Abstract: Acute and Critical Care 2021 November 36(4):300-307 https://doi.org/10.4266/acc.2021.00605 | pISSN 2586-6052 | eISSN 2586-6060 Association of vitamin D deficiency with COVID-19 severity and mortality in Iranian people: a prospective observational study Alireza Fatemi1, Seyed Hossein Ardehali2, Ghazaleh Eslamian3, Morvarid Noormohammadi4, Shirin Malek5 1 Men’s Health and Reproductive Health Research Center, Shahid Beheshti University of Medical Sciences, Tehran; 2Department of Anesthesiology and Critical Care, Shohadaye Tajrish Hospital, Shahid Beheshti University of Medical Sciences, Tehran; 3Department of Cellular and Molecular Nutrition, Faculty of Nutrition and Food Technology, National Nutrition and Food Technology Research Institute, Shahid Beheshti University of Medical Sciences, Tehran; 4Student Research Committee, Faculty of Nutrition and Food Technology, National Nutrition and Food Technology Research Institute, Shahid Beheshti University of Medical Sciences, Tehran, Iran; 5Department of Nutrition and Food Science, California State University, Chico, CA, USA Background: As the coronavirus disease 2019 (COVID-19) pandemic continues to escalate, it is important to identify the prognostic factors related to increased mortality and disease severity. To assess the possible associations of vitamin D level with disease severity and survival, we studied 248 hospitalized COVID-19 patients in a single center in a prospective observational study from October 2020 to May 2021 in Tehran, Iran. Methods: Patients who had a record of their 25-hydroxyvitamin D level measured in the previous year before testing positive with COVID-19 were included. Serum 25-hydroxyvitamin D level was measured upon admission in COVID-19 patients. The associations between clinical outcomes of patients and 25-hydroxyvitamin D level were assessed by adjusting for potential confounders and estimating a multivariate logistic regression model. Results: The median (interquartile range) age of patients was 60 years (44–74 years), and 53% were male. The median serum 25-hydroxyvitamin D level prior to admission decreased with increasing COVID-19 severity (P=0.009). Similar findings were obtained when comparing median serum 25-hydroxyvitamin D on admission between moderate and severe patients (P=0.014). A univariate logistic regression model showed that vitamin D deficiency prior to COVID-19 was associated with a significant increase in the odds of mortality (odds ratio, 2.01; P=0.041). The multivariate Cox model showed that vitamin D deficiency on admission was associated with a significant increase in risk for mortality (hazard ratio, 2.35; P=0.019). Conclusions: Based on our results, it is likely that deficient vitamin D status is associated with increased mortality in COVID-19 patients. Thus, evaluating vitamin D level in COVID-19 patients is warranted. Original Article Received: May 12, 2021 Revised: August 5, 2021 Accepted: August 6, 2021 Corresponding author Ghazaleh Eslamian Department of Cellular and Molecular Nutrition, Faculty of Nutrition and Food Technology, National Nutrition and Food Technology Research Institute, Shahid Beheshti University of Medical Sciences, No 7, Hafezi St, Farahzadi Blvd, P.O. Box 19395-4741, Tehran 1981619573, Iran Tel: +98-21-22360658 Fax: +98-21-22360657 E-mail: gh.eslamian@sbmu.ac.ir Key Words: 25-hydroxyvitamin D; COVID-19; critical care outcomes; SARS-CoV-2 Copyright © 2021 The Korean Society of Critical Care..