Influenza vaccine strain selection: equivalence of two antigenically distinct haemagglutinin variants of 1989 H3N2 influenza A virus in protection of mice.

Precise antigenic analysis with haemagglutinin-inhibition (HI) tests of 1989 H3N2 influenza A viruses with polyclonal ferret, rabbit and mouse antisera has shown, first, significant differences among 1989 wild-type isolates, second, antigenic differences between two high-yield vaccine candidate reassortant viruses, third, significant antigenic differences of one reassortant (X-105) from the wild-type virus (A/Guangdong A/39) from which it was derived, and fourth, dependence of antigenic characterization of viruses upon the host species used in immunization. Nevertheless, the two reassortant viruses (only 43% similar by HI test) were equally protective in preventing homovariant or heterovariant infection in either previously unimmunized or infection-primed mice. These results not only confirm the known antigenic heterogeneity of influenza A viruses, but raise questions about the adequacy of current methods of antigenic characterization of influenza viruses and the basis for decisions on vaccine strain selection.