Vaccine-type human papillomavirus and evidence of herd protection after vaccine introduction

OBJECTIVES: The aims of this study were to compare prevalence rates of human papillomavirus (HPV) in young women before and after HPV vaccine introduction to determine the following: (1) whether vaccine-type HPV infection decreased, (2) whether there was evidence of herd protection, and (3) whether there was evidence for type-replacement (increased prevalence of nonvaccine-type HPV). METHODS: Young women 13 to 26 years of age who had had sexual contact were recruited from 2 primary care clinics in 2006-2007 for a prevaccination surveillance study (N = 368, none were vaccinated) and 2009-2010 for a postvaccination surveillance study (N = 409, 59% were vaccinated). Participants completed a questionnaire and were tested for cervicovaginal HPV DNA. HPV prevalence rates were compared in the pre- versus postsurveillance studies by using χ² tests. Propensity score weighting was used to balance differences in covariates between the 2 surveillance studies. RESULTS: The mean age was ∼19 years for both groups of participants and most were African American and non-Hispanic. After propensity score weighting, the prevalence rate for vaccine-type HPV decreased substantially (31.7%-13.4%, P < .0001). The decrease in vaccine-type HPV not only occurred among vaccinated (31.8%-9.9%, P < .0001) but also among unvaccinated (30.2%-15.4%, P , .0001) postsurveillance study participants. Nonvaccine-type HPV increased (60.7%-75.9%, P < .0001) for vaccinated postsurveillance study participants. CONCLUSIONS: Four years after licensing of the quadrivalent HPV vaccine, there was a substantial decrease in vaccine-type HPV prevalence and evidence of herd protection in this community. The increase in nonvaccine-type HPV in vaccinated participants should be interpreted with caution but warrants further study.