But the large study found no benefit in vaccinating against recurrent anogenital warts.
The prevalence of anogenital warts dropped 80% among those vaccinated at age 14 or younger. But protection weakened substantially with each year people delayed getting the vaccine.
In fact, there was no protection from genital warts afforded from the quadrivalent vaccine if women received it over the age of 20, a large Norwegian study found.
According to the registry study of 2.2 million individuals, being vaccinated also offered no protection from subsequent anogenital warts if women had already had an infection prior to vaccination
âIt is not known to which extent recurrent anogenital warts are caused by reactivation of a non-cleared HPV virus, or infection with a new virus,â the authors wrote in Vaccine.
âIf reactivation is the mechanism, the prophylactic Gardasil vaccine is not expected to have any effect. If recurrent anogenital warts are caused by a new HPV infection, the Gardasil vaccine is more likely to have an effect.â
The lack of benefit indicated recurrent warts were caused by reactivation of a latent infection, they said.
The quadrivalent vaccine Gardasil was offered to seventh grade schoolgirls in Norway from 2009 to 2017, after which vaccines were offered to boys and young women who had missed out.
Researchers found the vaccine had a âstrongâ benefit in reducing warts in females, but it was strongly dependent on age at vaccination.
âPredicted cumulative hazard at 40 years of age was reduced from around 10% if no vaccination was given to 2â3 % if vaccination was given at 12 years of age,â the authors wrote.
âThere were minor differences between vaccination ages of 12â16 years, but after 16 years the cumulative hazard was increasing markedly with vaccination age.â
Over the study period, there was a gradual decline in warts among the unvaccinated males, consistent with herd protection from the vaccinated females.
