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Aim. Evaluation of herd immunity prior to the 2017-2018 influenza season, and characterization of influenza viruses isolated from severe or fatal influenza cases and from influenza cases in people vaccinated in the fall of 2017. Materials and methods. Evaluation of herd immunity in hemagglutination inhibition assay. Isolation of influenza viruses. Antigenic and genetic analysis. Results. Prior to epidemic season 33-47% of blood sera samples collected on the territory of Russia showed presence of protective antibody titers against vaccine strains of influenza A, 24-30% of samples — against B/Victoria. During 2017-2018 epidemic season 87 influenza A and B viruses were isolated. A(H1N1)pdm09 strains belonged to clade 6B.1, B/Yamagata strains to clade 3, and B/Victoria strains to clade 1A; they were antigenically similar to corresponding vaccine strains. A(H3N2) viruses belonged to clade 3C.2a and were difficult to characterize antigenically. One strain of influenza virus А(H1N1pdm09) was resistant to oseltamivir and had H275Y amino acid substitution in neuraminidase. All other isolates were susceptible to neuraminidase inhibitors. Conclusion. Influenza vaccination with vaccine effective against current circulating strains and treatment with neuraminidase inhibitor drugs at first manifestation of clinical signs of influenza disease are effective means of population protection against influenza.

Aim. Evaluation of seroprevalence of antibodies to influenza A and В viruses and analysis of specimens from severe or fatal influenza cases in Russia in 2015 - 2016 and 2016 - 2017 flu seasons. Materials and methods. Determination of antibody titer in human serum samples in hemagglutination inhibition assay with reference antigens. Isolation of influenza viruses from nasopharyngeal swabs and autopsy material in cell culture. Characterization ofisolated strains. Results. In 2016, compared to 2015, the proportion of serum samples, containing antibodies to influenza viruses A(H 1N1 pdm09) and A(H3N2), increased. During the 2015-2016 season, elevated number of severe and fatal cases of influenza were registered. The majority of circulated strains belonged to the new clade 6B.1 of A(HlNippdm09 viruses. 1% of analyzed isolates carried H275Y amino acid substitution in neuraminidase and were resistant to oseltamivir. In the 2016 - 2017 season, there were less severe cases of influenza. The most prevalent were influenza viruses A(H3N2) and B/Victoria. Isolated H3N2 viruses belonged to the 3C.2a subclade and B/Victoria isolates were from the 1A genetic group. All tested strains were susceptible to neuraminidase inhibitors. Conclusions. Flu seasons 2015 - 2016 and 2016 - 2017 differed in intensity of influenza activity and in the dominant influenza A virus subtype. Immunization with vaccine, comprising new HlNlpdm09-component, is crucial for prophylaxis of influenza infection with viruses from 6B. 1 subclade in the next season. Neuraminidase inhibitors are recommended for influenza treatment.

Aim. Control for the population herd immunity against seasonal influenza viruses as well as for emergence of antibodies against influenza with pandemic potential in human blood sera. Materials and methods. HAI reaction against vaccine and epidemic influenza viruses as well as HPAI viruses A/rook/Chany/32/2015 (H5N1) (clade 2.3.2.1c.) andA/Anhui/01/2013 (H7N9). Results. Among all the sera samples collected in the autumn of 2014 and 2015, none had reacted in HAI against A(H5N1) and A(H7N9) antigens even at 1:10 dilution. Among samples collected in autumn 2014, 41% were positive to A/Califorrna/07/09(HlNlpdm09) virus, 36% - A/Texas/50/2012 (H3N2), 40% - B/Brisbane/60/2008 (Vict.lin.) and 47% reacted in HAI against the B/Massachusetts/2/2012 (Yam.lin.) strain. 22% of all the samples had a titer of at least 40 against all the antigens and only 10% in HAI had a titer of 40 or more against all the vaccine strains. Among the samples collected in autumn 2015, the number of seropositive against A/Califorrna/07/09(HlNlpdm09) varied from 31% in the Urals FD to 46% in the Southern FD. The amount of seropositive against A/Switzerland/9715293/13 (H3N2) strain was at the level of 4 - 13% in all the FDs except Urals, where this parameter was slightly above 30%. The amount of seropositive against vaccine influenza В viruses varied from 23 to 76%. Only 2% of sera had titers in HAI of 40 or above against all the vaccine strains, 29% of all the samples were seronegative. Conclusion. Population immunity in Russia against influenza A(H3N2) is at a very low level, thus socially significant consequences of influenza epidemics in many aspects will depend on the vaccination campaign of autumn 2016.

Assessment of influence of annual seasonal vaccination on flu incidences of the Russian Federation population is carried out. The strong inverse correlation between the preventive flu vaccination coverage and the incidence of this infection is established. It is shown that the season vaccination in 2013 - 2014 result that about 40% of the samples of blood serum collected in 25 regions of Russia were positive to the vaccine influenza strains. It is necessary to vaccinate annually not less than 50% of total population, with not less than 90% immunization coverage of risk groups for ensuring necessary level of population immunity to stop a flu epidemic and to promote decrease in incidence of this infection in the Russian Federation