More than a third of sexually experienced young women who are not infected with human papillomavirus (HPV) become infected during a two-year period.1 In a cohort study of female university students who initially did not have genital HPV infection, women who were sexually experienced (had had penetrative vaginal sex with a male partner) at the study's start and those who became sexually experienced during the study were similarly likely to become infected. The risk of genital HPV infection was elevated for women who smoked, used oral contraceptives or acquired a new male sex partner, especially one a woman had known for less than eight months before having sex with him or one who had had other partners. The risk was not reduced for women who reported always using condoms with new partners. Sexually inexperienced women seldom became infected, but their risk was elevated if they reported any type of nonpenetrative sexual contact.
Between 1990 and 1997, researchers mailed study invitations to a random sample of female university students aged 18-20 in Washington State. Participants made a clinic visit at enrollment and at four-month intervals thereafter. At each visit, nurse practitioners interviewed women to obtain sexual history and health information, and collected genital tract (cervical and vulvovaginal) specimens that were tested for HPV by polymerase chain reaction; a subsample of women provided oral specimens for HPV testing. Kaplan-Meier analysis was used to estimate cumulative probabilities of acquiring HPV infection. Risk factors for infection were assessed by Cox proportional hazards analysis.
Analyses were based on 444 women who had no detectable HPV in genital tract specimens at the time of enrollment. On average, the women were 19 years old when they entered the study and participated for 41 months. Two-thirds of the women were sexually experienced at the time of enrollment, and they had had an average of about two sex partners. Nearly two-thirds of the women who were sexually inexperienced at enrollment had first penetrative vaginal sex during the study.
In the two years after enrollment, the women overall had a cumulative probability of genital HPV infection of 32%. Women who were sexually experienced at the start of the study had the same probability of infection over the next two years that women who first engaged in penetrative sex during the study had over the two years following that experience (39%). In the two years after acquiring a new or first sex partner, the probability of infection did not differ by women's sexual experience at enrollment.
HPV-16, the type of the virus most closely associated with cervical cancer, occurred more frequently than three other high-risk types studied. The cumulative probability of HPV-16 infection in the first two years after study enrollment (for women who were already sexually experienced) or initiation of penetrative sex was 10%. (For the other high-risk types of the virus, probabilities were 1-5%.)
Of the new HPV infections, 54% were detected in the vulvovaginal region only, 10% were detected in the cervix only and 35% were detected in both regions. Detection of HPV was significantly more likely in the vulvovaginal region than in the cervix regardless of the interval since enrollment or since first sexual experience.
Women who reported acquiring a new partner during the last year were at higher risk for genital HPV infection than women who did not, and the risk was highest when the new partner had been acquired in the 5-8 months before a study visit (hazard ratio, 3.0). No significant increase in risk was seen among women who reported acquiring a new partner 13-16 months before a study visit. After acquisition of a new partner during a given interval and lifetime number of partners were taken into account, women who had penetrative sex during that interval did not have a significantly elevated risk of genital HPV infection.
Sixty percent of sexually active women who reported having vaginal sex since their last visit also reported having oral-penile contact. However, HPV was detected in only 0.2% of the oral specimens collected, and women reporting oral-penile contact in the past year were not at increased risk for oral HPV infection.
HPV was detected in only 2% of genital tract specimens from sexually inexperienced women. Overall, the cumulative probability of HPV infection during two years was 8% for women who were sexually inexperienced at study enrollment; the probability was 15% for women who subsequently had first penetrative sex and 2% for women who did not.
After vaginal sex and acquisition of a new partner were taken into account, sexually experienced women who reported finger-vulvar or penile-vulvar contact were not at increased risk for genital HPV infection. In contrast, sexually inexperienced women were more likely to acquire genital HPV infection if they reported having any type of nonpenetrative sexual contact (finger-vulvar, penile-vulvar or oral-penile) than if they did not (10% vs. 1%).
In a multivariate analysis, women who initially were or became sexually experienced had a moderate increase in the risk of genital HPV infection if, during the past year, they used oral contraceptives (hazard ratio, 1.4), smoked (1.5) or had known a new partner for less than eight months before having sex (1.8); their risk also rose moderately as their number of partners increased (1.1). They had a sharp increase in risk if they reported having a new partner who had had at least one previous partner (5.2) or who had had an unknown number of partners (8.0). Women who reported always using condoms with new partners did not have a significantly reduced risk of infection.
Young women who acquire a new sex partner are at high risk for genital HPV infection, regardless of their prior sexual experience, the investigators conclude. The findings "suggest that the better and longer a woman knows her partner before intercourse, the less her risk of becoming infected with HPV." When women acquire a new partner, the first year is likely to be the critical time for detecting HPV infection if it occurs, the investigators note. Furthermore, although sexually inexperienced women rarely acquire HPV infection, "nonpenetrative sexual contact is a plausible route of transmission" in these women.--S. London
REFERENCE
1. Winer RL et al., Genital human papillomavirus infection: incidence and risk factors in a cohort of female university students, American Journal of Epidemiology, 2003, 157(3):218-226.