Antibody responses against influenza A decline with successive years of annual influenza vaccination.

There was a clear trend of decreasing seropositivity with increasing numbers of prior vaccinations in 2021 but not 2020 (Supplementary Fig. 3B ); however, a clearer trend emerged after adjustment for pre-vaccination titre (Fig. 2B ; Supplementary Fig. 6 , Supplementary Table 3 ).

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