Lower COVID-19 incidence in Bulgaria in 2022 can be attributed to the Delta variant's belated intense spread.

Abstract Objectives The first COVID-19 wave caused case fatality rates (CFRs) of above 7% globally. In 2021 and 2022 the values of the same indicator went down to 2%. Differences between countries are still observed. The present study aimed to assess the CFRs dynamics in Bulgaria in relation with the specificity of the epidemic process in the country. Methods Descriptive analysis of cumulative incidence, mortality rates, and hospital admissions during the Delta and Omicron waves in Bulgaria and in the EU was performed. Differences in stringency index and testing capacities were analyzed to provide reasoning behind the findings. Indicator values were compared temporally and quantitatively. Results Data was reviewed for 27 EU countries. Opposing trends in Bulgaria and in the EU were reported. During the Delta wave incidence in Bulgaria was 25% higher than in the EU and increased by 67% during the Omicron period. In the EU, the increase during the Omicron wave was five-fold. Mortality in Bulgaria decreased by 38% between the two waves but it continued to be above the median mortality rate in the EU. Hospital admissions followed similar trends. Out of 16 countries only 5 experienced a drop of the number of new hospital patients. In Bulgaria admissions shrank by 45% during the Omicron predominance. Conclusions Higher incidence during the Delta wave resulted in accumulation of population with recently obtained natural immunity. Given evidence from countries with good vaccination coverage, it is reasonable to conclude that some of the mortality in Bulgaria could have been averted. Due to the observed positive association between acceptance of vaccines and other anti-epidemic measures, we recommend that future risk communication efforts highlight the importance of personal decision-making and social behaviour.