[Supplementary data on the theory concerning the cyclic evolution of influenza viruses].

The distribution per age groups of antibodies to influenza viruses A/Hong Kong/1/68, A/England/837/69 and A/England/42/72 in the sera collected in 1970 demonstrates that antigenically similar strains also dominated the epidemiologic picture of influenza in the past century. The results suggest that the pandemic of 1893 and infection in persons born before 1885 were caused by a strain similar to A/Hong Kong/1/68 and in those born in 1895-1897 by the antigenic variant A/England/42/72. The anamnestic response to the A/England/69 variant, following upon vaccination with the Hong Kong strain shows that the primary infection in persons born before 1900 was caused by a strain antigenically similar to the first variant. The data obtained lend support to the hypothesis of antigenic recycling of influenza virus and of the finite character of the antigenic variation of influenza viruses.