Clade 2.3.4.4b H5N1 neuraminidase has a long stalk, which is in contrast to most highly pathogenic H5N1 viruses circulating between 2002 and 2020.

Summary: Bird flu is changing in a worrying way. For nearly two decades, the dominant version of the H5N1 virus had a "short stalk" structure in its neuraminidase protein. This structure made it very deadly to poultry but limited its ability to spread among mammals. However, since 2020, a new variant called Clade 2.3.4.4b has taken over globally. Unlike its predecessors, this version features a "long stalk" structure. This change is significant because the long stalk is associated with better transmission in mammals. This adaptation explains why we are seeing infections in dairy cattle and other mammals, raising concerns that the virus is evolving to spread more easily between humans.