A pandemic toolbox for clade 2.3.4.4b A(H5N1) influenza virus risk assessment

Since 1997, more than 1000 cases of highly pathogenic avian influenza A(H5N1) have been reported in humans.1 During this time, A(H5N1) viruses have evolved into dozens of clades, showing ever-expanding phylogenetic diversification and geographical spread.2 Over the past 5 years, clade 2.3.4.4b A(H5N1) viruses have exhibited rapid global expansion in migratory birds, with spillover to an increasing range of marine and terrestrial mammalian species. Among these species are dairy cattle, not previously recognised as a permissive host for influenza A viruses (IAV), leading to widespread clade 2.3.4.4b A(H5N1) outbreaks in herds in the USA and sporadic human infections linked with exposure to infected animals since March 2024.