Genetic and antigenic variation in the haemagglutinin of recently circulating human influenza A (H3N2) viruses in the United Kingdom.

Variation in the haemagglutinin (HA) gene of influenza A (H3N2) viruses isolated in the U.K. and abroad from 1992-1994, was determined by nucleotide sequencing of the HA1 domain of the HA gene. Viruses isolated in the U.K. early in the 1992-93 season were from the A/Beijing/353/89 lineage and were replaced later that season by viruses from the A/Beijing/32/92 lineage. Viruses from the new lineage continued to be isolated during the 1993-94 season, but were heterogeneous. Most of these isolates were more closely related to an A/Beijing/32/92 variant, A Hong Kong/23/92, but could be distinguished into three groups by serology (of which one group was circulating during the previous season) and four groups based on sequence variation in the HA gene. However, phylogenetic analysis of antigenically-distinct isolates showed that the HA gene is evolving along one lineage. Sequence analysis identified mainstream, subgroup and strain specific amino acid substitutions. There was a broad correlation between the observed amino acid changes and the antigenic sites of the HA. The results of this study highlight the value of regular molecular analysis of circulating viruses.