Abstract: DOI: 10.4149/BLL_2021_034 Bratisl Med J 2021; 122 (3) 200 – 205 CLINICAL STUDY The relationship between vitamin D and the severity of COVID-19 Basaran N1, Adas M2, Gokden Y2, Turgut N3, Yildirmak T4, Guntas G1,5 Department of Biochemistry, Prof.Dr. Cemil Tascioglu City Hospital, Istanbul, Turkey. gulcanguntas@gmail.com ABSTRACT AIM: Vitamin D, which has immunomodulatory effect, can reduce risk of infections and concentrations of proinflammatory cytokines. The aim of this study was to investigate the relationship between the levels of vitamin D and severity of COVID-19. METHODS: A total of 204 patients with COVID-19 disease were enrolled in the study. All patients had viral pneumonia, which was confirmed with chest computer tomography. All cases were divided in two groupsmild (outpatients); and serious (inpatients)- according to their clinical and laboratory data. Serum vitamin D levels were measured by chemiluminescence method. RESULTS: Vitamin D deficiency was found in 41.7 % (n = 85) of cases and insufficiency was found in 46.0 % (n = 94), while in 12.3 % (n = 25) of cases normal vitamin D levels were found. The odds of having a serious clinical outcome were increased for vitamin D insufficiency patients 5.604 times (%95 CI:0.633–49.584) and for vitamin D deficiency patients 38.095 times (%95 CI:2.965–489.50) for each standard deviation decrease in serum 25(OH)D. CONCLUSION: Adequate levels of vitamin D could suppress inflammation and reduce the severity of COVID-19. Vitamin D supplementation may have an important role in decreasing the impact of the pandemic (Tab. 5, Fig. 2, Ref. 27). Text in PDF www.elis.sk KEY WORDS: COVID-19, CRP, d-dimer, ferritin, vitamin D.