Table I.

Observational studies of the association between vitamin D and COVID-19.

First author, year	Study design	Subjects or countries	Main outcome	Conclusion	(Refs.)
Rhodes et al, 2020	Ecological	120 countries	Correlation of latitude degrees with mortality from COVID-19 per million in different countries: rho=0.53; P≤0.0001	Mortality per million is higher in countries with a latitude above 35° n; above this latitude, people do not receive sufficient sunlight to retain adequate vitamin D levels during winter	(57)
Ilie et al, 2020	Ecological	20 countries	Correlation of mortality from COVID-19 per million with a mean 25(OH)D concentration in different countries: r=-0.43; P=0.05	The association may explain the possible protection of vitamin D from the negative consequences of SARS-CoV-2 infection	(79)
Hastie et al, 2020	Ecological	1,474 subjects	Association of 25(OH)D concentration with positive COVID-19: OR=1.00; 95% CI, 0.998-1.01; P=0.208	The results do not support the potential of 25(OH)D concentration for susceptibility to COVID-19 infection	(80)
D'Avolio et al, 2020	Retrospective	107 subjects	Difference of 25(OH)D concentration in the positive and negative SARS-CoV-2 groups: 11.1 ng/mla (IQR, 8.2-21.0) vs. 24.6 ng/mla (IQR, 8.9-30.5); P=0.004	Low concentration of 25(OH)D may represent a risk factor for infection with SARS-CoV-2	(81)
Meltzer et al, 2020	Retrospective	499 subjects	Association of 25(OH)D deficiency with a positive test for COVID-19: RR=1.77; P=0.015	Individuals with vitamin D deficiency have a higher risk of a positive test for COVID-19 compared with those with sufficiency	(82)
Whittemore, 2020	Ecological	88 countries	Correlation of latitude with death rates per million from COVID-19 in different countries: r=0.40; P≤0.00005	Mortality per million is lower in populations closest to the Equator. The correlation supports a possible association between latitude, sunlight exposure, vitamin D and COVID-19 mortality	(83)
Panagiotou et al, 2020	retrospective	134 subjects	Difference in the prevalence of 25(OH)D sufficiency between ICU and non-ICU patients: 19 vs. 39.1%; P=0.02	25(OH)D deficiency was more prevalent in patients who required admission to the ICu than those who only needed management in medical wards; therefore, vitamin D could be a determinant of the severity of the disease. There was no significant association between 25(OH)D concentration and mortality	(84)

^aMedian. rho, Spearman's correlation coefficient; r, Pearson's correlation coefficient; IQR, interquartile range; 25(OH)D, 25-hydroxyvitamin D; COVID-19, coronavirus disease 2019; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; ICU, intensive care unit. 1 ng/ml=2.5 nmol/l.