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 %.

Modernity raises the most important questions for researchers: what will happen to patients who have undergone COVID-19? What complications threaten them? What happens to the immune system after this disease? To what extent do the changes caused by the virus disrupt the formation of protective immunity in the future? The study found a causal relationship between SARS-CoV-2 infection, immune dysfunction, and the manifestation of chronic inflammatory diseases. Proven multitarget action of the new drug Mercurid, aimed at modulating the activity of several critical target proteins, such as CD3, CD4, CD8, CD25, CD38, CD54, CD95. On the one hand, this has reduced the severity of postpartum complications, and on the other - to increase the ability of the immune system to eliminate the virus from the body. The therapeutic efficacy of Mercurid was 75.1%. The second research finding is that patients with a history of chronic inflammatory disease with a history of SARS-CoV-2 are more likely to develop impaired specific protective antibodies. This conclusion follows from the pathologically low level of CD4, CD8, CD25 and overexpression of CD38, ICAM-1, CD95, which indicates apoptosis of immune cells, lymphopenia and the formation of the phenotype of depleted T cells with activation of inhibitory receptor expression. Thus, for this cohort of patients, vaccination may be ineffective due to the presence of a compromised immune system. Accordingly, corrective multi-target immunotherapy targeting multiple target proteins critical to the development of long-term effective post-viral immunity to SARS-CoV-2 is an extremely important therapeutic task. Immunoprophylaxis can be performed both before and after vaccination, in order to achieve the maximum protective effect against the vaccine.