. Variables significantly related to HPV infection in univariate analyses were included in various multivariate models to deterARTICLE 999
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infected. HPV infection was most common among black women and least common among Asian women. Hispanic ethnicity was not associated with HPV infection. Single women were more likely to be positive for HPV, although the sample of currently married women available for comparison was small (n = 30). With regard to sexual factors (Table 2), the HPV prevalence increased with increasing lifetime number of sexual partners. The reported lifetime number of male sexual partners ranged from 0 to at least 99 (median = 4). Of the 15 (3%) individuals in this study who had never had vaginal intercourse with a man, three (20%) were positive for HPV at the vulva; none of the 12 from whom cervical specimens were available were positive at the cervix. Lifetime number of sexual partners increased with age and with decreasing age at first intercourse. However, the increase in HPV prevalence with increasing lifetime partners persisted within each age group (Fig. 1) and within each level of age at first intercourse, as well as among all racial groups (data not shown). Both increasing age at menarche and decreasing age at first intercourse were positively associated with HPV. However,
Table 3. Prevalence of HPV infection with crude odds ratios and 95% confidence intervals, by smoking and contraceptive method No. of women
% positive for HPV
Crude OR (95% CI)
History of smoking Never 374 Past 65 Current 28
43.6 49.2 64.3
1.0 1.3(0.7-2.1) 2.3(1.1-5.2)
History of oral contraceptive use Never 142 134 Past Current 191
26.8 52.2 55.0
1.0 3.0(1.8-5.0) 3.3 (2.1-5.3)
26.8 50.5 52.8 62.5 45.8 72.7
1.0 2.8(1.6-4.7) 3.1 (1.8-5.1) 4.6 (2.4-8.8) 2.3(1.0-5.6) P trend < .01
History of spermicidal barrier method use* Never 207 Past 183 Current 77
Adjustedt (95% CI)
1 1.8(0.8,3-5) 4.5 (2.4,8.7) 4.8 (2.4,9.6) 8.2(4.2, 15.8)
1 1.5(0.7,3.1) 4.5 (2.2,9.2) 5.0(2.3, 11.2) 11.2(4.9,25.4)
Age.y 17-19 20-21 22-23 24-25 26+
1 0.8(0.5,1.5) 1.1 (0.7,2.4) 1.2(0.6,2.3) 0.6(0.3,1.0)
1 0.6(0.3,1.3) 0.9(0.4, 1.9) 0.5(0.2, 1.2) 0.2(0.1,0.4)
Race White Asian Black
1 0.6(0.4, 1.0) 1.7(0.8,3.6)
1 0.9 (0.5, 1.6) 3.5(1.3,9.7)
1 2.8(1.6,4.7) 3.1 (1.8,5.1) 3.7(2.1,6.6)
1 2.2(1.2,4.0) 2.1 (1.2,3.9) 2.5(1.2,5.1)
Lifetime No. of male sexual partners 1 2-3 4-5 6-9 10+
Years of oral contraceptive use 47.3 47.0 37.7
1.0 1.0 (0.7-1.5) 0.7(0.4-1.2)
•Includes spermicidal foam, diaphragm, cervical cap, and contraceptive sponge.
mine independent risk factors. The strongest independent predictor of HPV infection was lifetime number of male sexual partners (Table 4). This variable alone explained the apparent association of infection with age at menarche, age at first intercourse, known intercourse with an uncircumcised man, a history of other sexually transmitted diseases, and a history of smoking. In the final analysis, the risk of HPV infection appeared to decrease steadily with age, despite adjustment for age-related covariates, such as lifetime number of partners, oral contraceptive use, and marital status. Black race persisted as an independent risk factor for HPV infection; however, the association of Asian race with decreased infection rate was no longer significant after adjustment for other factors. Oral contraceptive use also remained a risk factor, but duration of use had little effect. Single marital status remained a weak, although not significant, risk factor for HPV infection, unexplained by lifetime number of partners, oral contraceptive use, or age. Exclusion of women with abnormal cytologic diagnoses or current genital warts had little effect on the observed associations. In addition, when women were classified according to HPV status based only on the findings at the cervix or at the vulva, the models produced virtually identical results (data not shown). Similar results were found when women infected only at the vulva (n = 50) were compared with uninfected women (data not shown). The same independent risk factors were found when women (n = 108) infected with known HPV types (6, 11, 16, 18, 31, 33, 39, 45, 51, or 52) were compared with all uninfected women and when those (n = 96) infected only with unidentified HPV types, clinical isolates and "not typed" (25), were compared with uninfected women. However, risk relationships were stronger, in general, for known types. In particular, 1000
OR Crude (95% CI)
o1 2-3 4+
•Excludes virgins (n = 15), other races (n = 12), and unknown duration of oral contraceptive use (n = 11). tFor all variables presented in this table.
