Seasonal Influenza Vaccine 2021/2022 Provides Limited Cross Reactivity Against Contemporary Swine Influenza A Virus Strains in Spain

Swine influenza A viruses (SIVs) pose a zoonotic risk, with variants detected in humans in Europe. This study evaluated the efficacy of the 2021/2022 seasonal influenza vaccine against SIVs. Forty-six postvaccination human sera were tested via hemagglutination inhibition (HI) assay against Spanish SIV genotypes. Seroprotection rate (SPR, HI titer ≥ 40) was 76% (95% CI: 64-88) and 91% (95% CI: 83-99) for human vaccine strains H1 and H3, respectively. SPRs were 67% (95% CI: 53-81) for pandemic swine H1, 64% (95% CI: 50-78) for human seasonal-like H1, 17%-46% for Eurasian avian-like H1, 15% (95% CI: 5-25) for human seasonal-like H3 from the 1970s, and 83%-93% for human seasonal-like H3 from the 2000s SIVs.