According to the WHO data, the number of infected people exceeded 765.2 million people during the COVID-19 pandemic,. The severity of patient's condition is determined by immune system hyperactivation. Activation of T- and B-lymphocyte subsets plays a prominent role in the control of infectious process. A content of small circular DNA molecules - T-cell receptor excision circles (TREC - T-cell receptor excision circles) and B-cell (kappa) excision rings (KREC - Kappa-deleting recombination excision circles) - in the peripheral blood can be used as a marker of the functionally active T and B cells maturation. The purpose of this work is to quantify peripheral blood TREC and KREC level in patients with the new coronavirus infection COVID-19 of varying severity. Materials and methods. The material consisted of 1028 blood samples from patients with a confirmed diagnosis of COVID-19, as well as 717 blood samples from apparently healthy volunteers. The content of TREC and KREC DNA fragments in the total DNA fraction was assessed by quantitative Real-time PCR using the TREC/KREC-AMP PS test system (Saint-Petersburg Pasteur Institute, Russia). Blood cell phenotyping was carried out using flow cytometry. Results. TREC/KREC levels were significantly reduced in COVID-19 patients (p0.0001 at 95% CI). A significant direct correlation was established between the levels of peripheral blood TREC molecules and level of CD45+CD3+CD19- T cells (r = 0.59, p 0.0001), as well as between KREC level and count of CD45+CD3-CD19+ B cells (r = 0.66, p 0.0001). The level of TREC molecules in patients with severe vs. moderate infection was significantly reduced in patients aged 30-39 years old (p = 0.0404) and 40-49 years old (p = 0.0356). The negative correlation between severity of COVID-19 clinical manifestations and TREC level in the blood of patients in 30-49 year age group indicates about an opportunity of using this analyte as a diagnostic and prognostic laboratory marker of patients condition. A simple PCR analysis algorithm makes it relevant to use the described method for assessing a state of immunity in coronavirus patients in the context of systemic negative impact of the SARS-CoV-2 virus on human organism.