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Background: Severe cases of coronavirus disease 2019 (COVID-19) often experience hyper-inflammatory reactions, acute respiratory distress syndrome (ARDS), blood clotting, and organ damage. The most prominent immunopathology of advanced COVID-19 is cytokine release syndrome, or "cytokine storm" which is attributed to a defect of immune-regulating mechanisms. This study aimed to evaluate the role of regulatory T cells (Tregs) as one of the main cells that maintain immune homeostasis. Methods: A systematic search was performed on PubMed, Scopus and Google Scholar. All English articles related to Treg’s role in COVID-19 were extracted and evaluated by two researchers independently. Study eligibility was assessed based on modified Evidence-based librarianship (EBL) checklist. Results: Nineteen eligible studies comparing Treg cells in COVID-19 patients with the control group or comparing alterations of this cell in severe and moderate patients were evaluated. Currently, there is no consensus regarding the increase or decrease of Tregs in COVID-19 patients compared to the control group. However, it was observed that Tregs in severe COVID-19 patients were significantly lower than moderate patients, resulting in uncontrolled inflammation and cytokine storm. Conclusion: Regulatory T cells can be one of the determinants of disease severity and prognosis in patients with COVID-19 by inhibiting rampant inflammation and preventing cytokine storms.

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The global crisis caused by the SARS Corona virus-2 infection is continuing through 2021, with more than 3.5 million deaths. Several risk factors for this virus’s severity and death were documented, including diabetes, hypertension, and ischemic heart disease. To evaluate the relation between serum vitamin D3 level, the disease severity, and prognosis of the patients with SARS Corona virus-2 infection. Patients with COVID-19 were evaluated for serum vitamin D levels and laboratory data. Correlation between vitamin D levels and laboratory data with disease severity and prognosis was assessed. Cox and logistic regression tests, as well as ROC curves, were used for data analysis. Ninety-eight patients with Corona virus-2 disease (COVID-19), which consisted of sixty patients with moderate COVID-19 in the general wards, and thirty-eight patients with severe COVID-19 in the intensive care unit (ICU), were evaluated. The mean age in the general wards was lower than in ICU (60.96±14.86 compared to 67.94±16.46, P=0.001), and the mean serum vitamin D level in the patients admitted in the general wards was higher than in the ICU (31 ng/mL compared to 20.57 ng/mL, P=0.003). Furthermore, vitamin D deficiency (25 (OH) D <25 ng/ml) significantly increased the risk of severe disease. (odds ratio=2.91, P=0.019) and mortality (odds ratio=3.64, P=0.026). Vitamin D deficiency is a risk factor for disease severity and poor prognosis in COVID-19. Vitamin D levels of 25 ng/mL can be used as a cut-off value for predicting severity and prognosis.