Severe and critical COVID-19 in children: a simple single-center, cross-sectional study - Riga, Olena; Korsunov, Vladimyr; Penkov, Andriy; Michaylova, Olexandra; Urivaeva, Marina; Ischchenko, Tetyana; Georgiyants, Marine (Pediatria Polska - Polish Journal of Paediatrics, 2021)