Pre-existing cross-subtype serological responses against H5Nx in health care workers

ABSTRACT A rise in cases of highly pathogenic avian influenza A (H5N1) clade 2.3.4.4b in humans has been reported since 2023. Although influenza seasonal vaccination (H1N1/H3N2) was shown to be effective against cases of reinfection with endemic influenza A viruses (IAVs), its capacity to elicit cross-reactive antibodies against (H5N1) remains unclear. Here, we measured vaccine-elicited antibody responses against seasonal (H1N1/H3N2) and various avian subtypes A (H5Nx) using plasma samples from the CRCHUM Health Care Workers cohort in Montreal, Canada, during the 2024 influenza seasonal vaccination. The capacity of their plasma to recognize hemagglutinin (HA) and neuraminidase (NA) antigens from ten influenza strains was measured. Their neutralizing activity, as well as their capacity to mediate antibody-dependent cellular cytotoxicity (ADCC) against cells stably expressing HA and NA, was measured. Overall, seasonal vaccination against H1N1/H3N2 elicited antibodies able to recognize several HA and NA, and in some individuals, this translated to increased neutralization and ADCC activity, including against H5Nx. This study provides insightful information on how vaccination shapes cross-reactive antibody responses against avian H5Nx. IMPORTANCE Influenza A viruses (IAVs) are endemic in the human population and responsible for hundreds of thousands of deaths every year, occasionally leading to influenza pandemics. Since 2023, there has been a rise in avian IAV infections in humans, with some deaths reported, mostly related to the H5N1 subtype clade 2.3.4.4b. Seasonal vaccines against IAVs are effective, but their efficacy against avian IAVs is not well known. Here, following influenza seasonal vaccination in a cohort of adults, we report highly heterogeneous serological responses, possibly due to pre-existing immunity and HA sequence similarity. These findings provide insight into the breadth of pre-existing and vaccine-induced immunity against H5 influenza viruses and may inform the development of vaccination strategies aimed at providing broader protection against emerging influenza viruses.