The study of cross-protection and type replacement following the introduction of a human papillomavirus (HPV) vaccine is critical for guiding vaccination recommendations and policy. The goals of this study were to look at trends in non-vaccine-type HPV that were 1) genetically related to vaccine types and 2) genetically unrelated to vaccine types in young women aged 13–26 years after the introduction of the HPV vaccine. Between 2006 and 2017, participants were recruited from a hospital-based teen health center and a community health department for four cross-sectional surveillance studies. Participants filled out a survey on their sociodemographic features and habits, and cervicovaginal swabs were taken and analyzed for 36 HPV genotypes. Researchers looked at changes in the proportions of non-vaccine-type HPV prevalence and used logistic regression to calculate the chances of infection across the surveillance studies, which were propensity-score adjusted to account for selection bias. Vaccination status was used to stratify the analyses. From 2006 to 2017, the adjusted frequency of HPV types genetically linked to HPV16 fell by 45.8 percent among vaccinated women who got only the 4-valent vaccination, indicating indications of cross-protection. 

The corrected prevalence of HPV types that are genetically linked to HPV18 remained stable. The adjusted frequency of HPV types genetically unrelated to vaccination types did not change considerably, indicating that type replacement was not present. Following the release of the 9-valent HPV vaccination, further research is needed to monitor for cross-protection and possible type replacement.