Objective. To assess the prevalence of antibodies to SARS-CoV-2 in different age groups of the population of the Republic of Belarus and to determine the general trends in the population immunity in 2021.

Materials and methods. A three-stage study was performed (stage 1: February-March, stage 2: May-June, stage 3: September-October), which included a questionnaire and assessment of seroprevalence to SARS-CoV-2 among 1-70 and more years old population of all 7 regions of the Republic of Belarus (5243 people: 3769 from general population (769 children and 3000 adults) and 1474 healthcare workers). The presence of total (IgM and IgG) antibodies to RBD of SARS-CoV-2 was determined using the Wantai SARS-CoV-2 Total Ab kit (Beijing Wantai Biological Pharmacy Enterprise, Beijing, China).

Results. At the first stage of the study the seroprevalence of the general population was 61.4 % (95 % CI: 59.8—63.0), at the second — 72.1 % (95 % CI: 70.4—73.7), at the third — 79.6 % (95 % CI: 77.9—81.2). In all age groups seroprevalence increased statistically significantly from stage to stage (p<0.001). In each of the regions there was also a significant increase in seroprevalence

(p<0.001) throughout all stages. Seroprevalence among healthcare workers was about 10 % higher than among adults from general population at each stage of the study. The seroprevalence was formed both by persons who had a manifest form and an asymptomatic infection. Among 30 year olds and older people the manifest forms of COVID-19 were significantly more common than among younger ones (CMH-OR (95 % CI OR) 1.8 (1.5—2.0); p<0.001). The fraction of asymptomatically infected persons ranged from 39.0 to 69.4 % in different age groups and was the highest among children, that confirms their significant role in the pandemic. A high contribution of vaccination to seroprevalence in adults was shown. The proportion of vaccinated among seropositive people increased from stage to stage and at the third stage reached 63.3 % in healthcare workers and 48.1 % in other adult population.

Conclusion. The study improved the understanding of the epidemiology of COVID-19 in the Republic of Belarus.
A high level of seroprevalence already at the beginning of the study and its increase at subsequent stages, a significant fraction of asymptomatically infected individuals and a high contribution of vaccination to the population immunity were demonstrated.