Epidemiologic evaluation of vaccination efficacy during different periods of COVID-19 epidemic

The emergence of new genovariants of SARS-CoV-2 leads to the need to monitor the epidemic potential of the virus, COVID-19 morbidity and to assess the effectiveness of vaccination against this infection. Aim of the study was to assess the incidence of COVID-19 in unvaccinated and vaccinated population against this infection in different periods associated with the change in the leading SARS-CoV-2 genovariants. Material and methods . The data of registration of laboratory-confirmed cases of COVID-19 in St. Petersburg, Leningrad, Kaliningrad, Pskov regions and the Komi Republic in 2020–2024 were used in this work. To assess the effectiveness of vaccination, data on 2 443 331 cases in the population with different vaccine status in 2021–2024 were included in the study. Research methods: epidemiologic analysis, real-time PCR, statistical methods. Data on molecular genetic studies of 14 033 samples were obtained from GISAID and VGARus databases. Results. The incidence of COVID-19 among the fully vaccinated in different years in the regions was 5–9 times lower compared to the unvaccinated. During the period of circulation of different sublineages of the Omicron genovariant, a decrease in the efficacy of full vaccination was observed. In 2022, a decrease in the efficacy coefficient of 2.1–3.5 % and an increase in the odds of infection of 1.2–3.3 % were observed. In 2023, the efficacy coefficient decreased by 1.9–2.6 % and the odds of infection increased by 1.5–5.5 %. With an incomplete immunization scheme, there was a 28.3–36.6 % increase in the odds of infection, with 95 % lower odds of recurrence among those fully vaccinated. An increase in the probability of re-infection was found with an incomplete vaccination course. In 2024, an increase in the epidemiologic efficiency of vaccination was observed in the conditions of application of updated vaccines. Conclusions. The results of the study indicate the efficacy of a full course of vaccination against primary and recurrent COVID-19 incidence. During the emergence of genovariant Omicron and its sublines, a decrease in the effectiveness of the full course of immunization was established; violation of the vaccination scheme also led to a decrease in its effectiveness.