ABSTRACT
Young adults vaccinated in late spring of 1978 with one dose of vaccine containing either 7 microgram or 20 microgram of A/USSR (H1N1) hemagglutinin, followed by a dose of trivalent (A/USSR, A/Texas, B/Hong Kong) vaccine, were observed through an epidemic of A/Brazil influenza in the winter of 1978-1979, and of influenza B in 1979-1980. Influenza infection was diagnosed by virus isolation or serological titer rises between the spring seasons of 1978, 1979, and 1980. During the A/Brazil epidemic, rates of reported influenza-like illness and serologically confirmed H1N1 influenza infections were similar for all vaccine groups and two control groups. Naturally acquired antibody, but not vaccine-induced hemagglutination-inhibiting antibodies (HAI), appeared protective. During the influenza B epidemic, a lower rate of serologically confirmed infections was observed in the 1978 vaccine cohort than in one control group.