Abstract: nutrients Article 25 Hydroxyvitamin D Serum Concentration and COVID-19 Severity and Outcome—A Retrospective Survey in a Romanian Hospital Adriana Topan 1,2 , Mihaela Lupse 1,2, *, Mihai Calin 2 , Cristian Jianu 2 , Daniel-Corneliu Leucuta 3 and Violeta Briciu 1,2 1 2 3 * Citation: Topan, A.; Lupse, M.; Calin, M.; Jianu, C.; Leucuta, D.-C.; Briciu, V. 25 Hydroxyvitamin D Serum Concentration and COVID-19 Severity and Outcome—A Department of Infectious Diseases, “Iuliu Hatieganu” University of Medicine and Pharmacy, 400348 Cluj-Napoca, Romania The Clinical Hospital of Infectious Diseases, 400348 Cluj-Napoca, Romania Department of Medical Informatics and Biostatistics, “Iuliu Hatieganu” University of Medicine and Pharmacy, 400349 Cluj-Napoca, Romania Correspondence: mihaela.lupse@yahoo.com Abstract: Interest in the immunomodulatory function of vitamin D has grown since the COVID-19 pandemic started. Our study investigated the possible association between vitamin D deficiency and COVID-19 severity, intensive care needs, and mortality in patients hospitalized with COVID19. A prospective cohort study was performed on 2342 COVID-19 hospitalized patients between April 2020 and May 2022 in a Romanian tertiary hospital for infectious diseases. A multivariate generalized linear model for binary data was fit with dependent variables: severe/critical form of COVID-19, intensive care need, and fatal outcome as a function of vitamin D deficiency, controlling for age, comorbidities, and vaccination status. More than half of the patients (50.9%) were classified with vitamin D deficiency based on a serum concentration of less than 20 ng/mL. There was a negative association between vitamin D and age. Vitamin D-deficient patients presented with more cardiovascular, neurological, and pulmonary diseases, as well as diabetes, and cancer. In multivariate logistic regression models, vitamin D-deficient patients had higher odds of severe/critical forms of COVID-19 [OR = 1.23 (95% CI 1.03–1.47), p = 0.023] and higher odds of death [OR = 1.49 (95% CI 1.06–2.08), p = 0.02]. Vitamin D deficiency was associated with disease severity and death outcome in hospitalized COVID-19 patients. Keywords: 25(OH)D deficiency; vitamin D deficiency; COVID-19; severity form; ICU need; mortality; comorbidities Retrospective Survey in a Romanian Hospital. Nutrients 2023, 15, 1227. https://doi.org/10.3390/nu15051227 Academic Editor: Jose M. Valdivielso Received: 11 February 2023 Revised: 25 February 2023 Accepted: 26 February 2023 Published: 28 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).