Immunological interrelationships of Hong Kong, Asian and equi-2 influenza viruses in man.

Sera from an elderly population drawn prior to the 1968 Hong Kong influenza epidemic were found to have a high prevalence of Hong Kong antibody, indicating the circulation of a Hong Kong-like virus before 1900. This report compares an elderly (birth dates before 1892) and a young adult population with regard to serum antibody response to Hong Kong variant vaccine, avidity of Hong Kong antibody, and antibody absorption with Asian, A/Equi-2, and Hong Kong viruses.The demonstration of doubly-absorbable Hong Kong and A/Equi-2 antibodies in sera where both are present and of doubly-absorbable Hong Kong and Asian antibodies in young adult sera after Hong Kong variant vaccine lends new support to the view that there is an immunological relationship between these viruses.The difference in the response of the two age-groups to Hong Kong variant vaccine-specifically the anamnestic boosting of Hong Kong antibody together with the lack of Asian antibody rise in the elderly population in contrast to the striking Asian antibody response in the young adult population-leads to the conclusion that a Hong Kong-like virus may have been the original antigenic sin in the elderly population with initial Asian exposure in 1957. The excellent avidity of Hong Kong antibody in pre- and post-vaccine sera from the elderly suggests a close similarity if not identity of the haemagglutinins of the Hong Kong-like virus that circulated in human populations prior to 1900 and of the 1968 Hong Kong virus.