Distinctive Antigenic Drift in the Influenza A(H3N2) Virus Hemagglutinin

Variation in the H3 hemagglutinin (HA) among influenza A(H3N2) strains isolated in the USSR since the emergence of a novel shift variant, A/Hong Kong/1/68, has been comprehensively analysed. It has been found that the antigenic composition of the H3 HA changed during the period of observation due to changes in the primary structure of the gene coding for the HA protein. The progressive heterogeneity of the antigenic composition of the H3 HA in the strain population has been noted. However, regardless of population heterogeneity, each epidemic wave was associated with distinct leader strains that probably featured a combination of the most favourable mutations in the gene providing for their epidemicity. During the period of observation from 1968 to 1986, all such strains originally arose abroad, and were subsequently introduced into the USSR at various dates. The appearance of antigenic strains — revertants and relicts — with the HA’s higher antigenic affinity for drift variants, including the prototype A/Hong Kong/1/68 strain, has been demonstrated; though previously dominant in active circulation, these variants were ousted from it.