Introduction: For example, the amino acid substitution at position
D222G/E/N (H1 numbering) in the
HA molecule was associated with increased viremia, severe
pneumonia, and deaths; a substitution (
I219K) in the glycan receptor-binding site of
HA quantitatively increased its human receptor-binding affinity; and substitutions
D131E, S186, and
A198E contribute to virulence in mice.