Abstract: Clin Chem Lab Med 2021; 59(6): 1155–1163 Laurence Orchard*, Matthew Baldry, Myra Nasim-Mohi, Chantelle Monck, Kordo Saeed, Michael P. W. Grocott and Dushianthan Ahilanandan on behalf of the University Hospital Southampton Critical Care Team and the REACT COVID Investigators Vitamin-D levels and intensive care unit outcomes of a cohort of critically ill COVID-19 patients https://doi.org/10.1515/cclm-2020-1567 Received October 21, 2020; accepted December 28, 2020; published online January 19, 2021 Abstract Objectives: The pattern of global COVID-19 has caused many to propose a possible link between susceptibility, severity and vitamin-D levels. Vitamin-D has known UHS Critical Care Clinical Team: Dr. Sanjay Gupta, Dr. Julian Nixon, Professor Michael P W Grocott, Professor Denny ZH Levett, Dr. Michael Stewart, Dr. Ahilanadan Dushianthan, Dr. David Sparkes, Dr. Robert Chambers, Dr. Kathleen Nolan, Dr. Suzie Tanser, Dr. Jonathan Fennell, Dr. Michael Celinski, Dr. Dominic Richardson, Dr. Rebecca Cusack, Dr. Benjamin Skinner, Dr. Timothy Nicholson-Robert, Dr. Mai Wakatsuki, Dr. Ben Thomas, Dr. Francois Wessels. REACT Investigators: Professor Tom Wilkinson, Dr. Anna Freeman, Dr. Hannah Burke, Dr. Ahilanadan Dushianthan, Dr. Michael Celinski, Professor James Batchelor, Professor Saul Faust, Professor Gareth Thomas, Professor Christopher Kip. *Corresponding author: Dr. Laurence Orchard, General Intensive Care Unit, University Hospital Southampton NHS Foundation Trust, Tremona Road, Southampton, UK, Phone: +44 7791190726, E-mail: laurence.orchard@nhs.net Matthew Baldry and Myra Nasim-Mohi, General Intensive Care Unit, University Hospital Southampton NHS Foundation Trust, Southampton, UK Chantelle Monck, Southampton Specialist Virology Centre, University of Southampton School of Medicine, University Hospital Southampton NHS Foundation Trust, Southampton, UK Kordo Saeed, Microbiology Innovation and Research Unit, Department of Microbiology, University Hospital Southampton NHS Foundation Trust, Southampton, UK; Faculty of Medicine, University of Southampton University, Hospital Southampton, Southampton, UK Michael P. W. Grocott and Dushianthan Ahilanandan, General Intensive Care Unit, University Hospital Southampton NHS Foundation Trust, Southampton, UK; Faculty of Medicine, University of Southampton University, Hospital Southampton, Southampton, UK; and NIHR Southampton Clinical Research Facility and NIHR Southampton Biomedical Research Centre, University Hospital Southampton/University of Southampton, Southampton, UK immune modulatory effects and deficiency has been linked to increased severity of viral infections. Methods: We evaluated patients admitted with confirmed SARS-COV-2 to our hospital between March-June 2020. Demographics and outcomes were assessed for those admitted to the intensive care unit (ICU) with normal (>50 nmol/L) and low (<50 nmol/L) vitamin-D. Results: There were 646 SARS-COV-2 PCR positive hospitalisations and 165 (25.5%) had plasma vitamin-D levels. Fifty patients were admitted to ICU. There was no difference in vitamin-D levels of those hospitalised (34, IQR 18.5–66 nmol/L) and those admitted to the ICU (31.5, IQR 21–42 nmol/L). Higher proportion of vitamin-D deficiency (<50 nmol/L) noted in the ICU group (82.0 vs. 65.2%). Among the ICU patients, low vitamin D level (<50 nmol/L) was associated with younger age (57 vs. 67 years, p=0.04) and lower cycle threshold (CT) real time polymerase chain reaction values (RT-PCR)..