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 one year after introduction of vaccination to better understand the true extent of the COVID-19 pandemic 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 4,463 individuals from the Republika Srpska. Total anti-SARS-CoV-2 antibodies were determined in serum specimens using the Wantai total antibody ELISA assay and Kantaro Quantitative IgG assays. Results: The overall cumulative seroprevalence was 94.6%. Of all participants, 61.3% were vaccinated against COVID-19. Significantly higher seroprevalence rates were observed among vaccinated participants. Among various professions, the highest seroprevalence was found in the service industry (98.1%), education (98.0%) and healthcare (96.9%). This study found that 2.2% of vaccinated participants, and 3.6% of individuals with SARS-CoV-2 positivity during 2021, had no detectable IgG antibodies. Both seroprevalence and antibody titres were significantly higher in 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 one 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 for assessment of surveillance system performance and public compliance with vaccination policies. Keywords: COVID-19, SARS-CoV-2 antibodies, seroprevalence, population-based study, Bosnia and Herzegovina, UNITY, SEROPREV.

Background Sarajevo Canton in the Federation of Bosnia and Herzegovina has recorded several waves of high SARS-CoV-2 transmission and has struggled to reach adequate vaccination coverage. We describe the evolution of infection- and vaccine-induced SARS-CoV-2 antibody response and persistence. Methods We conducted repeated cross-sectional analyses of blood donors aged 18-65 years in Sarajevo Canton in November-December 2020 and 2021. We analyzed serum samples for anti-nucleocapsid (anti-N) and anti-spike (anti-S) antibodies. To assess immune durability, we conducted longitudinal analyses of seropositive participants at 6 and 12 months. Results 1015 participants were included in Phase 1 (November-December 2020), and 1152 in Phase 2 (November-December 2021). Seroprevalence increased significantly from 19.2% (95% CI: 17.2-21.4%) in Phase 1 to 91.6% (95% CI: 89.8-93.1%) in Phase 2. Anti-S IgG titers were significantly higher among vaccinated (58.5%) than unvaccinated infected participants across vaccine products (p<0.001), though highest among those who received an mRNA vaccine. At 6 months, 78/82 (95.1%) participants maintained anti-spike seropositivity; at 12 months, 58/58 (100.0%) participants were seropositive and 33 (56.9%) had completed the primary vaccine series within 6 months. Among 11 unvaccinated participants who were not reinfected at 12 months, anti-S IgG declined from median 770.1 (IQR 615.0-1321.7) to 290.8 (IQR 175.7-400.3). Anti-N IgG antibodies waned earlier; from 35.4% seropositive at 6 months to 24.1% at 12 months. Conclusions SARS-CoV-2 seroprevalence increased significantly over 12 months from end of 2020 to end of 2021. Although individuals with previous infection may have residual protection, COVID-19 vaccination is vital to strengthening population immunity.