when women (n = 55) infected with the most common cancerassociated HPV types (16 or 18) were compared with uninfected women, the relationships of HPV infection with lifetime number of partners and oral contraceptive use were markedly strengthened (odds ratio for 10+ partners versus one partner = 60.0, with 95% confidence interval = 6.8-531.9; odds ratio for 4+ years' oral contraceptive use versus no use = 7.1, with 95% confidence interval = 1.8-27.8). When HPV infection status was based on ViraPap assay results, only lifetime number of partners and black race were identified as significant predictors of HPV infection.
Discussion There is growing evidence that HPV infection is the sexually transmitted cause of cervical neoplasia, as predicted by several decades of epidemiologic research. If this suspicion is correct, then the risk of HPV infection should correlate with the same sexual variables that have been firmly linked to the risk of cervical cancer. In fact, HPV infection should explain the risk of neoplasia associated with these sexual variables. Accordingly, a number of investigations have attempted to correlate genital HPV infection with sexual variables, such as lifetime number of partners and age at first intercourse, and with other risk factors for cervical neoplasia, such as race, smoking, and oral contraceptive use. Clinically manifest HPV infection, i.e., genital condylomata, has been shown to be more common among women with multiple sexual partners and among women using oral contraceptives for extended periods (31\32). However, studies examining the correlates of subclinical genital HPV inJournal of the National Cancer Institute
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Total No. of years of oral contraceptive use 0 142 1 109 2-3 125 4-5 56 6+ 24 Unre ported 11
Table 4. Crude and adjusted odds ratios for HPV infection and 95% confidence intervals, for independent risk factors*
In our data, the use of oral contraceptives was strongly associated with an increased likelihood of detectable HPV infection, independent of correlated variables such as sexual activity. Vol. 83, No. 14, July 17, 1991
This finding corroborates three recent reports {21J7J8). If oral contraceptive use increases the biological activity of HPV, this result may explain the association of oral contraceptive use and risk of cervical cancer seen in many {5,9-11), but not all {39), previous investigations. The mechanism whereby oral contraceptive use might increase the risk of detectable HPV infection remains speculative, although hormonal perturbation of the immune response or steroid-responsive regulation of HPV activity {40,41) has been suggested. Residual confounding by unmeasured sexual behaviors correlated with both oral contraceptive use and HPV infection cannot be excluded as an explanation for this finding. We found no evidence to suggest that barrier contraception or spermicide use was associated with a decreased risk of HPV infection, in contrast with the protective effect of each method on risk of cervical cancer reported in some studies {12,42,43). However, the small sample of women using neither spermicidal barrier methods nor oral contraceptives, a necessary control group, limited our analysis; small protective effects of spermicidal barrier methods may have been missed. Smoking was correlated with HPV infection in univariate analyses, but this association was explained by taking into account sexual behavior. The strength of association between smoking and risk of cervical neoplasia is reduced by adjustment for sexual variables {6,44), although a residual independent role for smoking has been seen in most investigations, especially those that focused on high-grade cervical intraepithelial neoplasia and invasive cervical cancer. Our data suggest that the effect of smoking on the risk of cervical neoplasia is not mediated by the risk of HPV infection. Smoking may promote the development or progression of cervical neoplasia among HPVpositive women {45), or it may act independently of HPV. HPV infection was less prevalent in women who were currently married. This difference, although not statistically significant, persisted even after we considered sexual behavior, oral contraceptive use, and age. While this result may be due to chance, being married may reflect a predictor variable that we did not measure. In the past, marital status was used as an indicator of sexual activity; this may require re-evaluation in light of our data. An age trend in HPV prevalence was observed in this study. Women 22-25 years of age had the highest prevalence of HPV infection, while younger and older women were less likely to be infected. This peak prevalence corresponds well to the average age of women with condylomatous atypia of the cervix, the earliest cytopathologic change thought to be caused by HPV {46). After adjustment for other variables, the prevalence of HPV infection decreased markedly with increasing age. This trend has been observed in other studies in diverse geographical areas [{14,16); Schiffman M, Bauer H, Manos M: unpublished observations]. Increasing age was strongly and positively associated with increasing lifetime numbers of sexual partners, the most powerful predictor of HPV positivity in our data. Therefore, the decrease of HPV prevalence with age may suggest that many HPV infections are transient. This transience might parallel the spontaneous regression of many cases of condylomatous atypia. Whether the disappearance of detectable virus with age reflects ARTICLE 1001
