Human papillomavirus: high herd immunity may be affected

The prevalence of the four main types of this virus is less than 1% in young Quebecers aged 16 to 20 who have not received the vaccine.

Less than 20 years after its introduction in Quebec, the human papillomavirus (HPV) vaccination program is so effective that even unvaccinated young people are benefiting from a form of immunity. "This shows that the program is working very well, and that we need to stay the course," comments Université Laval Faculty of Medicine professor Chantal Sauvageau , who, with four colleagues, has just published a study on the subject in the Journal of Infectious Diseases.

The HPV vaccine is designed to prevent health problems caused by this sexually-transmitted virus. There are over 200 HPV genotypes, at least 12 of which cause cancer. HPV is present in virtually all cases of cervical cancer. It also causes genital or anal condyloma, as well as lesions that can lead to cancers of the throat, vagina, vulva, anus and penis.

In Quebec, the HPV vaccination program was introduced in 2008. Originally, the vaccine was offered only to girls aged 9 to 17. At the time, the vaccine covered the 4 types of the virus that pose the greatest health risks. In 2016, the Quebec program was expanded to include boys. Since September 2024, the vaccine, which now protects against 9 types of the virus, has been available to young people aged 9 to 20.

Initially, we wanted to compare the efficacy of these different vaccination schedules," points out Prof. Sauvageau, who is also a researcher at the Centre de recherche du CHU de Québec - Université Laval and medical advisor to the Institut national de santé publique du Québec. Normally, this efficacy is measured by comparing the prevalence of the virus in vaccinated and unvaccinated individuals. This approach is appropriate when there is no herd immunity. In the case of HPV, the efficacy of the vaccine combined with the high vaccination coverage for those who received one dose or more - we’re talking 90% of girls and 86% of boys when measured at age 15 - suggested that herd immunity may have set in. This is what we wanted to verify."

To do this, the team led by Prof. Sauvageau measured HPV prevalence in 369 sexually active, unvaccinated young men aged 16 to 20, recruited between September 2020 and August 2022. The results? In these subjects, the prevalence of the four HPV types covered by the first vaccines used in Quebec was 0.5%.

By way of comparison, a Quebec study carried out among people aged 18 to 24 prior to the introduction of the vaccination program revealed that HPV type 16 was the most prevalent in this group, and was detected in 16% of men.

"HPV circulation has become so low that even unvaccinated people benefit from a form of protection attributable to herd immunity," summarizes Prof. Sauvageau.

"If we maintain this level of vaccination coverage, we can envisage the elimination of the main types of HPV that cause cancer. The impact of this vaccine is major."

-- Chantal Sauvageau The presence of herd immunity means, however, that public health authorities will have to find another way of monitoring the effectiveness of the HPV vaccination program. Quebec is in the process of replacing the Pap test used to screen for cervical cancer with an HPV detection test," says Professeure Sauvageau. We could use this new program to monitor the prevalence of the types of HPV most harmful to health. If the vaccine’s effectiveness were to decline, it would be possible to correct the situation by reintroducing a second dose of the vaccine."

The authors of the study, published in The Journal of Infectious Diseases, are Catherine Wolfe, Iulia Gabriela Ionescu, Marie-Hélène Mayrand, François Coutlée and Chantal Sauvageau.