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.

Keywords: United Kingdom; highly pathogenic avian influenza; influenza; influenza A; neutralization; quadrivalent influenza vaccine; viruses; zoonoses; zoonotic transmission.

Figure

Live-virus neutralization of preseasonal and postseasonal quadrivalent influenza vaccination in a study of influenza A(H5N1) virus neutralization by quadrivalent seasonal vaccines, United Kingdom, 2021–2024. Violin plots show paired serum samples before and after each seasonal vaccine (top row, 2023–24, n = 59; middle row, 2022–23, n = 54; bottom row, 2021–22, n = 30) tested against pandemic H1 and 2 highly pathogenic avian influenza H5 strains: H1N1 strain (A), H5N1 Cambodia strain (B), and H5H1 Texas dairy strain (C). Virus neutralization reported as log₂-transformed IC₅₀. Black diamonds indicate median IC₅₀. Nonspecific cross-reactivity levels are shown as shaded areas. p values from McNemar tests are shown, comparing the number of persons above background (dashed line) before and after quadrivalent influenza vaccination. IC₅₀, reciprocal titer at which 50% of viral infection is inhibited; NE, not estimated.