Table 1.

Summary of observational study information.

Study	Design	Participants	Conclusion
Ling et al. (3)	Multicenter retrospective study	986 participants with COVID-19	Treatment with cholecalciferol booster therapy, regardless of baseline serum 25(OH)D levels, appears to be associated with a reduced risk of mortality in acute in-patients admitted with COVID-19.
Diaz-Curiel et al. (4)	Retrospective observational study	1549 patients hospitalized for COVID-19	Vitamin D deficiency in patients with COVID-19 is correlated with an increased risk of hospital admission and the need for critical care, vitamin D levels do not influence the rate of mortality.
Carpagnano et al. (2020) (5)	Retrospective, observational study	42 patients with acute respiratory failure due to COVID-19	High prevalence of hypovitaminosis D was found in COVID-19 patients with acute respiratory failure, and severe Vitamin D deficiency significantly related to higher mortality risk.
Subramanian et al. (6)	Observational study	472 patients with COVID-19	Extremely low ( < 25 nmol/L) and high (>100 nmol/L) levels of Vitamin D may be associated with mortality risks of patients with COVID-19.
Luo et al. (7)	Cross-sectional study	335 COVID-19 patients	Vitamin D deficiency impacts COVID-19 hospitalization and severity in the Chinese population.
Charoenngam et al. (8)	Retrospective chart review study	287 COVID-19 patients aged ≥ 18 year	There was an independent association between Vitamin D sufficiency defined by serum 25(OH)D ≥ 30 ng/mL and decreased risk of mortality from COVID-19 in elderly patients and patients without obesity.
Hernández et al. (9)	Retrospective case–control study	216 COVID-19 patients and 197 control population	The study did not find any relationship between Vitamin D concentrations or Vitamin deficiency and the severity of the disease.
Vanegas-Cedillo (10)	–	551 COVID-19 patients	Low Vitamin D may contribute to a pro-inflammatory and pro-thrombotic state, increasing the risk for adverse COVID-19 outcomes.