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Characteristics and pathogenicity of influenza A viruses
| Viral features | |
| Number of HA subtypes | 16 |
| Number of NA subtypes | 9 |
| Predominant human subtypes | H1, H2, H3 |
| Avian subtypes | H1–H16 |
| HPAI subtypes | H5 and H7 |
| Conversion to HPAI | Basic amino acid insertion in HA |
| Avian sialic acid-galactose linkages | α-2,3 linkages |
| Human sialic acid-galactose linkages | α-2,6 linkages |
HA, hemagglutinin; HPAI, highly pathogenic avian influenza; NA, neuroaminidase.
