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.