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 herd immunity in the Republic of Armenia (RA) by late 2022. Materials and Methods. A randomized study was conducted from 28 November to 2 December (2022) by the Saint Petersburg Pasteur Institute (Russia) in collaboration with the National Center for Disease Control and Prevention (Armenia). This 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 (1 April 2021 to 1 May 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. Conclusions. 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.