There is a paradox when protective IgG antibodies to the SARS-CoV-2 virus are detected only in a part of patients who have contracted COVID-19. At the same time, some persons who were in contact with the infected, but did not get sick with COVID-19 (without disease symptoms, PCR negative), still produce IgG antibodies to the SARS-CoV-2 virus. The presence of IgG to the SARS-CoV-2 virus correlates with a changed immune response when infected with seasonal viral diseases - in particular, the ability to produce protective antibodies decreases. All this determines the different clinical course of the disease and the features of the patient’s post-COVID conditions (PCC). To study various immune responses of patients and asymptomatic carriers of COVID-19, 414 people were examined. The novelty of the study is that, for the first time, not only the levels of protective IgG antibody titers to the SARS-CoV-2 virus were determined, but also their correlation with the markers of lymphocyte activation CD25, CD54 (ICAM-1) and CD95 in patients who did not suffer from COVID- 19, got sick with COVID-19 and just had a heart attack. Research has scientific and practical goals, including the search for new therapeutic opportunities. Our attention was drawn to the domestic multitarget immunotherapy drug Mercurid (RP UA6098/01/01). Its ability to control T-lymphocyte activation molecules and modulate their activity was studied. The therapeutic efficiency of Mercurid was 75.6 %.