[Laboratory studies of an epidemic outbreak caused by influenza A1 viruses].

Virological and serological studies of an epidemic influenza outbreak in December, 1977, carried out in the clinical department of D. I. Ivanovsky Institute of Virology of the USSR AMS established the disease to have been caused by influenza Al viruses. The marked incidence of the disease among younger age groups was due to a low humoral immunity level to the strains which had circulated before the young people's birth. The reduced effectiveness of protection by antibody against viruses which had caused influenza disease 20--30 years ago was conducive to the spread of influenza Al cases among middle-aged and senile population.