medwireNews: Human papillomavirus (HPV) vaccination programs may only be effective when targeted at individuals aged under 20 years, show findings from a Swedish study.
In a large, nationwide study, quadrivalent HPV (qHPV) vaccination offered significant protection against genital warts (GW) among girls and women who were fully vaccinated before age 20 years, but this effect declined substantially in women immunized after age 20 years, with no effect at all observed among women who were vaccinated after the age of 22.
"By including more than 2.2 million girls and women ranging in age from 10 to 44 years, we could, for the first time, discern nuance defects of age at vaccination," say Lisen Arnheim-Dahlström (Karolinska Institute, Stockholm, Sweden) and colleagues.
Individual-level data collected from the entire Swedish population using the Prescribed Drug Register, the Total Population Register, and the Swedish Vaccination Register, showed that of 124,000 girls and women aged 10-44 years who received on-demand qHPV vaccination between 2006 and 2010, 33,178 had GW during a mean follow-up period of 4.4 years.
As reported in the Journal of the National Cancer Institute, the vaccine was effective in 76% of those who received three doses of the vaccine, with their first dose received before age 20 years. Effectiveness was highest, at 93%, in girls vaccinated before age 14 years, followed by 80% in girls aged 14-16 years, 71% in those aged 17-19 years, and 48% in women aged 20-22 years.
Time-to-event analysis of the crude incidence rate (IR) of GW in women and girls fully vaccinated versus not vaccinated showed that the greatest reduction in IR was for complete vaccination among the earliest age group considered (age 10-13 years). This maximum reduction in IR of GW decreased with each subsequent category of older age and was undetectable among those aged 27 or older.
"We found that among women first vaccinated at age 20 years or older, there was low to immeasurable effectiveness," writes the team."This suggests that vaccinations in this age group were not adequate for achieving the intended health benefit."
Interestingly, the researchers found that girls and women who had at least one parent with a university education were 15 times more likely to be vaccinated before age 20 years than those whose parents did not complete high school. "That parents' education influences vaccine uptake when out-of-pocket costs are involved may be anticipated, but the magnitude of the relative effect found in this study was surprising," they remark.