Table 3.

Results of studies included in National Institute for Health and Care Excellence (NICE) Evidence Review exploring correlations between vitamin D levels and COVID-19.

Study (Reference Number)	Country/Region	Sample size	Study design	Study population	Outcome studied	Results
Ilie et al. 2020 [16]	20 countries in Europe	n ≥ 45,000 (as of 8 April 2020)	Cross sectional-Observational prognostic	General Population	The correlation between mean serum 25(OH)D level and COVID-19 cases and mortalities by country	A negative correlation observed between 25(OH)D levels and the number of COVID-19 cases (p = 0.050) and mortalities per million population (p = 0.05) respectively
D’Avolio et al. 2020 [73]	Switzerland	n = 1484 (107 COVID-19 positive/negative cohort from 1 March-14 April 2020 AND n = 1377 control patients from 1March-14 April 2019	Retrospective Cohort	General Population	Serum 25(OH)D concentrations in SARS-CoV-2 PCR positive patients compared with PCR-negative patients from 2020 cohort and historic controls from 2019. The association between serum 25(OH)D status and COVID-19 infection	Significantly lower 25(OH)D levels seen in 2020 SARS-CoV-2 PCR positive cohort [11.1 ng/ml] compared with 2020 PCR negative cohort [24.6 ng/ml] (p = 0.004) A similar association seen when 2020 SARS-CoV-2 PCR positive cohort [11.1 ng/ml] compared to 2019 historic controls [24.6 ng/ml] (p < 0.001)
Fasano et al. 2020 [74]	Italy	n = 2693 (1486 patients with Parkinson’s disease [PD] and 1207 controls)	Case control Survey	Patients with Parkinson’s Disease	Primary Outcome: Risk of Developing COVID-19 in PD patients Secondary Outcome: The risk factors for COVID-19 infection in PD patients	22.4% of PD patients on vitamin D supplementation did not develop COVID-19 while 12.4% of PD patients who did not take vitamin D supplementation developed COVID-19 Age adjusted OR: 0.56 (95% CI:0.32–0.99); (p = 0.048)
Hastie et al. 2020 [75]	UK	n = 348,598 (UK Biobank participants aged 37–73 years between 16 March 2020 to 14 April 2020)	Observational Prognostic using univariate and multivariate regression	General Population- UK Biobank participants across England, Scotland, and Wales	The prediction between baseline 25(OH)D status and subsequent COVID-19 infection	25(OH)D levels showed a significant association with COVID-19 infection in univariate analysis (p = 0.013) but not after adjustment for confounders (p = 0.208)
Laird et al. 2020 [76]	12 countries in Europe	n = 21,769	Cross sectional -Observational Prognostic using correlation	Older adults	The correlation between serum 25(OH)D status by country and COVID-19 mortalities	A statistically significant correlation between low mean serum 25(OH)D levels and higher rate of COVID-19 mortalities per million population (p = 0.046)