Evolution of seroprevalence to SARS‐CoV‐2 in blood donors in Sarajevo Canton, Federation of Bosnia and Herzegovina: Cross‐sectional and longitudinal studies

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: One thousand fifteen 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 re‐infected 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.