"The pandemic of 1957 provided the first opportunity to observe vaccination response in that large part of the population that had not previously been primed by novel HA and NA antigens not cross-reactive with earlier influenza A virus antigens. As summarized by Meiklejohn at an international conference on Asian influenza held 3 years after the 1957 onslaught of H2N2, more vaccine was required to initiate a primary antibody response than with the earlier H1 vaccines (almost always observed in heterovariant primed subjects). In 1958, 1959, and 1960 (as recurrent infections occurred), mean initial antibody levels in the population increased (i.e., subjects were primed) and response to vaccination was more readily demonstrated. Divided doses given at intervals of <4 weeks were more beneficial than a single injection. Less benefit was derived from this strategy as years passed. Intradermal administration of vaccine provided no special advantage over the conventional subcutaneous/intramuscular route, even when the same small dose was given. The Asian influenza experience provided the first opportunity to study how the postpandemic infection and disease into an endemic phase subsided. In studies conducted in separate and disparate populations, the populations compared were Navajo school children and New York City medical students. In both groups, subclinical infections occurred each year during the 3-year study period, and clinically manifested infections decreased in conjunction with an increasing level of H2N2-specific hemagglutination inhibition antibody."