A field strain of influenza A (H3N2) virus isolated in embryonated eggs during the 1984–5 influenza outbreak (A/Finland/13/85E) was compared in an antigenic analysis with virus from the same clinical specimen isolated in MDCK cell cultures (A/Finland/13/85M). The M-virus appeared to be more sensitive to haemagglutination-inhibiting antibodies against heterologous viruses than did the Evirus. The results of propagation and plaque purification experiments support the hypothesis that a single clinical specimen may consist of distinct antigenic variant subpopulations promoted selectively by the host during isolation procedures. Receptor-binding properties are discussed as a possible explanation for this selectivity.
A set of 471 paired sera consisting of pre-epidemic and post-epidemic specimens taken from the same subjects in 1984–5 was studied for haemagglutinationinhibiting antibodies to six influenza A (H3N2) virus strains, including the E-virus and the M-virus from A/Finland/13/85. Of the antigens used, the M-virus detected significant antibody increases more frequently than did the E-virus (10·0 v. 5·9%). The superiority of the M-virus may rest primarily in its ability to pick out anamnestic antibody responses. Irrespective of this cross-reactivity, preepidemic antibody to the M-virus was fairly well associated with protection. In the set of sera (230 specimens) collected in summer 1985 to represent different age groups, the antibody status against the M-virus was significantly better than the status against the E-virus. The results suggest that, at least in some instances, antibody to MDCK-grown virus is a more accurate indicator of the immune status of a community than antibodies to egg-grown virus variants.