EVALUATION OF POPULATION-LEVEL IMMUNITY TO SARS-COV-2 ACROSS BULGARIA (END OF 2023)

Background: The COVID-19 pandemic in Bulgaria was characterized by a high mortality rate and vaccination efforts yielded suboptimal results. Understanding population immunity is important as new SARS-CoV-2 variants continue to emerge. This study aimed to assess the seroprevalence of SARS-CoV-2 antibodies in the general population of Bulgaria and examine demographic variations in antibody presence. Materials and methods: In December 2023, 1895 serum samples were randomly collected from healthy individuals across all 28 provinces. Samples were tested for SARS-CoV-2 spike protein-specific IgG antibodies using ELISA method. A subset of the positive samples was subsequently tested for neutralizing antibodies. Seroprevalence was analyzed by sex, age group, and geographic region. Statistical analyses were conducted using SPSS v.26. Results: Overall seroprevalence was 91.9%, with similar rates between males (91.6%) and females (92.0%). Seroprevalence was highest in the 18-39 age group (95.1%) and lowest in those over 65 (89.3%). Regional seroprevalence ranged from 80.0% to 98.3%. Among samples tested for presence of neutralizing antibodies, 90.5% were positive, indicating effective immune response. Conclusions: The high seroprevalence suggests widespread prior exposure to SARS-CoV-2 and/or vaccination among the Bulgarian population. The strong presence of neutralizing antibodies might provide potential protection against severe disease. Targeted interventions towards older age groups could be appropriate in order to sustain immunity as COVID-19 remains a public health concern.