Background
Clade 2.3.4.4b highly pathogenic avian influenza A(H5N1) (HPAI H5N1) viruses are widespread globally and have transmitted from birds to dairy cattle at least four times in the United States, including once by a genotype B3.13 virus and three times by genotype D1.1 viruses. Despite their prevalence and known ability to infect humans, only a few studies have examined respiratory droplet transmission capabilities of clade 2.3.4.4b viruses in mammalian models of influenza infection.
Methods
Here, we assessed respiratory droplet transmission of two recent human clade 2.3.4.4b HPAI H5N1 viruses – A/Michigan/90/2024 (‘MI90-H5N1’), a B3.13 isolate, and plaque-purified A/British Columbia/PHL2032/2024 (‘BC2032-H5N1’), a D1.1 isolate – in the ferret model.
Findings
We found that MI90-H5N1, in contrast to earlier findings, causes severe disease and partial lethality in ferrets, with virus spread to extra-respiratory organs and no respiratory droplet transmission. BC2032-H5N1 caused less severe disease with no lethality in ferrets and, consistent with a recent report, failed to transmit via respiratory droplets.
Interpretation
Together with other reports, our results suggest that respiratory droplet transmissibility of clade 2.3.4.4b viruses is variable. Therefore, continued monitoring and risk assessment for emerging HPAI H5N1 viruses is essential to better understand their pandemic potential.