Immunogenicity and safety of 1 versus 2 doses of quadrivalent-inactivated influenza vaccine in children aged 3–8 years with or without previous influenza vaccination histories

This study assessed the immunogenicity and safety of a quadrivalent influenza split-virion vaccine (IIV4) in children with and without prior influenza vaccination. An open-label, phase IV clinical trial was conducted in healthy children aged 3-8 years in Pizhou, Jiangsu Province, China. Children with ≥2 doses of prior influenza vaccination (Influenza Vaccination Group) and those with no prior vaccination (Influenza Vaccine-naïve Group) were given two doses of IIV4, 4 weeks apart. The primary immunogenicity endpoints were seroprotection rates (SPRs) of hemagglutination inhibition antibody 28 days after each dose against influenza A/H1N1, A/H3N2, B/Yamagata (BY), and B/Victoria (BV). Safety endpoints included adverse events (AEs) within 28 days and serious adverse events (SAEs) within 6 months. Between September 19-25, 2021, 278 participants were screened, and 240 were enrolled (120 per group). In the Influenza Vaccination Group, SPRs after the 2nd dose were 98.28%, 92.24%, 99.14%, and 87.93%, similar to those after the 1st dose (all <i>p</i> > .05). In the Influenza Vaccine-naïve Group, SPRs after the 2nd dose were 99.12%, 96.49%, 99.12%, and 92.11%, significantly higher than after the 1st dose (<i>p</i> < .001 for all strains except BY, <i>p</i> = .070). Most AEs were mild or moderate, with no serious AEs related to the vaccine. Children aged 3-8 years with prior influenza vaccination need only one dose of IIV4, while those without prior vaccination require two doses. ClinicalTrials.gov, the identifier is NCT05144464.