Surveillance of the antibody status to current influenza strains in the Norwegian population. 2: Antibodies to the A/USSR/90/77 (H1N1) virus in various age groups after the second epidemic wave.

Over 950 randomly selected sera from various regional bacteriological laboratories, collected in October 1979, were assayed for the presence of haemagglutination-inhibition antibodies to current influenza strains. It was shown that in contrast to the previous year (1978), most age groups had significant levels of antibodies to the A/USSR influenza variant. In October 1978 only children, adolescents, and adults born between 1931 and 1950 had such antibodies. As there was little laboratory evidence of A/USSR infections in other age groups than among children and adolescents, we conclude that the majority of these infections must have been inapparent or generally mild.