Herd Immunity to SARS‐CoV‐2 Among the Armenian Population in the Second Half of 2022

Aim.This study aimed to assess the SARS-CoV-2 collective immunity in the Republic of Armenia (RA) by late 2022. Materials and methods. A randomized study was conducted from Nov. 28 to Dec. 2 (2022) by the Saint Petersburg Pasteur Institute (Russia) in collaboration with the National Center for Disease Control and Prevention (Armenia). The study was approved by the ethics committees at both organizations. A volunteer cohort (N=2974) was formed and grouped by participant age, region, or activity. Antibodies (Abs) to viral nucleocapsid antigen (Nc) and receptor-binding domain (RBD) in plasma were determined by ELISA. The statistical significance of differences was calculated using a p< 0.05 threshold, unless noted. Results. At the end of 2022, estimated SARS-CoV-2 seroprevalence (Nc and/or RBD Abs) among the Armenian population was 99% (95%CI: 98.5-99.3). It was evenly distributed throughout the cohort without any significant differences by age, region, or activity. Volunteers with low (32-124 BAU/ml) or medium (125-332 BAU/ml) anti-Nc Ab levels prevailed: 32.4% (95%CI: 30.7-34.1) and 25.5% (95% CI: 24.0-27.1), respectively. Regarding anti-RBD Abs, maximum levels (>450 BAU/ml) were detected in 40% of children. The share of individuals with high anti-RBD Abs levels increased with age, reaching 65% among those aged 70+ years. The important contribution to the formation of herd immunity to coronavirus infection was made by vaccination in the preceding period (01.04.2021 to 01.05.2022). The contribution from individuals with post-vaccination immunity was estimated to be above 80%. Hybrid immunity, formed after vaccination of those who had earlier experienced COVID-19, was characterized by greater effectiveness than post-vaccination immunity alone. Conclusion. Within the context of mass prophylactic vaccination, effective herd immunity to SARS-CoV-2 was formed, which helped to stop epidemic spread in the Republic.