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Novel highly pathogenic avian influenza (HPAI) H5Nx viruses are predominantly circulating worldwide, with an increasing potential threat of an outbreak in humans. It remains largely unknown how the stably maintained HPAI H5N1 suddenly altered its neuraminidase (NA) to other NA subtypes, which resulted in the emergence and evolution of H5Nx viruses. Here, we found that a combination of four specific amino acid (AA) substitutions (S123P-T156A-D183N- S223 R) in the hemagglutinin (HA) protein consistently observed in the H5Nx markedly altered the NA preference of H5N1 viruses. These molecular changes in H5N1 impaired its fitness, particularly viral growth and the functional activities of the HA and NA proteins. Among the AA substitutions identified, the T156A substitution, which contributed to the NA shift, also dramatically altered the antigenicity of H5N1 viruses, suggesting an occurrence of antigenic drift triggered by selective pressure. Our study shows the importance of how HA and NA complement each other and that antigenic drift in HA can potentially cause a shift in the NA protein in influenza A virus evolution.

In 2014, novel highly pathogenic avian influenza A H5N2, H5N5, H5N6, and H5N8 viruses caused outbreaks in Asia, Europe, and North America. The H5 genes of these viruses form a monophyletic group that evolved from a clade 2.3.4 H5N1 variant. This rapid emergence of new H5Nx combinations is unprecedented in the H5N1 evolutionary history.