Immunogenicity and Safety of Extended-Interval 2-Dose Regimens of 9vHPV Vaccine

To evaluate the immunogenic antibody response and safety of extending the interval between the 2-dose 9-valent human papillomavirus (9vHPV) vaccine series. Current guidelines recommend administering the 2 doses to adolescent patients 6 to 12 months apart.The study included 146 sexually naive adolescents—85 boys and 61 girls—aged 10 to 15 years (median age: 13). Participants received the first dose of the 9vHPV vaccine at least 12 months prior to enrollment (range: 12-53 months, mean: 25 months after dose 1). Data were collected between March 15 and October 28, 2021.Serologic antibody responses to the vaccine-specific HPV subtypes (HPV 6, 11, 16, 18, 31, 33, 45, 52, and 58) were measured before and 1 month after administration of the second 9vHPV vaccine. Regression models were used to estimate the effect of varying dose intervals on antibody geometric mean titers. Safety data were collected from the day of vaccination through 5 days after vaccination, and serious adverse events were monitored for 12 months following the second dose.All participants (100%) were seropositive for each HPV vaccine subtypes 1 month after the second dose, with similar increases in antibody responses across the dosing interval (12- to 53-month range). The most common adverse event was a mild to moderate injection site reaction, with no serious adverse events or deaths reported.Extending the dosing interval beyond 12 months between the first and second doses of the 9vHPV vaccine elicited robust immunogenicity to all the HPV vaccine subtypes. No significant adverse events were associated with the extended dosing schedule.Flexibility in the 9vHPV vaccine schedule may help inform public health and vaccine advisory recommendations, while encouraging increased patient adherence. It also has broader global health implications by encouraging HPV vaccine initiation in areas with limited health care access, thereby improving equity, accessibility, and allocation of resources to protect a larger adolescent population.