Abstract: PRevention of COVID-19 with Oral Vitamin D supplemental Therapy in Essential healthCare Teams (PROTECT): Ancillary study of a randomised controlled trial Banafsheh Hosseini (  banafsheh.hosseini@umontreal.ca ) University of Montreal: Universite de Montreal Cecile L Tremblay University of Montreal: Universite de Montreal Cristina Longo University of Montreal: Universite de Montreal Shirin Golchi McGill University John White McGill University Caroline Quach University of Montreal: Universite de Montreal Louis-Georges Ste-Marie University of Montreal: Universite de Montreal Robert Platt McGill University Francine Ducharme University of Montreal: Universite de Montreal https://orcid.org/0000-0001-5096-0614 Research Article Keywords: Vitamin D, Hybrid study, Healthcare workers, Randomized controlled trial Posted Date: July 19th, 2022 DOI: https://doi.org/10.21203/rs.3.rs-1588325/v1 License:   This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Page 1/20 Abstract Background The study objectives were to ascertain the efficacy of vitamin D supplementation in rapidly increasing serum vitamin D and of implementation of a hybrid (virtual and in-person) trial. Methods In a randomised triple-blind controlled trial, healthcare workers were allocated to receive an oral bolus of 100,000 IU with 10,000 IU/week vitamin D 3 or placebo. The co-primary outcomes of this ancillary study were the change from baseline in serum 25-hydroxyvitamin D [(Δ) 25(OH)D] and proportion with vitamin D sufficiency (25(OH)D ≥ 75nmol/L), at endpoint. Adherence to supplements and procedures as well as adverse event rates were documented. Results Thirty-four (19 intervention, 15 control) subjects were randomised, with 28 (41%) virtual visits. After 44.78 ± 11.00 days from baseline, a significant adjusted group difference of 44.2 (34.7, 53.8) nmol/L was observed in the Δ 25(OH)D (95%CI) in favor of supplementation; 77.8% of intervention, and 13.3% of control, patients were vitamin D sufficient (OR:6.11, 95%CI:1.6, 22.9). The adherence to intervention was 94.7% in the intervention and 100% in the control groups. Irrespective of visit type, high adherence was observed in sampling procedures and completion of fortnightly online questionnaire. No adverse events attributable to vitamin D were reported. Conclusion The vitamin D supplementation rapidly and safely raised 25(OH)D levels to sufficient levels for a biological effect. Similarly high adherence to study procedures was observed with virtual and in-person participation. Trial registration: This trial was registered at https://clinicaltrials.gov on July 23, 2020 (#NCT04483635) Background The global pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been regarded as the largest public health problem in recent decades1. Concurrent with the development of Page 2/20 COVID-19 vaccines, vitamin D supplementation emerged as a potential primary prevention approach to reduce the incidence and severity of SARS-CoV-2 infection. Indeed, a meta-analysis of 27 observational studies showed a higher odds of COVID-19 related hospitalisation and mortality in subjects with vitamin D deficiency than their counterparts2. Several mechanisms support the potential benefits of vitamin D for COVID-19, particularly via its immunomodulatory effects that enhance antiviral immune response and anti-inflammatory properties3. A reduced risk of several infections (such as..