Properties of the Hong Kong influenza virus. Antigenic relationship of the Hong Kong virus haemagglutinin to that of other human influenza A viruses.

The magnitude of antigenic change of the Hong Kong strains from the earlier influenza A2 strains was compared with previous antigenic changes among the type A viruses. Representative type A strains recovered from man during the years 1933 through 1969 were examined by reciprocal haemagglutination-inhibition, neutralization and neuraminidase-inhibition tests, using antisera produced in chickens. Phenograms or "family trees" were constructed, using numerical taxonomic methods based on cluster analysis of similarity coefficients. By the use of numerical taxonomic methods the inequality of the antigenic relationships among presently recognized classes becomes evident. Antigenic dissimilarities between A0 and A1 strains seen in phenograms constructed from reciprocal haemagglutination-inhibition and neutralization tests were of lower magnitude than dissimilarities between A1 and A2 strains and should not be given equal emphasis. The degree of antigenic dissimilarity between the Hong Kong-like strains and the early A2 strains was even less than that between A0 and A1.The authors stress the need for a new system for identifying and classifying human influenza A strains.