[Neutralizing and inhibiting activity of hemagglutination of serum antibodies after natural infection and after vaccination with influenza virus A/USSR/90/77 (H1N1) strain].

Sera with different content of hemagglutination-inhibiting antibody (h.i.a.) collected from healthy subjects in late summer 1978, from influenza cases which occurred in winters 1977-78 and 1978-79 and from vaccinees in autumn 1978 were checked for neutralizing antibodies (n.a.) to A/USSR/90/77 influenza virus strain. Neutralizing activity (Geometric mean titer of n.a.) of h.i.a. was found dependent chiefly on the previous antigenic experience of the subjects. This because, given the same h.i.a. level, geometric mean titers of n.a. were lower in children and adolescents than in adults more than 24-year-old. For the age of the subjects and for time of collection of sera, the presence of antibodies in the former group is very likely due to the first antigenic stimulus. Moreover among the vaccinated adults the Geometric mean titers of n.a. in sera with the same h.i.a. content were higher in the age group 25-40 years. With any probability this is the cohort with an original antigenic sin due to H1N1 variants. The results have been also discussed in relation to the epidemiological behaviour of H1N1 strains in the first years of the second prevalence era. The above observations suggest caution in adopting the only hemagglutination-inhibition test for evaluating the immune status of populations and the immunizing efficacy of influenza virus vaccines.