Changes in the antibody status of a population following epidemic infection by influenza virus A2-Hong Kong-1-68.

The haemagglutinin of influenza virus A2/Hong Kong/1/68 was shown to be markedly different from that of previously isolated A2 virus strains. No haemagglutination-inhibiting (HI) antibody to A2/Hong Kong/1/68 virus was detected in serum specimens collected in 1966 from persons aged 60 years or less. In contrast, HI antibody tests with 270 sera collected in 1968 indicated that 9.6% had demonstrable HI antibody at low titres, and 35.2% of 454 postepidemic (1969) sera had demonstrable HI antibody at relatively high titres. Most sera from persons aged 80 years and more collected in 1968 and 1969 had demonstrable HI antibody to influenza virus A2/Hong Kong/1/68. No HI antibody to the Hong Kong virus was detected in pre-epidemic sera from children aged 6 months to 3 years, whereas 32% of postepidemic sera had HI antibody. The acquisition of HI antibody to A2/Hong Kong/1/68 was not accompanied by an increase in the incidence or titres of HI antibody to heterotypic A2 influenza viruses. For sera from children aged 4-11 years, an increase of HI titre to heterotypic A2 influenza was found.