Genetic variability of the Influenza A H5N1 virus

Influenza A H5N1 virus represents a global zoonotic threat, capable of infecting a wide range of hosts, including (besides birds) non-human mammals and, occasionally, humans. In recent years, the virus has exhibited unprecedented geographic expansion and genetic diversification, with numerous documented cases of infection in non-avian species, particularly those associated with clade 2.3.4.4b. The primary aim of this study was to evaluate the potential of the H5N1 virus to adapt to human hosts. In pursuit of this goal, all available nucleotide sequences of the genes encoding Hemagglutinin (HA) and Neuraminidase (NA) from the H5N1 strain—isolated from birds, non-human mammals, and humans and accessible via the GISAID platform—were analysed. Evolutionary dynamics and adaptive potential were assessed across the resulting datasets. The results reveal a significant expansion of clade 2.3.4.4b, along with the presence of mutations known to promote infection and replication in mammals. However, no distinct genetic signature was identified that would indicate spillover or confirmed adaptation to the human host. These findings suggest that, although the H5N1 virus has infected numerous mammalian species in recent years and several human cases have been reported, there is currently no evidence of sustained human-to-human transmission, and birds still remain the main hosts. Rather, humans appear to be incidental hosts, acquiring infections from strains typically circulating in avian or bovine populations, depending on geographic region, sanitary conditions, and lifestyle factors.