Herd effect of human papillomavirus vaccination on incidence of high-grade cervical lesions: a population-based cohort study in Sweden
Menée à partir de données de registres suédois portant sur 857 168 femmes nées entre 1985 et 2000, cette étude évalue l'immunité collective via l'analyse de l’incidence des lésions cervicales de haut grade chez les femmes non vaccinées contre le papillomavirus humain
Background: Human papillomavirus (HPV) vaccination has substantially reduced the incidence of high-grade cervical lesions (HSIL+) among vaccinated individuals. However, indirect effects on unvaccinated populations remain unclear. We assessed herd effects by examining age-varying HSIL+ incidence among unvaccinated women in Sweden.
Methods: We conducted a nationwide, retrospective, register-based cohort study including 857 168 girls and women born between 1985 and 2000, using data from the Swedish National Cervical Screening Registry and several national health and population registries. Participants were grouped by birth cohorts exposed to different HPV vaccination strategies: opportunistic vaccination (1985–88; reference group), subsidised vaccination (1989–92), catch-up vaccination (1993–98), and school-based vaccination (1999–2000). Participants were followed up from age 10 years or from Jan 1, 2006, whichever came later, until their first HPV dose, an HSIL+ diagnosis, emigration, death, their 35th birthday, or Dec 31, 2022. Poisson regression was used to estimate age-constant and age-varying incidence rate ratios (IRRs) of HSIL+ among unvaccinated individuals across cohorts.
Findings: The study population, followed up from Jan 1, 2006, to Dec 31, 2022, consisted of 857 168 girls and women who had not been previously vaccinated for HPV or received a diagnosis of HSIL+ at baseline. We identified 42 274 cases of HSIL+, with cumulative incidence differing across birth cohorts and lowest in the 1999–2000 cohort. For participants aged 23 years, with the 1985–88 cohort as the reference group, the IRR of HSIL+ was 0·53 (95% CI 0·39–0·73) in the 1999–2000 cohort, 1·26 (1·19–1·34) in the 1993–98 cohort, and 1·26 (1·18–1·34) in the 1989–92 cohort. IRRs in the older cohorts were reduced with increased age, declining to 1·00 (0·87–1·05) by the age of 29 years for the 1993–98 cohort and 0·89 (0·80–0·99) by the age of 33 years for the 1989–92 cohort.
Interpretation: HSIL+ incidence in unvaccinated women declined in the birth cohort eligible for HPV vaccination through a school-based programme. This finding shows that the herd effect can be achieved through high-coverage HPV vaccination.
The Lancet Public Health , article en libre accès, 2026