HPV vaccine may protect unvaccinated people through herd immunity

A Swedish nationwide cohort study of more than 850,000 unvaccinated girls and women found that those eligible for high‑coverage school-based HPV vaccination had about half the rate of high‑grade cervical lesions compared with earlier unvaccinated cohorts, suggesting community-level herd protection; experts caution that precisely measuring the size of this effect is difficult.

A nationwide Swedish study examined whether broad HPV vaccination programs reduced precancerous cervical lesions among people who were not vaccinated. The study appears in The Lancet Public Health and used national vaccination registries. Researchers from Karolinska Institutet compared birth cohorts born 1985–2000 exposed to different vaccination strategies, including opportunistic, subsidized, catch-up, and school-based programs. The analysis focused on rates of high‑grade cervical lesions (HSIL+) among more than 850,000 unvaccinated girls and women. Key findings: - Unvaccinated women born in 1999–2000 who were eligible for school-based vaccination programs with greater than 80% coverage had about a 50% lower incidence of HSIL+ than unvaccinated women from birth cohorts before vaccination was widespread. - At age 23, the incidence rate ratio for the 1999–2000 cohort was 0.53 (95% CI, 0.39 to 0.77). - The subsidized vaccination cohort (1989–92) had an age‑adjusted incidence rate ratio of 1.18 (95% CI, 1.15 to 1.20), and the catch‑up cohort (1993–98) had an age‑adjusted incidence rate ratio of 1.03 (95% CI, 0.99 to 1.06). - Nearly all cervical cancers are caused by HPV infection, and vaccination has been shown to reduce infections and precancerous lesions; herd protection refers to reduced community transmission when vaccination coverage is high. - The study’s strengths include its large size and use of registry data, which help limit selection bias and allowed comparison across different vaccination strategies. Summary: The study’s results are consistent with a herd effect from high‑coverage, school-based HPV vaccination, showing lower rates of precancerous cervical lesions among unvaccinated women in those cohorts. Experts writing in an accompanying commentary caution that estimating the exact magnitude of herd effects is challenging because of changing sexual behavior, screening practices, and the absence of clear counterfactual models. Undetermined at this time.