Many females receive the human papillomavirus (HPV) vaccine after the recommended ages, often after sexual activity has started, according to a research letter published in JAMA Network Open. Didem Egemen, PhD, and colleagues estimated the proportion of females who were vaccinated before sexual debut and examined the association of delayed vaccination with the prevalence of HPV 16/18. The prevalence of cervical HPV-16/18 decreased from 6% in the unvaccinated group to 3% in the group vaccinated after the start of sexual activity and less than 1% in the group vaccinated before the start of sexual activity among 4,727 females ever eligible for vaccination. The prevalence of HPV-16/18 was 89% lower in the preadult vaccination group but was not significantly lower (41%) in the post-debut group versus the vaccinated group. Vaccination before sexual activity was associated with an 82% reduction in HPV-16/18 prevalence compared with post-debut vaccination. Only 21% of vaccinated, routine vaccination-eligible females reported receiving their first dose by age 12, per recommendations of the CDC Advisory Committee on Immunization Practices (mean age at first dose, 14.5).