Serologic responses to nonprevalent influenza A viruses during intercyclic periods.

Sera collected at different times in the course of the Tecumseh Study of Respiratory Illness have been tested for the presence of antibodies to a variety of nonprevalent human and animal type A influenza viruses. In sera collected during the 1966-1968 period of H2N2 prevalence, rises in antibody titer were detected against the H3 antigen. The frequency of these rises in titer increased gradually and significantly as the time of emergence of the H3N2 virus approached. Occasional rises in Hsw1 antibody titer and presence of such antibody in sera obtained from young children were also detected in specimens collected at that time. In pre-vaccination 1976 serum collection, the prevalence of antibody against Hsw1N2 was significantly higher in certain age groups than that found in 1966-1968. During the 1976-1977 period, evidence for increase in H1N1 antibody frequency was found in nonvaccinated children, again before the emergence of the H1N1 strain. Possible interpretations of the results are discussed.