The COVID-19 pandemic that swept the world in 2019 radically changed not only the social sphere of life, subordinating everything around to its influence, but also the healthcare sector. It affected both its practical and scientific sides, globally changing the approach to medicine. Numerous studies around the globe are being conducted on the prevention and treatment of this infection. A significant amount of them study the role of vitamin D in the pathogenesis of the disease. Cholecalciferol (Vitamin D) is a fat-soluble vitamin that is involved in maintaining the serum calcium-phosphorus gradient. Vitamin D supplementation significantly reduces the risk of osteoporosis, and also affects the regulation of immune responses by modulating innate and adaptive immune responses. It has been shown to inhibit the proliferation of T cells, namely the subsequent transformation of Th1 cells to Th2 cells. This effect on cell proliferation and differentiation contributes to a significant decrease in the level of proinflammatory cytokines, thereby reducing the severity of immune-mediated damage. It was these mechanisms that formed the basis of the theory about the ability of vitamin D to prevent the severe course of coronavirus infection and even prevent it. At the moment, the effect of vitamin D on reducing COVID-19 morbidity is not fully confirmed.

Patients and Methods: Baseline characteristics will be recorded for all patients including medical history, height, weight, body mass index (BMI), antibodies to SARS-CoV-2 (IgG) and 25(OH)D. Serum 25(OH)D level, IgG to SARS-CoV-2 will be repeated at 12-14 weeks. Also, we will check information about clinical manifestation of COVID -19 and CT results in cases positive Ab. The research format is a prospective controlled single-center study. The study was carried out based on the Federal State Budgetary Institution, V. A. Almazov Federal North-West Medical Research Centre, of the Ministry of Health of the Russian Federation, October 30, 2020, to August 31, 2021. During the study, medical care workers who are working in an infectious hospital and who taking vitamin D in a controlled manner, having previously signed informed consent, will be observed. The study will be involved minimum 120 medical staff, including women and men. All subjects will be divided into three groups according to the criterion of having a higher, secondary, or no medical education. All participants in the study will be assessed twice for the level of 25-hydroxyvitamin D \[25(OH)D\], IgG to SARS-CoV-2 in their blood: before the start of therapy with cholecalciferol and after 3 months of Vitamin D administration. After the screening, the subjects will be randomized into 2 groups. In the Group 1, vitamin D therapy will be initiated at a dosage of 50,000 IU on the first and seventh week, followed by a switch to a daily intake of 5,000 IU for 12 weeks. In the Group 2, vitamin D therapy will be prescribed for 12 weeks at a dosage of 2,000 IU. After 3 months of taking vitamin D supplementation, all participants will undergo to repeat testing of the 25(OH)D level in the blood with an assessment of the effectiveness of the therapy. IgG titer and PCR results will be compared in two groups of participations taking different doses of vitamin D supplementation.