Abstract
OBJECTIVES: A single dose of human papillomavirus (HPV) vaccine would simplify logistics and reduce costs of vaccination programs worldwide. We conducted a phase IIa trial to determine the stability of HPV type-specific antibody responses after a single dose of the nonavalent HPV vaccine, Gardasil9. METHODS: Two hundred-and-one healthy 9 to 11-year-old girls and boys were enrolled at 2 centers in the United States to receive a prime dose of the nonavalent vaccine at baseline, a delayed dose at month 24, and an optional third dose at month 30. Blood samples were collected to measure HPV type-specific antibodies at baseline and at 6, 12, 18, 24, and 30 months after the prime dose. The primary outcomes were serum HPV16 and HPV18 antibody responses. RESULTS: In both girls and boys, geometric mean concentrations of HPV16 and HPV18 antibodies increased at 6 months, declined between months 6 to 12, and then remained stable and high (at 20- and 10-times those at baseline for HPV16 and HPV18, respectively) throughout months 12, 18, and 24 (prebooster) visits. Both HPV16 and HPV18 antibody responses demonstrated anamnestic boosting effect at 30-months after the delayed (24-month) booster dose. CONCLUSIONS: A single dose of the nonavalent HPV vaccine induced persistent and stable HPV16 and HPV18 antibody responses up to 24 months. This study contributes important immunogenicity data to inform feasibility of the single dose HPV vaccination paradigm. Further research is needed to assess the long-term antibody stability and individual clinical and public health benefit of the single dose schedule.