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Background. The course of the COVID-19 epidemic process depends on population immunity which prevents pathogen spread. Aim: to study an evolution of SARS-CoV-2 humoral immunity in the Belarusian population rela- tive to COVID-19 pandemic dynamics. Materials and methods. The work was carried out according to a methodology for assessing herd immunity developed by Rospotrebnadzor (Russia) and the Belarusian Ministry of Health involving the St. Petersburg Pasteur Institute (SPPI) by taking into account the WHO recommendations. The study was approved by the Bioethics Committee of Belarus and the SPPI Bioethics Committee. Participant selection was carried out by ques- tionnaire using a cloud (internet server) service. To monitor herd immunity, a cohort of 4661 subjects (involved at all stages of seromonitoring) was formed from the total volunteer group. Study subjects were randomized into groups based on age (117, 1829, 3039, 4049, 5059, 6069, 70+ years), geographic region, and occupation. For the detection of antibodies (Abs) against SARS-CoV-2 nucleocapsid (Nc) and S glycoprotein receptor-binding domain (RBD), rel- evant assay systems were used according to the manufacturers instructions. A four-stage study was conducted according to a unified scheme. Results. At stage 1 (pandemic month 15), herd immunity was mainly accounted for by Nc+RBD+ Ab status alone. By stage 2 (4 months later), its specific proportion decreased by 1.2-fold, whereas percentage of subjects solely bearing RBD-specific Abs increased by 1.7-fold. At stages 3 and 4 (9 and 19 months after the onset) vs. stage 2, percent- age of subjects with RBD+Nc decreased by 3.5%; the proportion of persons with Nc+RBD Abs increased by 1.5-fold. The most important contributor in herd immunity turned out to be due to population vaccination, with coverage reaching 70% by stage 4. Among vaccines, compared with whole-virion, inactivated BIBP-CorV vaccine the Sputnik V and Sputnik Light vector were used most often. Conclusion. The evolution of herd SARS-CoV-2 humoral immunity included a series of changes in circulating Ab levels (Nc, RBD). The hybrid immunity formed helped to reduce the incidence of COVID-19 to sporadic level.

Introduction. In late 2019 - early 2020, an outbreak of infection caused by a novel strain of beta coronavirus SARS-CoV-2 was reported. The World Health Organization defined the disease as coronavirus disease 2019 (COVID-19). In the Tyumen Region, the first case of COVID-19 was diagnosed on 31/1/2020. The source of infection was a female student who came from Jinan, Shandong province (China). The number and rate of cases were steadily increasing from the 16th week through 28th week in 2020. The highest rate was 36.87 cases per 100 thousand people. Afterwards, the cumulative incidence kept increasing gradually, but not as quickly.The purpose of the seroepidemiological study was to measure the level and to identify the structure of herd immunity against the SARS-CoV-2 virus among the population of the Tyumen Region during the rapid spread of the COVID-19 outbreak.Materials and methods. Volunteers for participation in the study were selected through questionnaire surveys and random sampling. The exclusion criterion was an active COVID-19 infection at the time of the survey. A total of 2,758 individuals were tested for SARS-CoV-2 specific antibodies. The age of the surveyed volunteers ranged from 1 year to 70 years and older.Results of the study. During the active phase of the COVID-19 incidence, the population of the Tyumen Region showed moderate (24.5%) seroprevalence of SARS-CoV-2. At the same time, the tests revealed a high (97.8%) rate of asymptomatic infection cases in seropositive individuals who had never been diagnosed with COVID-19 and did not have history of positive PCR test results or acute respiratory infection symptoms on the day of testing. The maximum level of herd immunity was identified in children aged 1-6 years (34.7%), which was significantly higher compared to the average level of seroprevalence in the entire cohort. In recovered COVID-19 patients, antibodies were detected in 68.2%. In individuals with positive PCR test results, antibodies were detected in 64%. Conclusion. The results of the assessment of the level of herd immunity against the SARS-CoV-2 virus are crucial for prediction of the development trend of the epidemic and for planning specific and non-specific COVID-19 prevention measures.