HPV vaccination programs have been launched around the world in hopes of preventing cervical cancer and other HPV-related cancers. While incidence of genital warts is the earliest possible disease outcome to measure the efficacy of the HPV vaccine, the results of such efficacy trials may not be fully generalizable to real-life HPV vaccination programs. Furthermore, incidence of genital warts after on-demand vaccination with quadrivalent HPV vaccine using individual-level data remains unknown.
In order to determine the efficacy of maximizing HPV vaccines at a young age, Amy Leval, R.N., of the Department of Medical Epidemiology and Biostatistics at Karolinska Institutet in Stockholm, Sweden and colleagues, looked at a cohort of females ages 10 to 44 years living in Sweden between 2006-2010 who were linked to multiple population registers to identify genital warts incidence in relation to HPV vaccination. Incidence rate ratios of genital warts were estimated using time-to-event analyses with adjustment for attained age and parental education level, stratified by age at first vaccination.
The researchers found that of the cohort, females with at least one university-educated parent were 15 times more likely to be vaccinated for HPV before age 20 years than females whose parents had not completed high school. For females over the age of 20, the rates of genital warts incidence decreased among the unvaccinated, suggesting that women with a high risk of genital warts favorably used HPV vaccines. The authors note their limitations, however, saying, "Interpreting the crude estimates of effectiveness for those aged 20 years or older at first vaccination is difficult because we found evidence suggesting a self-selection bias with women at high risk preferentially seeking vaccination."
In an accompanying editorial, Jennifer S. Smith, Ph.D., M.P.H., of the Gillings School of Global Public Health, University of North Carolina, and Elisabete Weiderpass, Ph.D., of Karolinska Institutet, write that the low HPV vaccination coverage before the national school-based program in Sweden was implemented gave the opportunity to compare the incidence of genital warts in vaccinated cohorts with that in unvaccinated cohorts. "Future studies in Sweden—and elsewhere where HPV vaccine coverage rates are high in target populations, such as Australia and Canada—may need to compare vaccinated birth cohorts with older birth cohorts that were previously unvaccinated to evaluate the impact of vaccination on the population level."