This study for the first time assessed quadrivalent human papillomavirus (qHPV) vaccine effectiveness against HPV6/11/16/18-related high-grade cervical disease in Japanese women (16-26 years old), as previously demonstrated in overseas trials, and vaccine safety in a longer term (48-month) open-label study (NCT01544478). Participants received three doses of qHPV vaccine (Day 1, Month 2, Month 6). Effectiveness endpoints, assessed in the per-protocol population, included incidence of

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... 8-related cervical intraepithelial neoplasia (CIN) Grade 2 or worse (CIN Grade 2 and 3, adenocarcinoma in situ, and/or cervical cancer) as primary endpoint and incidence of external genital lesions (EGLs). Disease related to other high-risk HPV types was also assessed. Adverse events (AEs) and serious AEs (SAEs) were collected from Days 1-15 after any vaccination; vaccine-related SAEs, deaths, and new medical conditions were collected throughout the study. A total of 1030 women received at least one vaccination. No cases of CIN2 or worse or EGLs were reported in the per-protocol population. Injection site-related AEs were reported in 14.5% of participants; most were mild and resolved within 15 days. Vaccine-related systemic AEs occurred in 8.6% of participants, most commonly headache (2.3%), malaise (1.7%), and pyrexia (1.3%). There were no vaccine-related SAEs; one participant discontinued due to a vaccine-related AE of mild uticaria. Overall, qHPV vaccine effectiveness against HPV6/11/16/18-related high-grade cervical disease and EGLs was indicated in Japanese women. The vaccine was well-tolerated, without new safety signals throughout the 48-month study period. Findings are consistent with overseas qHPV vaccine pivotal trials. CLINICAL TRIAL REGISTRY: clinicaltrials.gov; NCT01544478.