The characteristic ability of influenza viruses to undergo variation in antigenic structure has given rise to a new family of influenza A strains, now officially referred to as the "Asian family." 1 With the first recognized appearance and isolation of this new virus, early in 1957, and the decision to develop vaccines for active immunization against this new antigenic variant,2 emphasis was placed anew on problems which required a review of our previous knowledge of the antigenic behavior of Type A influenza viruses as well as of the immunologic response in man when exposed to new variants of the Type A group of viruses.
Early studies on antigenic variants of influenza A viruses showed that protection induced by active immunization was greatest against the homologous virus strain and that the efficacy of immunization against heterologous strains depended upon antigenic relationships between the immunizing and the infecting virus strains.3
BRUYN HB, LENNETTE EH. Immunological Evaluation of Monovalent Influenza Vaccine in Adults and in Children. AMA Arch Intern Med. 1959;103(6):914-923. doi:10.1001/archinte.1959.00270060066009