Table 1. The antigenic sites and attributes of amino acids in the HA of avian H1N1 IAVs.
| Attributes of amino acids | Number of amino acids at antigenic sites | ||||
|---|---|---|---|---|---|
| Sa | Sb | Ca | Cb | Total | |
| fully conserved | 4 | 2 | 6 | 2 | 14 |
| largely conserved* | 5 | 0 | 4 | 0 | 9 |
| 1918 & 2009 avian-like (not human-like) | 3 | 6 ‡ | 1 ‡ | 2 | 12 |
| 1918-like only | 0 | 0 | 2 | 1 | 3 |
| 2009-like only | 1 | 0 | 0 | 0 | 1 |
| avian-like † | 0 | 3 | 5 | 1 | 9 |
| Total | 13 | 11 | 18 | 6 | 48 |
| Revised total ‡ | 13 | 12 | 19 | 6 | 50 |
*Residues were considered largely conserved when the same amino acid residue observed in many isolates over the years.
†“avian-like” category indicates the number of amino acids at the HA antigenic sites of 31 avian IAVs of Anseriformes and Charadriiformes origin that were not seen in seasonal or pandemic human H1N1 IAVs.
‡Positions 158 in Ca and 212 in Sb are “pandemic-like only” in the H1N1 IAVs of Anseriformes origin but not in those of Charadriiformes origin. The total number of amino acids is 50 with the addition of these 2 positions.