Table 2. Amino acid substitutions accompanying antigenic jumps, and their associated charge changes (see [5]).
| Year | Amino acid substitutions | Overall charge effect |
| 1972 | T122N; (G144D); T155Y; (N188D); R207K | 2 − |
| 1975 | N137S; S145N; L164Q; Q189K; (S193D); (N53D); I278S; F174S; R102K; I213V; I217V; I230V | 1 − |
| 1977 | S137Y; (G158E); Q164L; D193N; K50R; D53N; S174F; K201R; V213I; V230I; E82K; M260I | 3 + |
| 1979 | N133S; P143S; G146S; (K156E); T160K; Q197R; (N53D); N54S; D172G; V217I; V244L; 162K; (K82E) | 1 − |
| 1987 | (G124D); Y155H; K189R | 0 |
| 1989 | N145K | 1 + |
| 1992 | (S133D); (K145N); E156K; E190D | 0 |
| 1995 | N145K | 1 + |
| 1997 | (K156Q); E158K; V196A; N276K; (K62E); | 0 |
| 2002 | A131T; (H155T); Q156H; (R50G); (H75Q); E83K; L25I; V202I; W222R; (G225D) | 0 |
While successive antigenic groups overlap in time, ‘year’ denotes the timing of the first vaccine strain incorporating the given substitutions. Substitutions shown in bold denote a net positive change, and vice versa for those in parentheses.