Non-specific inclusion criteria  Participants aged 18 and above Participants agreeing to follow the study procedures Participants able to understand the purpose, nature, and methodology of the study Participants having signed the informed consent  Healthy donors inclusion criteria  No self-reported COVID-19 infection No vaccination against COVID-19 No immunoglobulin (Ig) IgM or IgG antibodies against SARS-CoV-2 by three tests (IgM to SARS-CoV-2 detection: "SARS-CoV-2-IgМ-ELISA-BEST", Ref #D-5502, IgG to SARS-CoV-2 S protein detection: "SARS-CoV-2-IgG-quantity-ELISA-BEST", "Vector-Best", Ref #D-5505), IgG to SARS-CoV-2 N protein detection: "SARS-CoV-2 IgG Reagent Kit", ARCHITECT, Abbott Laboratories, EU)  Convalescents inclusion criteria  Self-reported COVID-19 infection PCR confirmed COVID-19 in a period 14-45 days before recruitment Detectable IgG antibodies against SARS-CoV-2 (IgG to RBD domain of SARS-CoV-2 S protein detection: "SARS-CoV-2-IgG-ELISA", "National Medical Research Center for Hematology", Moscow, Russia. Ref #K153G)  Vaccinated inclusion criteria  SARS-CoV-2 vaccination with 'Sputnik V' (Gam-COVID-Vac) in a period 7-14 days after the second vaccination before recruitment Detectable IgG antibodies against SARS-CoV-2 (IgG to receptor-binding domain (RBD) domain of SARS-CoV-2 S protein: "SARS-CoV-2-IgG-ELISA", "National Medical Research Center for Hematology", Moscow, Russia. Ref #K153G)  Exclusion criteria  - Age under 18 y.o.