The Second Round of a Population-Based Seroprevalence Study of Anti-SARS-CoV-2 Antibodies and COVID-19 Vaccination Assessment in the Republika Srpska, Bosnia and Herzegovina.

Introduction: The aim of the study was to assess the seroprevalence of SARS-CoV-2 in the Republika Srpska, Bosnia and Herzegovina, after five waves of COVID-19 and 1 year after introduction of vaccination to better understand the true extent of the COVID-19 pandemic in the population of the Republika Srpska and role of vaccination in achieving herd immunity.

Methods: The population-based study was conducted from December 2021 to February 2022 in a group of 4463 individuals in the Republika Srpska. Total anti-SARS-CoV-2 antibodies were determined in serum specimens using the Wantai total antibody ELISA assay. Quantitative analysis, using Kantaro IgG assays, was performed in subsamples (1273 specimens) to asses and compare levels of IgG among vaccinated, recovered and participants with hybrid immunity. To adjust for age and gender distribution in sample, poststratification method is applied.

Results: The overall cumulative seroprevalence was 94.6% (95% CI = 93.9-95.3). Significantly higher seroprevalence rates were observed among vaccinated 97.8% (95% CI = 97.3-98.4) comparing to unvaccinated participants (89.5%, 95% CI = 88.0-91.0). Seroprevalence increases with the number of received doses. Among various professions, the highest seroprevalence was found in the service industry (98.1%), education (98.0%) and healthcare (96.9%). We found that 2.2% of vaccinated and 3.6% of participants with SARS-CoV-2 positivity during 2021 had no detectable IgG antibodies. Both seroprevalence (98.6%) and antibody titres (1094.4 AU/mL) were significantly higher among people with hybrid immunity.

Conclusion: Our findings reveal a 2.3-fold increase in seroprevalence of SARS-CoV-2 antibodies due to infection and vaccination, comparing to the first study performed 1 year earlier. This study provides better understanding of the SARS-CoV-2 transmission and highlights the important role of the vaccination in achieving the population immunity. Periodically conducted population-based seroprevalence studies are important to understand temporal trends and assess surveillance system performance and public compliance with vaccination policies.