Studies of influenza virus type A and B strains isolated during the epidemics of 1990 and 1991 in Novgorod from sick children previously vaccinated with influenza vaccines demonstrated no influence of postvaccination immunity on the antigenic properties of the isolates from patients with different vaccination history. Under conditions of the concurrent circulation of viruses with previously known and new antigenic structures the difference in the immune response consisted in its greater stimulation of this response to new influenza virus strains.