Early vaccination effect: Genital wart incidences in Norway
Genital warts come from the sexually transmitted virus Human Papillomavirus (HPV). Genital warts are the most common clinical manifestation of an HPV infection, and about 11% of all Nordic women get genital warts during their lifetime.
The HPV vaccine used in the Norwegian Childhood Immunisation Programme protects against 2 of the HPV types that cause 90% of genital warts.
From the time you are infected with HPV, it can take up to 10 years for cervical cancer to occur. Precursors to cervical cancer occur within three years. Genital warts, on the other hand, have a relatively short incubation period of about 2-3 months.
The incidences of genital warts have never before been described in Norway. In order to study the epidemiology of this disease, and to assess the effect of the HPV vaccine, data from the Norwegian Patient Registry, SYSVAK and the Norwegian Prescription Database will be combined.
In Denmark, a significantly larger proportion of women have received the HPV vaccine compared to Norway. In order to compare the HPV vaccine effect on genital warts at the population level, a similar analysis is also made with Danish data.
In this study, we look at the prevalence of genital warts among Norwegian women and men born in the period 1975-2003.
This is a registry-based, non-invasive study. This means that all information used in the study has already been collected, and the study will therefore not require active participation from those included in the study's data material.
In the first part of the study, we compared the incidences of genital warts in Denmark and Norway in 2015, six years after the HPV vaccine became a free offer to 12-year-old girls. The study shows that fewer people get genital warts in both countries and it decreases to a greater extent in Denmark.
The explanation the researchers point to is that only Denmark had collected vaccination of older girls during this period.
Furthermore, there are also clear differences between Norwegian and Danish men. It is interesting to see that the incidence of genital warts is decreasing among men, even though they have not been offered a vaccine systematically in any of the countries during the period we have studied. This can be interpreted to mean that men receive indirect protection against genital warts in that fewer women have HPV infection after vaccination.
The first results have been analysed and published. New analyses are in progress.
In the new analyses, we take a closer look at vaccine efficacy at an individual level. We find that the HPV vaccine is about 80% effective if given at 12 years of age, but the effect decreases considerably with vaccine age. The effect fades away after the first genital wart occurrence. We also find that the vaccine is effective already two months after the first dose. These results are presented at The Eurogin 2021 Congress.
An article describing the findings is in progress.