Numerous articles have been published on COVID-19 cases in the child population now. Children become infected and get sick with a new infection much less often and more easily than adults. The supposed relative resistance of children to SARS-CoV-2 can be explained by a number of reasons, one of which is the immaturity of the ACE2 receptor, or by the fact that children have certain features of innate immunity that disappear in ontogenesis. On the contrary, the maturity of the immune system may explain the unfavorable type of triggered immune response associated with the development of acute respiratory distress syndrome in adult patients. The article describes a case of a new coronavirus infection in a 5-year-old child with a severe secondary immunodeficiency condition associated with hematopoietic stem cell transplantation. By describing this clinical case, we wanted to draw attention to a mild course of COVID-19 infection in a child with a burdened premorbid history in the form of severe secondary immunodeficiency associated with hematopoietic stem cell transplantation in the treatment of acute promyelocytic leukemia.