The quadrivalent human papillomavirus vaccine (QHPV) became available in Manitoba in 2006, and was introduced into the publicly-funded school-based program in 2008. Anogenital warts (AGWs) are one of the earliest clinical outcomes of HPV infection. They tend to occur at a younger age than other HPV-related disease, and can therefore provide an early indication of the success of school-based vaccination programs. However, to date, few studies have assessed the effectiveness of these programs in preventing AGWs using population-based, individual-level data. We used a historical matched cohort study to assess the effectiveness of the QHPV program in Manitoba, in reducing the incidence of medically-attended AGWs and, whether effectiveness depends on age at vaccination, and evidence of prior sexual activity.
Using Manitoba's population-based vaccine registry, we identified females > 9 years old who received QHPV between September 2006 and March 2013 (n=31,464). They were matched, with replacement, on age and area of residence to three unvaccinated females (n=94,327). Information on incident AGWs was obtained from hospital, physician and drug prescription databases using validated algorithms. Evidence of prior sexual activity was determined using a composite of codes for pregnancy, sexually transmitted infection, or contraceptive drug use. We used Cox regression models, stratified to account for matching, to determine hazard ratios for AGW among the vaccinated, compared to the unvaccinated.
We identified 500 cases of incident AGWs. QHPV was associated with 40% reduction in AGW risk (HR 0.6, 95% CI 0.4-0.8) among females vaccinated before they turned 18. Among Females vaccinated at >18 years of age, QHPV was associated with increased AGW risk, especially among those who were sexually active (HR 2.8, 95% CI 2.1-3.7), likely because of increased QHPV use among high-risk women. Adjustment for socioeconomic and medical history covariates did not alter these estimates.
In order to optimize our current publicly-funded QHPV vaccination program in Manitoba, further efforts should be targeted at increasing vaccine uptake in young adolescents, prior to the initiation of sexual activity.