Immunological evaluation of monovalent influenza vaccine in adults and in children.

1With 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 Apparently no completely new antigen has appeared within the influenza A group of viruses, and the antigenic components of strains isolated over a period of years have not yet disappeared.4 Thus, while the sera of young Americans have been singularly devoid of antibodies against the Asian strain,1 "older" adults, especially those over 70 years of age, have occasionally been found to possess antibodies against this 1957 strain.5 (See especially Reference 6.) Successive experience with the diversity of antigenic components encountered among strains of influenza A virus even¬ tuates in a broadened antibody response; the humoral antibody spectrum engendered by these repeated antigenic experiences constitutes a "recapitulation," readily dem¬ onstrable by serologie means, of past infections produced by members of the different families of influenza A viruses, i. e., by variants of the Type A virus.7 It has been shown that prior infection with antigenic variants of influenza A virus re¬ sults in an antibody-orienting effect and that the subsequent use of a monovalent vaccine elicits predominantly a "booster" response directed against the antigenic com¬ ponents of the strains with which the person has had previous experience.7 Consequent¬ ly, monovalent vaccines prepared from re¬ cently isolated strains may call forth only a poor antibody response in young subjects