Abstract
This study aimed to evaluate the humoral and cell-mediated immune responses induced by an inactivated influenza vaccine (IIV4) during the 2019/2020 influenza season in Japan. We collected blood samples from 25 healthy adults before vaccination, 2 weeks post-vaccination, and 5 months post-vaccination. Humoral and cell-mediated immunities were assessed based on hemagglutination inhibition antibody titers and interferon-γ (IFN-γ) levels, respectively. The geometric mean titer ratio for A/H3N2 exceeded 2.5, meeting the criteria outlined by the European Medicines Agency guidelines; other strains did not achieve similar thresholds. IFN-γ responses indicated significant activation for all strains, with 32-36% of participants exhibiting ≥ 1.5-fold increases. Due to the implementation of infection control measures against COVID-19, influenza activity was not observed during the 2020/2021 and 2021/2022 seasons, potentially altering influenza immunity. Our findings highlight the importance of both humoral and cell-mediated immunity in evaluating vaccine immunogenicity.