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For the first time, a population-based study of a human papillomavirus (HPV) vaccine has proven that the vaccine is effective in reducing the incidence of genital warts.

In an accompanying commentary on the study, Jennifer Smith, PhD, MPH, research associate professor at the UNC Gillings School of Global Public Health and member of the UNC Lineberger Comprehensive Cancer Center, outlined how the data gave researchers the ability to compare results from a national school-based vaccination program that reached millions compared against a large unvaccinated sample population. The study and commentary were published in the March 13 Journal of the National Cancer Institute.

Given vaccine’s effectiveness at preventing high-grade cervical lesions, which are possible precursors to cervical cancer, nations across the world have instituted HPV vaccination programs. The Swedish study of more than two million women and young girls found that the age of initial vaccination played an important role in the vaccine’s effectiveness.

“The data from the Swedish study will help to clarify the impact of early HPV vaccination against genital warts on a large population,” said Dr. Smith.

Dr. Smith and colleagues write that the relatively 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,” said Dr. Smith.

In the study, Amy Leval, RN, 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 who had lived in Sweden between 2006 and 2010, Leval examined multiple population registers to identify genital warts incidence in relation to HPV vaccination. Incidence rates 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.

“Yet, vaccine effectiveness was shown to systematically decrease with increasing age, resulting in no clear vaccination effectiveness for women vaccinated when older than 22 years, indirectly demonstrating that increased exposure to HPV with increasing age results in notably lower vaccine benefit against genital warts. The question remains to what extent similar reductions in HPV vaccine effectiveness will be observed in the future for other HPV-associated clinical outcomes, such as high-grade precancerous lesions,” said Dr. Smith.