The magnitude of SARS-CoV-2 infection, the dynamic changes of immune parameters in patients with the novel coronavirus disease (COVID-19) and their correlation with the disease severity remain unclear. The clinical and laboratory results from 154 confirmed COVID-19 patients were collected. The SARS-CoV-2 RNA levels in patients were estimated using the Ct values of specific RT-PCR tests. The lymphocyte subsets and cytokines profiles in the peripheral blood were analyzed by flow cytometry and specific immunoassays. 154 confirmed COVID-19 patients were clinically examined up to 4 weeks after admission. The initial SARS-CoV-2 RNA Ct values at admission varied but were comparable in the patient groups classified according to the age, gender, underlying diseases, and disease severity. Three days after admission significant higher Ct values were found in severe cases. Significantly reduced counts of T cells and T cell subsets were found in patients with old age and underlying diseases at admission and were characteristic for the development of severe COVID-19. Severe COVID-19 developed preferentially in patients with underlying compromised immunity and was not associated with initial virus levels. Higher SARS-CoV-2 RNA levels in severe cases were apparently a result of impaired immune control associated with dysregulation of inflammation.