Duration of the Humoral Immune Response to Influenza Vaccination in a Cohort of Repeatedly Vaccinated Adults over Four Consecutive Epidemic Seasons: 2018/19–2021/22 Observational Study

Background/Objectives: The impact of repeated annual influenza vaccination on long-term immunogenicity and protection remains debated. Although reduced fold-increases in antibody titers are often observed in repeatedly vaccinated individuals, the effect on the duration of the humoral response remains poorly understood. This study evaluated the magnitude and persistence of the humoral immune response over four consecutive seasons. Methods: An open-label observational study was conducted between 2018 and 2022 and involved 28 healthy adults (median age, 55 years) immunized with seasonal inactivated influenza vaccines. Hemagglutination inhibition (HAI) titers were assessed at five time points annually. Secondary endpoints included neutralizing (MNA) and neuraminidase-inhibiting (ELLA) antibody responses measured at baseline and three weeks after vaccination. The duration of antibody responses above the seroprotection threshold (titer ≥ 40) was compared across seasons using the Mantel–Cox test. Results: Antibody dynamics differed by vaccine component. The highest GMT and fold-increase were observed for A/H1N1pdm09 in 2018/19, followed by a significant decline after the second vaccination, that was more pronounced for neutralizing antibodies. The response to N1 neuraminidase remained weak throughout the study. For A/H3N2, baseline HAI GMTs increased significantly after each subsequent vaccination, whereas N2 neuraminidase responses peaked after the first dose and declined thereafter. Influenza B responses were weaker than those to influenza A. A significant increase in B/Yamagata titers in 2020/21 was attributed to the introduction of quadrivalent vaccines. A/H1N1pdm09 HAI responses were long-lasting, with titers remaining at or above the seroprotection threshold for one year. The duration of the A/H3N2 antibody response increased significantly over four seasons, likely because of annual strain updates. A similar trend was observed for B/Victoria after strain changes. Conclusions: Repeated annual influenza vaccination did not negatively affect post-vaccination antibody titers to the HA vaccine antigens in the studied cohort. Regular updates to vaccine strains may enhance the persistence of HA-targeted humoral immune responses.