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

## TL;DR

The 2021/2022 seasonal flu vaccine offers limited protection against current swine flu strains in Spain, especially older H3 variants.

## Contribution

This study quantifies the cross-reactivity of seasonal influenza vaccines against contemporary swine influenza A virus strains in Spain.

## Key findings

- The seroprotection rate against pandemic swine H1 was 67% (95% CI: 53–81).
- The vaccine showed low seroprotection (15%) against human seasonal-like H3 from the 1970s.
- High seroprotection (83–93%) was observed against human seasonal-like H3 from the 2000s.

## Abstract

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.