Single center, open label randomized clinical trial.

Study location: tertiary hospital center (University Hospital Split, Croatia). All COVID-19 patients with positive PCR test admitted to ICU and in need for respiratory support will be eligible for inclusion in this study.

Patients admitted to ICU with severe COVID-19 disease and in need for invasive or non-invasive respiratory support with low levels of vitamin D (<50 nmol/l) measured on admission. All patients are older than 18 years and have confirmed COVID-19 disease with PCR test.

Intervention:

All patients included in this study will receive standard of care. Patients randomized into intervention group will be receiving 10 000 IU of cholecalciferol daily. Supplement will be administered orally or via gastric tube during ICU stay or for at least 14 days in case of ICU discharge before day 14. Supplementation will begin within 48 hours of admission to ICU. Supplement will be prepared and administered by experienced nursing staff. For patients receiving supplementation, vitamin D levels will be checked on days 7 and 14. In case that vitamin D levels are > 150 nmol/l or if the calcium levels are consistently > 2.6 mmol/l, further supplementation will be stopped.

Outcomes:

Primary outcome is number of days spent on ventilator.

Secondary outcomes: all-cause mortality on day 28, all-cause mortality on day 60, mortality at hospital discharge, clinical improvement at day 28 (WHO clinical progression scale), days spent in ICU, days spent in hospital after discharge from ICU, need for dialysis at day 28, bacterial superinfections, neutrophile to lymphocyte ratio, disease severity (CRP levels, PaO2/FiO2 ratio, D-dimer levels, fibrinogen, ferritin, PCT), adverse outcomes.

Hypothesis: patients receiving Vitamin D supplementation will have shorter number of days spent on mechanical ventilation.

Inclusion Criteria:

• Patients admitted to ICU with severe COVID-19 disease and in need for invasive or non-invasive respiratory support with low levels of vitamin D (<50 nmol/l) measured on admission. All patients are older than 18 years and have confirmed COVID-19 disease with PCR test.

Exclusion Criteria:

• Patients will be excluded from study in case of calcium levels that are consistently above normal serum range (> 2.6 mmol/L, > 10.5 mg/dL) or if vitamin D levels are > 150 nmol/L.