Our analysis of behavioral survey data for a college-age cohort reveals a nuanced story of sexual behavior and HPV vaccination. Although vaccinated participants had more sexual partners overall, after adjusting for time since sexual debut and other covariates, we found instead that a weak association between HPV-vaccination and fewer sexual partners approached statistical significance. Although we expect that this association may be spurious, it may also be that vaccination uptake and fewer sexual partners are both associated with risk avoidance behavior. Regardless, we interpret this finding as evidence that HPV-vaccination is not associated with increased sexual activity. Vaccination status at sexual debut was not significantly associated with the probability of sexual debut or the age at sexual debut, though alcohol use or being white were. Gender was not significantly associated with any of the main outcomes but was significant in certain sex-act-specific outcomes.

This study contributes to the emerging evidence that HPV vaccination does not promote sexual behavior in young adults. Previous studies have reported no differences in the number of sexual partners [19, 25], age of sexual debut [19], sexual activity and condom use [20], risk perceptions [22, 26], STI and pregnancy incidence or testing [21, 23, 24] in girls and young women. Many parents have hesitated to vaccinate their children against HPV out of fear of risky or adverse sexual behavior outcomes later in life [16]. For example, a 2009 study by Kahn et al. [29] found that only 48% of mothers intended to vaccinate daughters under the age of 13, the target age group for HPV vaccination. Therefore, evidence refuting these concerns has clinical implications, since parental concern has found to be an important barrier to physician recommendation of vaccination [30]. Physician biases and concerns related to teen sexual behavior have also been reported, particularly as a barrier to male vaccination [17, 31]. Since physician recommendations, as well as parental acceptance of the vaccine, are vital to vaccine uptake [18], strengthening the evidence for the lack of association between HPV vaccination and subsequent sexual behavior is imperative.

Our finding that sexual behavior (number of partners and sexual debut) is strongly associated with alcohol use is consistent with many previous studies citing a link between drinking and increased sexual activity. For example, the Harvard College Alcohol Study (CAS) provided national data showing that college students who engaged in binge drinking were 2–3 times more likely to engage in unplanned sex and unprotected sex [32]. In this study, we found that binge drinking was not associated with the outcomes after adjusting for any alcohol consumption, although the definition of binge drinking varied slightly between studies. Moreover, Metrik et al. [33] found both alcohol use and marijuana use were independently associated with a greater likelihood of sexual encounters; we found that alcohol, though not marijuana, use was associated with our sexual behavior outcomes in our cohort.

This study provided a first look at the cohort data generated by the M-HOC study, which, at its completion, has the potential to inform many other aspects of HPV infection, including the relationship between longitudinal sexual behaviors, including debut, and oral HPV infection incidence, prevalence, and clearance. This study is the first to investigate the impact of HPV vaccination on sexual behavior that we are aware of that has included men. Further, targeting a college-aged sample can investigate any effects that vaccination may have closer to the age of sexual debut. Studying both short-term and long-term effects of vaccination status is critical to understanding the interplay between sexual behavior and vaccination and ultimately increasing vaccine uptake.

Our analysis is based on self-reported questionnaire data, and it is not possible to independently confirm the reported sexual and substance-use behaviors or vaccination status. Since the majority of vaccinated participants in our cohort were vaccinated years prior to the baseline questionnaire, it is possible that individuals misremembered or were unsure of their HPV vaccination status. Although we accounted for HPV status at sexual debut by including it as an exposure, sexual debut and vaccination can both be viewed as survival processes, and thus future work could take a competing risks approach to assessing associations between them. Moreover, in this retrospective analysis, we tested for associations between substance use at study baseline and prior sexual debut; consequently, these associations are inherently not causative. Too, the location and recruiting method of this study may lead to a generalizability limitation. This cohort is composed of college-aged individuals, the majority of whom are white and attend a large university. These characteristics could potentially influence behavioral decisions in a manner not generalizable to other individuals of the same age. Moreover, because of the timing of vaccine introduction, this cohort was vaccinated later (mean age 15.5) than younger cohorts will likely be, since they did not (and largely could not) have received the vaccine as now recommended. Nonetheless, our results provide additional evidence of the lack of association between HPV vaccination and sexual behaviors across different populations.