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Background: Efficacy for cervical cancer prevention of opportunistic HPV vaccination in post-pubertal girls is lower than in 11-year-olds. Methods: Women born between 1986 and 1992 vaccinated at 15–25 years of age (at least one dose of 4-valent HPV vaccine) and screened at 24–27 years of age were included. Frequency of opportunistic vaccination, overall and by birth cohort, was calculated; screening outcomes were compared between vaccinated and unvaccinated women. Results: Overall, 4718 (4.9%) HPV-vaccinated, and 91,512 unvaccinated, women were studied. The frequency of vaccination increased by birth cohort, ranging between 1.8% and 9.8%; age at vaccination decreased progressively by birth cohort (<i>p</i> < 0.0001). Participation in screening was 60.8% among vaccinated, and 56.6% among unvaccinated, women (<i>p</i> < 0.0001). Detection rates (DR) for high-grade lesions were lower in vaccinated women (2.11‰ vs. 3.85‰ in unvaccinated, for CIN3+, <i>p</i> = 0.24; 0.0‰ vs. 0.22‰ for cancer). The DR of CIN3+ increased with age at vaccination, scoring respectively 0.0‰, 0.83‰, and 4.68‰ for women vaccinated when they were 15–16, 17–20, and 21–25 years old (<i>p</i> = 0.17). Conclusions: In comparison to unvaccinated women, higher compliance with cervical cancer screening invitation and lower CIN3+ DR among vaccinated women was observed. Age at vaccination was inversely correlated to vaccination efficacy.