Marinova, Lilly, et al. “OVERVIEW OF THE COVID-19 PANDEMIC AND ITS IMPACT ON THE MORBIDITY IN THE SOFIA CITY REGION DURING 2020-2021”. PROBLEMS of Infectious and Parasitic Diseases, vol. 50, no. 2, Feb. 2023, pp. 19-25, https://doi.org/10.58395/pipd.v50i2.91.