Population immunity in residents of the Kaliningrad region to current antigenic variants of influenza viruses over 2018-2021

The article presents the results of population immunity assessment in residents of the Kaliningrad region to current antigenic variants of influenza viruses over the period of 2018 to 2021. To assess spectrum of influenza types circulating in Kaliningrad region, the real-time PCR method was used using AmpliSense reagent kits. The biological material for the study was obtained from oropharyngeal swabs from the persons who applied to polyclinics with inflammatory diseases of upper respiratory tract, or underwent treatment at the out- or inpatient basis in a healthcare facilities of Kaliningrad region. Humoral immunity was assessed by testing blood sera obtained from healthy residents of Kaliningrad region during periodic prophylactic examinations at the city and regional polyclinics. The study of residents covered all age groups. For the period of 2018-2021, more than 14,000 studies have been carried out. Determination of specific serum antibody titers was carried out by staging the hemagglutination suppression reaction using local influenza antigens (LLC «PPDP», St. Petersburg). Antibody titers of 1/40 and higher were considered sufficient to reduce the risk of disease by > 50%. During the study period, 2165 cases of influenza were confirmed in the region by laboratory tests. Over the study period, the A(H1N1) pdm serotype proved to be the major strain causing influenza, its proportion reached 57.5%. However, along with A(H1N1) pdm, a significant contribution was also made the A(H3N2) strain to the epidemic process, being the etiological cause of influenza infection in 2019 (42.2%), type B influenza strains being actual in 2020 (42.5%). Over the period of 2018-2021, 420 samples of blood sera from vaccinated and non-vaccinated individuals were tested for specific antibodies. The sera were taken 1-2 months after vaccination and between epidemic rises of influenza (April-May). In vaccinated individuals, the antibody titer was protective in 58.3-64.5%. In this group of persons, the titer sufficient to produce immunity was maintained over the spring/summer period. Among those persons who refused vaccination, a protective antibody titer in the pre-epidemic period was noted in 41.2% of the examined, and during the off-season it was detected in 37.4% of the volunteers. Thus, the A(H1N1) pdm strain was the main etiological factor of influenza in Kaliningrad Region in 2018-2021. The protective level of antibodies in vaccinated population over the period preceding the epidemic peak, was observed 30% more often than in unvaccinated individuals. High incidence of influenza B strains noted in 2020 appears to require a change of specific vaccine preparation.