HPV Vaccination and CIN3+ Among Women Aged 25–29 Years in Northern Norway, 2010–2024: A Population-Based Time-Series Analysis

Background/Objectives: Cervical intraepithelial neoplasia grade 3 and worse (CIN3+) is a robust surrogate for cervical cancer risk. In Norway, organized cervical screening starts at 25 years of age (25–69 years). Norway introduced school-based HPV vaccination with the quadrivalent vaccine for 12-year-old girls in 2009 (birth cohorts ≥ 1997) with high 3-dose completion, and a catch-up program with the bivalent vaccine for women born 1991–1996 in 2016–2019 with lower uptake. We assessed whether increasing birth-cohort vaccination coverage (defined as ≥1 dose) was followed by reductions in CIN3+ at the age of entry to organized screening (25–29 years). Methods: We conducted a retrospective, population-based time-series of women aged 25–29 years in Troms and Finnmark screened in 2010–2024. CIN3+ was counted per unique woman and expressed per 1000 screened women per year. Cohort-level vaccination exposure was proxied by birth-year eligibility and national coverage (≥1 dose) by calendar year. Temporal trends were assessed using segmented linear regression (2010–2017; 2017–2024). Results: Among 42,253 screening tests, 865 women had CIN3+. CIN3+ rates were stable in 2010–2016 (≈15–24 per 1000), peaked in 2017–2018 (≈26–28 per 1000), and declined to 6.6 per 1000 in 2024 (~75% reduction from the peak). The 2010–2017 trend was not significant (p = 0.244), whereas 2017–2024 showed a significant annual decline (slope −3.04 per 1000 per year; p = 7.4 × 10−5). The decline coincided with an increase in the vaccinated share of the age group from an estimated 12% in 2017 to 78% in 2024. Cervical cancer was rare throughout and absent in 2024, and the 2023 transition to primary HPV testing did not interrupt the downward trend. Conclusions: As vaccinated birth cohorts—especially those vaccinated before sexual debut—entered organized screening at age 25, CIN3+ in women aged 25–29 years fell markedly. Estimates are based on coverage defined as ≥1 dose; future linkage to individual dose data and HPV type–specific CIN3+ is warranted.