Antigenic relationship between H1 and H2 of influenza A virus

Using cloned viruses of proven purity, and by the methods of hemagglutination inhibition, single radial hemolysis, strain-specific complement fixation and neutralization tests we have demonstrated the serological cross reaction between late H1N1 variant (Dutch/56) and H2N2 of influenza A virus with fowl and hamster antisera. Such a cross reaction is not detected with earlier H1N1 variants. Serological crossing covers variants of H2N2 virus isolated from 1957-1966 but in decreasing titers, and disappears with the last variant of H2N2 isolated late in 1967. Analysis with mono-specific antisera or antigens prepared with recombinants reveal that the hemagglutinins of late H1N1 and H2N2 are related, while their neuraminidases are distinct. We have discussed the bearing of such antigenic relationships to previous epidemiological observations on the partial protection of patients convalescent from late H1N1 disease against H2N2 and to the recombination theory for the origin of H2N2 virus.