Reactogenicity, safety, and immunogenicity of a quadrivalent seasonal influenza vaccine in adults aged 65 years and older: Phase 4 study results from India during December 2023–February 2024

<i>Fluarix Tetra</i> is a quadrivalent seasonal influenza vaccine that has been approved in India for immunization of adults and children aged ≥6 months. This study was conducted in adults aged ≥65 years in India to fulfill a post-approval regulatory commitment. In this prospective, multicenter, single-arm, open-label, Phase 4 study, participants were administered a single 0.5-mL dose of <i>Fluarix Tetra</i> (2023-2024 Northern Hemisphere). The primary objectives were to assess the reactogenicity and safety of <i>Fluarix Tetra</i> by monitoring solicited adverse events (AEs) within 7 days post-vaccination, as well as unsolicited AEs and serious AEs (SAEs) within 21 days post-vaccination. The secondary objective was to evaluate immunogenicity by monitoring the geometric mean titers, geometric mean increase, seroconversion rates (SCRs), and seroprotection rates (SPRs). Overall, 250 adults aged ≥65 years (mean age, 71.8 years) were included in the study. Pain (15.0%) was the most common solicited injection-site AE, and fever (6.5%) was the most common solicited systemic AE. No SAEs were reported within the 21-day post-vaccination period, and no deaths occurred during the study. The hemagglutination inhibition titers increased from Day 1 to Day 22 against all the four influenza strains, A/(H1N1), A/(H3N2), B/Yamagata, and B/Victoria, with SPRs > 80% and SCRs > 75% at Day 22. <i>Fluarix Tetra</i> was well tolerated and immunogenic in adults aged ≥65 years in India.<b>Trial registration</b>: NCT05648357.