Dimitrova, Teodora, et al. “VACCINE BREAKTHROUGH CASES AMONG HOSPITALISED PATIENTS IN THE INTENSIVE CARE UNIT FOR COVID-19 IN NORTH-EASTERN BULGARIA”. PROBLEMS of Infectious and Parasitic Diseases, vol. 52, no. 1, May 2024, pp. 29-33, https://doi.org/10.58395/hb45xe74.