The most recent guidelines from the Centers for Disease Control and Prevention recommend two-rather than the originally recommended three-doses of the human papillomavirus (HPV) vaccine when treatment is initiated before age 15. In a retrospective cohort study, researchers examined the association of one, two, and three or more doses of the quadrivalent HPV vaccine with the cumulative incidence of histopathologically confirmed cervical intraepithelial neoplasia 2 or 3 and high-grade cytology.

Data from a private insurance company database were used to identify girls and women nine to 26 years of age who had received one or more doses of the quadrivalent HPV vaccine from 2006 to 2015. A matched cohort of those who hadn't been vaccinated served as controls. A total of 133,082 girls and women (66,541 of whom were vaccinated, and an equal number unvaccinated) were included in the study. More than half of those vaccinated received three or more doses of the HPV vaccine.

A lower risk of high-grade cytology (hazard ratio, 0.84) was seen among 15-to-19-year-olds who had received three or more HPV vaccine doses. A lower risk of preinvasive cervical disease among those in this age group was associated with any number of HPV vaccine doses. The hazard ratios for those receiving one, two, or three doses were 0.64, 0.72, and 0.66, respectively.

The limitations of this study include outcomes information taken from diagnosis codes and the researchers' reliance on insurance claims, which are not always accurate or complete. In addition, participants may have been vaccinated through another insurance carrier, and the findings are applicable only to those who have private insurance, the authors say. Also, the researchers didn't account for other factors that may influence the risk of preinvasive cervical disease, and screening recommendations changed during the course of the study.