Table 1.

Recent randomized clinical trials and retrospective cohort studies evaluating a relation between vitamin D supplementation and COVID-19 outcomes.

Study type	Location	Size	Vitamin D dose	Outcomes	Ref.
Cross-sectional analysis	Iran	235		Potentially reduce the severity of morbidities and mortality of COVID-19	[20]
Retrospective Cohort	UK	444	Booster (high-dose) vitamin D therapy	Associated with a reduced risk of COVID-19 mortality	[32]
Retrospective Cohort	US	489	1000–3000 IU/daily for 14 were administered before COVID-19.	Deficient vitamin D status was associated with increased COVID-19 risk	[126]
Quasi-experimental	France	66	80,000 IU bolus in week following or previous month	Less severe COVID-19 and better survival in frail elderly	[30]
Quasi-experimental	France	77	50,000 IU/month or 80,000–100,000 IU every 2–3 months before COVID-19) vs. 80,000 IU within “few hours” of COVID-19	Regular bolus vitamin D3 supplementation was associated with less severe COVID-19 and better survival rate.	[31]
Randomized clinical trials	Spain	76	0.532 mg (21,280 IU) on the day of admission and 0.266 mg on day 3, 7 and weekly	Significantly reduce the need for ICU of COVID-19 cases (e.g. reduced the severity of COVID)	[129]
Randomized clinical trials	Brazil	240	Single oral dose of 200,000 IU of vitamin D3	Increase serum vitamin D levels (21–44 ng/mL) but did not significantly reduce hospital length of stay	[132]
Randomised clinical trials (SHADE study)	India	40	60,000 IU of cholecalciferol daily for 7 days	Greater proportion of vitamin D-deficient individuals with SARS-CoV-2 infection turned SARS-CoV-2 RNA negative with a significant decrease in fibrinogen	[128]