Abstract: The Effect of Vitamin D Deficiency on COVID-19 Patients Tohoku J. Exp. Med., 2021, 255, 127-134 127 Prevalence and Clinical Outcomes of Vitamin D Deficiency in COVID-19 Hospitalized Patients: A Retrospective Single-Center Analysis Siamak Afaghi,1,* Farzad Esmaeili Tarki,1,* Fatemeh Sadat Rahimi,2 Sara Besharat,3 Shayda Mirhaidari,4 Anita Karimi1 and Nasser Malekpour Alamdari5 Department of Internal Medicine, Shahid Modarres Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran 2 Critical Care and Quality Improvement Research Center, Department of General Surgery, Shahid Modarres Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran 3 Clinical Research and Development Center, Department of Radiology, Shahid Modarres Hospital, Shahid Beheshti University of Medical Sciences, Tehran, Iran 4 Clinical Instructor of Surgery, Northeastern Ohio University (NEOMED), Rootstown, OH, USA 5 Vascular and Laparoscopic Surgery, Clinical Research and Development Center, Shahid Modarres Hospital, Department of Surgery, Shahid Beheshti University of Medical Sciences, Tehran, Iran 1 Vitamin D attenuates inflammatory responses to viral respiratory infections. Hence, vitamin D deficiency may be a highly significant prognostic factor for severity and mortality in COVID-19 patients. To evaluate the complications and mortality in different vitamin D status groups in COVID-19 hospitalized patients, we conducted this retrospective study on 646 laboratory-confirmed COVID-19 patients who were hospitalized in Shahid Modarres Hospital, Tehran, Iran from 16th March 2020 until 25th February 2021. Overall, patients with vitamin D deficiency, insufficiency and sufficiency were 16.9%, 43.6% and 39.5%, respectively. The presence of comorbidity, length of hospitalization, ICU admission, and invasive mechanical ventilation requirement and overall complications were significantly more in patients with vitamin D deficiency (p-value < 0.001). 46.8% (51/109) of vitamin D deficient patients died due to the disease, whilst the mortality rate among insufficient and sufficient vitamin D groups was 29.4% (83/282) and 5.5% (14/255), respectively. In univariate analysis, age > 60 years (odds ratio (OR) = 6.1), presence of comorbidity (OR = 10.7), insufficient vitamin D status (OR = 7.2), and deficient vitamin D status (OR = 15.1) were associated with increase in COVID-19 mortality (p-value < 0.001). Finally, the multivariate analysis adjusted for age, sex, and comorbidities indicated vitamin D deficiency as an independent risk factor for mortality (OR = 3.3, p-value = 0.002). Vitamin D deficiency is a strong risk factor for mortality and severity of SARS-CoV-2 infection. Vitamin D supplementation may be able to prevent or improve the prognosis of COVID-19 during this pandemic. Keywords: COVID-19; prognosis; vitamin D; vitamin D deficiency Tohoku J. Exp. Med., 2021 October, 255 (2), 127-134. have entered a concerning second phase of the disease spread. Exceeding 30 different mutations of Severe Acute Respiratory Syndrome-Coronavirus (SARS-CoV2), the causative agent of COVID-19, have been recently revealed (Yao et al. 2020a). By rising discoveries in the number of new strains, the worldwide enthusiasm has been remarkably increased for studies evaluating the effects of antiviral drugs