Immunogenicity and Safety of a Full-Dose Regimen of Cell Culture-Derived Quadrivalent Inactivated Influenza Vaccine in Children Aged 6–35 Months: Results from a Multinational Phase 3 Randomised Controlled Trial

Background: Influenza causes substantial morbidity in young children, particularly those aged 6–35 months. In this age group, optimisation of vaccine dose regimens remains important to ensure adequate immunogenicity while maintaining acceptable safety. This study evaluated the immunogenicity and safety of a full 0.5 mL dose of quadrivalent inactivated influenza vaccine (NBP607-QIV) in young children. Methods: This Phase 3, randomised, double-blind, active-controlled, multicentre study was conducted in Korea, Thailand, and Malaysia. Healthy children aged 6–35 months were randomised 2:1 to receive NBP607-QIV (0.5 mL) or control vaccine (0.25 mL). Immunogenicity was assessed using the haemagglutination inhibition assay. Primary endpoints were non-inferiority of NBP607-QIV versus Agrippal for seroconversion rate (SCR) and adjusted post-vaccination geometric mean titre (GMT) ratio against three shared strains. Immunogenicity against the additional B/Yamagata strain was evaluated according to Committee for Medicinal Products for Human Use (CHMP) criteria. Safety was assessed based on adverse events. Results: A total of 676 participants were randomised, and 675 were included in the safety set. Non-inferiority of NBP607-QIV versus control vaccine was demonstrated for SCR for all shared strains and for the adjusted GMT ratio for A/H1N1 and B/Victoria, but not for A/H3N2. Immunogenicity against the B/Yamagata strain met CHMP criteria for SCR and geometric mean ratio (GMR). Immunogenicity was consistent across prespecified subgroups, and the incidence of adverse events was comparable between groups, with no clinically meaningful safety concerns. Conclusions: NBP607-QIV administered at a 0.5 mL dose demonstrated acceptable immunogenicity and a safety profile comparable to that of a licensed trivalent influenza vaccine in children aged 6–35 months, supporting its use in this paediatric population.