Inconclusive evidence for non-inferior immunogenicity of two- compared with three-dose HPV immunization schedules in preadolescent girls: A systematic review and meta-analysis

Background: The European Medicines Agency (EMA) recently approved two-dose immunization schedules for bivalent (HPV 16/18) and quadrivalent (HPV 6/11/16/18) human papillomavirus (HPV) vaccines in nine to fourteen and thirteen year-old-girls, respectively. Registration was based on trials comparing immunogenicity of two-dose schedules in girls 9-14 years to three-dose schedules in young women 15-26 years. We evaluate comparability of antibody levels between and within age groups and discuss potential implications for monitoring the effectiveness of HPV vaccination.

Methods: A systematic literature search was performed for studies comparing immunogenicity of two- to three-dose schedules of HPV vaccination. We compared geometric mean concentrations (GMCs) of vaccine-type antibodies between different dosing schedules across different age groups. Meta-analysis was used to estimate pooled GMC ratios (bivalent vaccine) of two- compared with three-dose schedules within girls.

Findings: For both vaccines, two-dose immunization of girls yielded non-inferior GMCs relative to a three-dose schedule in young women up to respectively 36 and 48 months follow-up. Pooled GMC ratios for the bivalent vaccine within girls showed the two-dose schedule becoming inferior to the three-dose schedule in girls for HPV 16 at approximately two years after the first dose. For the quadrivalent vaccine, antibody responses for HPV-18 became inferior from 18 months follow-up onwards when comparing the two-dose schedule with the three-dose schedule within girls.

Implications: Two-dose immunization of girls has non-inferior immunogenicity compared to a three-dose schedule among young women. However, non-inferior immunogenicity of two- compared with three-dose schedules within girls has not been shown at all time points. Due to this inconclusive evidence, implementation of two-dose HPV vaccination needs to be monitored closely.