Intraepidemic heterogeneity of influenza A (H3N2) viruses in 1985: antigenic analysis and sensitivity to non-specific inhibitors.

During the influenza outbreak of 1984-85 22 strains of H3N2 viruses were isolated in Finland. An intra-epidemic heterogeneity was demonstrated in an antigenic analysis by haemagglutination inhibition test with antisera produced in rats. The strains could be classified into three groups which corresponded to the following reference strains: group I: A/Hong Kong/1/84, A/Hong Kong/3/84; group II: A/Philippines/2/82; group III: A/Caen/1/84. Seven of the isolates were entirely insensitive to gamma-inhibitors of guinea-pig sera, which is in contrast to the small number of these viruses found among H3N2 strains isolated in the 1970s. The insensitive strains could not be isolated until the second or third passage through the eggs, whereas about half of the sensitive and intermediate strains were already isolated during the first passage. Conversions in reactivity with gamma-inhibitors could be detected only from an intermediate or an insensitive virus to a sensitive virus when several strains were passed serially in ovo and in MDCK cultures. The findings suggest that the gamma-inhibitor-insensitive strains corresponded well to the viruses of the human host or arose from dimorphic virus populations under an arbitrary selection of terminal dilution conditions prevailing during isolation in eggs. The insensitive strains did not differ substantially from the sensitive viruses in their ability to agglutinate erythrocytes of different laboratory animals or in their disagglutination patterns. On the other hand, propagation of viruses in MDCK cultures had an effect on these properties. The results are discussed with respect to Q phase variants and receptor binding properties.