Repeat Influenza Vaccination Effects in 2021/22 and 2022/23 in a Community-Based Cohort in Hong Kong

BACKGROUND: Repeated influenza vaccination has been associated with attenuated immune responses and reduced clinical effectiveness. Most studies have focused on the responses at 30 days post-vaccination. As influenza did not circulate in Hong Kong during the COVID-19 pandemic, we were able to investigate vaccine responses in the absence of infections. METHODS: In this community-based cohort study in Hong Kong, we investigated the impact of repeated annual influenza vaccination on antibody titer boosting and waning rates of 8 contemporary vaccine strains in 2021/22 and 2022/23, using hemagglutination inhibition assays. We estimated the impact of repeat vaccination on mean fold rises and on post-vaccination titers. We investigated differences by vaccination history in titer waning rates using a power law model. RESULTS: We found differences in post-vaccination geometric mean and mean fold rises in titers against several vaccine strains in repeat vaccinees in 2022/23 compared to 2021/22. However, although participants with higher vaccination uptake had significantly slower antibody waning against A/Victoria/2570/2019, B/Phuket/3073/2013, and B/Austria/1359417/2019, they reached similar antibody titers at 6 months post-vaccination. CONCLUSIONS: Our findings suggest that repeated influenza vaccination is associated with reduced titer increase at day 14 post-vaccination, but has less impact on antibody levels at 6 months post-vaccination.