Ngoc, Kim, Iva Trifonova, Teodora Gladnishka, Elitsa Panayotova, Evgenia Taseva, Vladislava Ivanova, Kutevа Eleonora, Iva Vladimirova, and Iva Christova. “EVALUATION OF POPULATION-LEVEL IMMUNITY TO SARS-COV-2 ACROSS BULGARIA (END OF 2023)”. PROBLEMS of Infectious and Parasitic Diseases 52, no. 3 (January 16, 2025): 13–18. Accessed April 16, 2025. https://pipd.ncipd.org/index.php/pipd/article/view/52-3-2-evaluation-of-population-level-immunity.