Background: In clinical trials, vaccines against human papillomavirus (HPV) have been highly effective against HPV16- or HPV18-associated cervical lesions. The quadrivalent HPV vaccine was licensed in 2006 and subsequently implemented in the Danish vaccination program. The study aim was to use individual information on HPV vaccination status to assess subsequent risk of cervical lesions.
Methods: Using a cohort study design, we identified all girls and women born in Denmark in the period from 1989 to 1999 and obtained information on individual HPV vaccination status in the period from 2006 to 2012 from nationwide registries. Incident cases of cervical lesions were identified by linkage to the nationwide Pathology Data Bank. We compared vaccinated and unvaccinated girls and women stratified by birth cohort in Cox proportional hazards models.
Results: Risk of atypia or worse (atypia+) and of cervical intraepithelial neoplasia grade 2 or 3 (CIN2/3) were statistically significantly reduced among vaccinated women in birth cohorts 1991 to 1994 (1991-1992atypia+: hazard ratio [HR] = 0.46, two-sided 95% confidence interval [CI] = 0.39 to 0.56; 1991-1992CIN2/3: HR = 0.56, 95% CI = 0.37 to 0.84; 1993-1994atypia+: HR = 0.40, 95% CI = 0.29 to 0.56; 1993-1994 CIN2/3: HR = 0.27, 95% CI = 0.10 to 0.67). The birth cohort 1989 to 1990 had a statistically significantly reduced risk of atypia+ (HR = 0.75; 95% CI = 0.65 to 0.86); the risk of CIN2/3 was also decreased but not statistically significant. No events occurred among girls in the birth cohort 1997 to 1999, whereas for the birth cohort 1995 to 1996 a hazard ratio could be calculated only for atypia+.
Conclusions: Six years after licensure of the quadrivalent HPV vaccine in Denmark, a reduced risk of cervical lesions is observed at the population level.