Evolution of herd SARS-CoV-2 humoral immunity in the Republic of Belarus

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.