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fection as measured by viral DNA detection have produced conflicting or even negative results. Several studies comparing HPV-positive and HPV-negative women have found no difference in age at first intercourse, in lifetime number of male sexual partners, or in use of oral contraceptives {17,18\20J3). For example, a recent study of adolescents in New York (55) showed that the lifetime number of sexual partners did not differ between HPV-positive and HPV-negative individuals. Recent studies in Latin America {17,18) did not show a statistically significant association between HPV infection and number of lifetime sexual partners. One study (20) surprisingly found that HPV-positive women had fewer lifetime sexual partners than HPV-negative women. In contrast to previous investigations, the findings in our study showed the expected strong correlation between lifetime number of male sexual partners and prevalence of HPV infection. Women with 10 or more sexual partners were 11 times more likely to be infected with HPV than monogamous women, and the trend of increasing prevalence with increasing numbers of sexual partners was seen at all ages and in all races. As a strong confounding variable, lifetime number of sexual partners explained the apparent correlation of HPV infection with age at first intercourse. Given the strength of association between number of sexual partners and HPV prevalence, we conclude that genital HPV infection is primarily transmitted in adulthood by sexual intercourse. Data from those previous studies (using filter in situ hybridization) that failed to find such an association were probably limited by misclassification of HPV infection status due to the nonspecificity and insensitivity of the method {17,1820). Although less sensitive than the PCR DNA amplification, the ViraPap assay results identified number of sexual partners and race to be risk factors for infection, but not oral contraceptive use or age. A recent study {34), which used ViraPap/ViraType HPV detection, found the lifetime number of sexual partners to be the only risk factor for infection. It appears that the full complement of risk factors for a given population may be revealed only with a specific and sensitive, broadspectrum HPV detection method. In this study, black women were more likely to be positive for HPV, even after we accounted for lifetime number of sexual partners, oral contraceptive use, and age. This finding suggests that these women were more likely either to encounter an HPVpositive partner or, given exposure, to acquire the infection. Confounding by socioeconomic status could explain this result; despite the fact that all women in this study were university students, racial groups may have differed in their socioeconomic backgrounds. This information, however, was not available. The finding of increased HPV infection in blacks appears inconsistent with two studies {35J6) in which genital warts, known to be caused by HPV types 6 and 11, were shown to be more frequent in whites than in blacks. Our finding is accordant, however, with the observation that black women are at greater risk of developing cervical cancer {4); a higher prevalence of genital HPV infection may play a role.
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Journal of the National Cancer Institute
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the immunologic elimination of HPV from the genital tract or merely the suppression of HPV to levels escaping detection, even by a technique as sensitive as the PCR method, cannot be determined from our data. Alternatively, the decrease in HPV prevalence with increasing age seen in this study and in other studies may reflect a cohort effect, in other words, that HPV prevalence is increasing. The associations of HPV infection with age, race, oral contraceptive use, and sexual behavior were strengthened by excluding unidentified HPV types from the analysis. Although such results might occur by chance, several other explanations are possible. Partial misclassification (nongenital HPV detection or false positives) within the "unidentified" HPV group could explain this finding. Alternatively, this group may contain genital HPVs that are not sexually transmitted. The results with unidentified types, as well as the dramatic increase in strength of sexual behavior associations with HPV16/HPV18 infection, may reflect true biological differences between virus types. Such differences could affect viral colonization or shedding in, or transmission by, both the female and male host and, thus, explain the observed variation in risk factors for infection. Further analyses are ongoing to investigate HPV type-specific risk factors. This study demonstrates that the determinants of genital HPV infection are consistent with those associated with an increased risk of cervical cancer, supporting a role for HPV in the etiology of cervical cancer. If our findings are replicated in other populations, the final epidemiologic test of the etiologic role of HPV will be to incorporate HPV measurements into natural history studies of cervical neoplasia. If the classical sexual risk factors for cervical neoplasia can be shown to influence the probability of disease via their effects on risk of acquiring HPV infection, then a central etiologic role for HPV will have been confirmed.
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NEW! CHEW OR SNUFF IS REAL BAD STUFF This free four-color brochure informs adolescents about the adverse health and social effects o1 using chewing tobacco and snuff. The brochure, with a foldout poster, is available in quantities for schools, health professionals, and community organizations.
To order, write the National Cancer Institute, Building 31, Room 10A24, Bethesda, MD 20892, or call the Cancer Information Service toll-free at 1 800 4 CANCER.
Vol. 83, No. 14, July 17, 1991
ARTICLE 1003
