“…However, in terms of cross-reactivity of the HI antibody acquired after repeated immunizations to the various antigenic drift strains, a difference was found between persons who had been naturally infected and persons who had only been exposed to HINI virus in the form of vaccine. Recent studies reported that following immunization with either inactivated whole or split vaccines, most adults responded mainly by producing a cross-reactive or common antibody among Hong Kong viruses (Webster et al 1976;Schild et al 1977;Oxford et al 1979), probably indicating original antigenic sin after vaccination with influenza viruses (Hennessy, Davenport & Francis, 1955; influenza (HJNJ) immunization in Japan Davenport & Hennessy, 1956, 1957. In the present studies, the poorer crossreactivity to some drift strains, including Hswl antigen, among the people who had not been infected may be explained by the lack of immunological memory to a cross-reactive antigenic determinant which the natural infection may induce but the artificial immunization may not.…”