The antibody pattern of equine-2/63 viruses has been more sharply defined using a large number of human sera collected in 1964. The birth dates of persons exhibiting the richest experience with equine-2/63-like viruses delineate a period of past prevalence in man of equine-2/63-like viruses. The period is believed to have begun in the mid-1870's and to have terminated in 1889–1890 at the time of the first Asian pandemic. The equine-2/63 antibodies found in human sera react specifically in the photometric test of Drescher. The equine-2/63 antibody pattern advances along the age scale in exact concordance with the passage of time. The homologous antibody response of the older subjects to equine-2/63 vaccine is more vigorous, reflecting the conditioning effects of prior exposures to equine-2/63 antigens. A "one-way cross" between equine-2/63 virus and A2 and A1 strains has been demonstrated. The antigenic ties between strains of influenza A isolated from humans, swine, horses, and birds is recognized and discussed. It is apparent that horses do not constitute an active reservoir for strains of human involvement. The epidemiologic significance of the antigenic linkages between strains isolated from different species remains obscure.

This content is only available as a PDF.