Antigenic and amino acid sequence analysis of the variants of H1N1 influenza virus in 1986

Since their reintroduction to human populations in 1977, influenza A viruses of the H1N1 subtype have undergone antigenic drift. Recently a distinct antigenic variant, A/Singapore/6/86, has been almost exclusively isolated internationally, and the antigenic properties and amino acid sequence of its haemagglutinin have been determined and compared with those of the haemagglutinins of other H1N1 viruses, in particular A/Chile/1/83. Fourteen amino acid sequence differences are detected between the HA1 components of these two viruses, ten of which are different from equivalent residues in the haemagglutinins of all H1N1 viruses isolated between 1982 and 1983, and seven of which are novel in the haemagglutinins of all H1N1 viruses sequenced to date. The results are discussed in relation to the three-dimensional structure of the haemagglutinin and the location of the previously defined antigenically important regions.