Background While phase 3 trials have shown that vaccination against human papillomavirus (HPV) types 16 and 18 prevents persistent HPV type 16 and 18 infections and most high-risk HPV type positive cervical intraepithelial neoplasia (CIN) grade 2+ lesions, long-term follow-up of the phase 3 cohorts is needed to demonstrate that HPV16/18 vaccination prevents CIN3 and invasive cervical carcinoma (CIN3+).

Methods We used data from the Finnish Cancer Registry for passive follow-up of cluster (age-cohort) and individually randomised cohorts of women born in 1984–1989 to assess incidence rates of CIN3+ in HPV16/18 vaccinated Finnish cohort of the bivalent HPV 16/18 vaccine PATRICIA trial participants (N=2404) and a reference cohort (N=7049) enrolled from the same communities. Six months after the Phase III trial was closed in 2009 the cohorts were linked with the Finnish Cancer Registry.

Results and Conclusions A pilot study in 2009 showed that the baseline incidence of CIN3+ was 41 per 100 000 women years in the reference cohort. Knowing that CIN3+ incidence rapidly increases as the cohorts age, the baseline incidence yields 80% power to show 70% vaccine efficacy against CIN3+ in just 5 years. The phase 3 trial included intensive clinical follow-up and thorough health education and counselling which may have modified subsequent risk of cervical neoplasia in all study participants, the incidence rates of CIN3+ need to be validated in a cohort not exposed to any clinical intervention. Preliminary data from such comparison of the incidence rates during passive follow-up the PATRICIA study participants (comprising 50 000 women years) and the reference cohort will be reported.