Influenza, which are yearly circulating in the cities registered from the last decade of 19^th century. The same time it is an emerging threat, because influenza A subtypes periodically are changing their genetic and antigenic structures by reassortment of the RNA segments called “shift” variations.

Especially peculiar epidemic behavior have been observed in the case of influenza A/H1N1 infections during the last 40 years. Variants circulating in the world during 1947–57 disappeared due to the emergence of subtype influenza A/H2N2. The latter has been also eliminated by the emerging next pandemic strain influenza A/H3N2 in 1968–69.

Influenza A/H1N1 reemerged, however, in 1977 causing pandemic among people mostly younger then 30 years of age. Elder cohorts had been protected probably by residual immunity since it happened that the antigenic structure of A/USSR/90/77-like A/H1N1 viruses possessed nearly identical antigenic properties than those circulating in 1950^th.

There were four epidemics of influenza A/H1N1 in Russia after 1977–78 (in seasons 1981–82, 1984, 1986–87 and 1989). Influenza A/H1N1 viruses continued to undergo antigenic “drift” in comparison to the reference strains from A/USSR/90/77-like strains to A/Singapure/6/86-like variants. The spread of influenza A/H1N1 viruses did not prevent the circulation of A/H3N2-caused influenza A epidemics. The epidemics caused by influenza A/H1N1 variants alternated with epidemics caused by influenza A/H3N2.

The A/H1N1) viruses disappeared from open epidemic circulation for 5 years from summer 1989 till summer 1995. The A/H1N1 viruses caused mostly local outbreaks among schoolchildren and youth during the epidemic season 1995–96. The isolated influenza A/H1N1/1995 strains were A/Texas/36/91-like. Their hemagglutinins (HA1) were different in 8–10 amino acid positions from those of strains isolated between 1986–1989. The antigenic “drift” during 1990–1995 moved into a different direction than those isolated from 1950 to 1956.

After 1996 no open epidemics were caused by influenza A/H1N1 again for 5 years. The influenza A/H1N1 reemerged in the epidemic season in 2000–2001. Antigenic modifications of these variations are discussed.