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CDC - Emerging Infectious Diseases - Investigation of Influenza A(H5N1) Virus Neutralization by Quadrivalent Seasonal Vaccines, United Kingdom, 2021–2

Commonground

Commonground

Senior Moderator
Volume 31, Number 6—June 2025
Dispatch
Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.​
Investigation of Influenza A(H5N1) Virus Neutralization by Quadrivalent Seasonal Vaccines, United Kingdom, 2021–2024

Abstract

We tested cross-neutralization against highly pathogenic avian influenza A(H5N1) virus in adults vaccinated with 2021–2023 seasonal quadrivalent influenza vaccine in the United Kingdom. Seasonal quadrivalent influenza vaccines are unlikely to protect vulnerable persons against severe H5N1 disease during widespread transmission. Enhanced measures are needed to protect vulnerable people from H5N1 virus infection.
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Continued: https://wwwnc.cdc.gov/eid/article/31/6/24-1796_article
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Excerpt from above article

Ongoing adaptation of HPAI H5N1 clade 2.3.4.4b virus in cows and other mammal hosts found on dairy farms, including rodents and cats, substantially increases the risk for a major HPAI H5N1 virus epidemic or pandemic in humans (2,4). The paucity of human serologic memory against either H5N1 virus strain raises the potential for widespread vulnerability to infection within the adult population. We observed a predictable boost to neutralizing titers against the contemporary seasonal influenza A(H1N1) virus (A/Wisconsin/67/2022) that was absent for the 2 clinically relevant H5N1 viruses tested in our high-throughput neutralization assay (Appendix). Neutralizing antibody titers have long been used as a correlate of protection against seasonal influenza (12); thus, our observations suggest seasonal QIVs are unlikely to offer adequate serologic protection against H5N1 virus.​

Immunity against influenza evolves throughout the lifespan, and early infection exposures influence subsequent antibody responses after infection and vaccination (13). Few participants in our study had detectable neutralizing titers above background to the 2.3.2.1c A/Cambodia/NPH230776/2023 virus and none to the 2.3.4.4b A/dairy_cattle/Texas/24-008749-002/2024 virus. Together with our observed lack of QIV boosting, our results suggest that strategies reliant on existing population-level or QIV-based immunity against H5N1 virus infection must be approached with caution.​
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