Each type of virus has a stable nucleoprotein group antigen common to all strains of the type, but distinct from that of the other type; each also has a mosaic of surface antigens (haemagglutinin and neuraminidase) which characterise the strains and which are subject to variations of two kinds: 1) a rather continual drift that occurs independently within the haemagglutinin and neuraminidase antigens; 2) after a period of years, a sudden shift (notably in type A virus of human origin) to a different haemagglutinin or neuraminidase antigen. The sudden major shifts are the basis of subdivisions of type A virus of human origin.
types a and B virus evolve continuously, resulting in changes in the antigenicity of their spike proteins, preventing the development of prolonged immunity to infection. The spike proteins, external haemagglutinin and neuraminidase have been studied as models of membrane glycoproteins.
strain notations indicate type, geographic origin, year of isolation, and, in the case of type A strains, the characterizing subtypes of haemagglutinin and neuraminidase antigens (e.g., A/Hong Kong/1/68 (H3 N2); B/Hong Kong/5/72).