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Materials and methods. The article presents the results of laboratory tests for the presence of IgG antibodies to the SARS-CoV-2 coronavirus in 2112 patients living in 5 regions of the Russian Federation. In the main group of the study, 50 people were under dynamic observation for 6 months. Depending on the severity, the main group was divided into two subgroups: group I1 (n=25), represented by patients with moderate coronavirus infection, and group I2 (n=25), consisting of patients with a severe course of coronavirus infection. Blood sampling was carried out on the 14th day of illness (from the moment the first symptoms appeared) in the conditions of the procedure room of the Infectious Diseases Clinical Hospital №1, subsequent blood sampling (44 and 74 days, 3–6 months). Results. During a three-month observation of patients in the main group, there were no significant changes in the content of IgG antibodies to the SARS-CoV-2 coronavirus, and all fluctuations were characterized only by a trend. By the 4th month of observation, the decrease in antibody titer relative to the maximum level (as a rule, 14–44 days) was 43.3% in subgroup I1, and 37.6% in subgroup I2. By the 5th month, the level of IgG antibodies to the SARS-CoV-2 coronavirus in sub-group I1 was 40.9% relative to the maximum value (on day 44), and in subgroup I2 – 35.0% relative to the maximum value (on day 14). After 6 months from the start of observation, the content of IgG antibodies to the SARS-CoV-2 coronavirus in both groups did not differ significantly from the control (p>0,05). Conclusion. We found that there were no significant changes in the level of IgG antibodies to the SARS-CoV-2 coronavirus from the 14th day of illness to the 3rd month of observation from the moment of the first symptoms of the disease. Starting from the 4th month of observation, the level of antibodies decreased in both groups by multiples in relation to the maximum values, decreasing by the 6th month of observation to the values of the control group.

An excessive inflammatory response to SARS-CoV-2 is thought to be a major cause of disease severity and mortality in patients with COVID-19. Longitudinal analysis of cytokine release can expand our understanding of the initial stages of disease development and help to identify early markers serving as predictors of disease severity. In this study, we performed a comprehensive analysis of 46 cytokines (including chemokines and growth factors) in the peripheral blood of a large cohort of COVID-19 patients (n=444). The patients were classified into five severity groups. Longitudinal analysis of all patients revealed two groups of cytokines, characterizing the “early” and “late” stages of the disease course and the switch between type 1 and type 2 immunity. We found significantly increased levels of cytokines associated with different severities of COVID-19, and levels of some cytokines were significantly higher during the first three days from symptom onset (DfSO) in patients who eventually required intensive care unit (ICU) therapy. Additionally, we identified nine cytokines, TNF-α, IL-10, MIG, IL-6, IP-10, M-CSF, G-CSF, GM-CSF, and IFN-α2, that can be used as good predictors of ICU requirement at 4-6 DfSO.