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.

Background: The effectiveness and safety of recently implemented COVID-19 vaccine platforms are often compared since the launch of the mass vaccination campaign worldwide. The aim of the present study is to assess the prevalence of adverse events (AEs) occurring after vaccination with the two viral vector-based vaccines authorized in the European Union to prevent COVID-19.  Materials and methods: A survey was carried out among adults who have completed vaccination with either of the viral vector-based vaccine approved for use in Bulgaria ChAdOx1-S nCoV-19 vaccine (commonly known as Astra Zeneca or Vaxzevria) or Ad26.COV2S vaccine (commonly known as Janssen or JCOVDEN). For the data analysis, standard descriptive statistics was used. Quantitative variables are presented by the mean and standard deviation (mean ± SD) or median (25th percentile; 75th percentile). Qualitative variables are presented as numbers absolute/relative frequencies totals and percentages. The Kolmogorov-Smirnov test was used to obtain information regarding the distribution of the sampled patients. The chi-square test for independence was used to determine whether differences between observed and theoretical distributions existed.  Results: In total 314 respondents took part in the study. Of them, 273 (86.9%) reported at least one local AE after the first dose of a vaccine, and the prevalence among the ChAdOx1-S vaccine group was significantly higher (88.5%) than in the Ad26.COV2.S vaccine group (59.2%) (Pearson χ2 test=19.942, p=0.000). The most common systemic AEs after administration of a viral vector-based vaccine were chills (77.3% for ChAdOx1-S and 25.9% for Ad26. COV2.S.) fatigue (76.3% for ChAdOx1-S and 25.9% for Ad26.COV2.S.), and headache (62.3% for ChAdOx1-S and 25.9% for Ad26.COV2.S.). Those adverse events appeared significantly more often in participants vaccinated with the ChAdOx1-S vaccine.  Conclusion: The analysis of the collected data proves that although common, AEs following vaccination with vector-based products are classified as mild and usually resolve within 48 hours of dose administration.