Cervical intraepithelial neoplasia rates in British Columbia women: A population level data linkage evaluation of the school-based HPV immunization program.

BACKGROUND: To understand real-world HPV vaccine impact, continuous evaluation using population-based data is critical. We evaluated the early impact of the school-based HPV immunization program on cervical dysplasia in females in British Columbia (BC), Canada.

METHODS: Data linkage was performed using records from the provincial cervical screening and immunization registries. Precancerous outcomes were compared between unvaccinated and HPV vaccinated females born in 1994-2005. Incidence rate (IR) and relative rate (RR) and vaccine effectiveness (VE), using unadjusted and adjusted Poisson regression, of cytology (HSIL) and histopathology (CIN2, CIN3 and CIN2+) outcomes were compared across vaccination status groups.

RESULTS: Females who received a complete series of vaccine on schedule between the ages of 9-14 had a adjusted RR=0.42 (95%CI 0.31 - 0.57) for CIN2+ over 7 years of follow-up compared to unvaccinated females, resulting in a VE of 57.9% (95%CI 43.2-69.0%). The adjusted RR for HSIL was 0.53 (95%CI 0.43-0.64), resulting in a VE of 47.1% (95%CI 35.6-56.7%).

CONCLUSION: Females vaccinated against HPV have a lower incidence of cervical dysplasia compared to unvaccinated females. Immunization between 9 - 14 years of age should be encouraged. Continued program evaluation is important for measuring long-term population impact.