Table 3.

Included studies in the systematic review.

Author	Date	Country	Design	Participants	Trimester (n)			Age (year)a	Vitamin D (ng/ml)a	Vit D normal range (ng/ml)	Outcome	Main finding
					1st	2nd	3rd
Yalcin Bahat et al., 2020 [27]	September 2020	Turkey	Case-control	44 confirmed COVID-19 cases	5	12	27	28.5	9.7 ± 59.14	30–100	Incidence	Serum vitamin D levels were significantly lower than normal cut-off ranges (p < 0.001)
Sinaci et al., 2021 [21]	August 2021	Turkey	Case-control	159 confirmed COVID-19 cases	Participants are from all trimesters			29.6	12.46 ± 6.46	Above 30	Incidence	Serum vitamin D levels were significantly lower in cases (p < 0.001)
				332 healthy pregnant women				27.4	18.76 ± 13.74
				128 mild COVID-19 cases	Participants are from all trimesters			NR	13.69 ± 9.72	Above 30	Severity	Serum vitamin D levels were significantly higher in mild patients compering to moderate and severe cases (p = 0.041)
				31 moderate & severe COVID-19 cases				NR	9.06 ± 8.82
Tekin et al., 2021 [20]	October 2021	Turkey	Prospective case–control	147 confirmed COVID-19 cases	17	46	84	27.9	14.64 ± 10.72	Above 50	Incidence	Serum vitamin D levels were significantly higher in cases (p = 0.001)
				300 healthy pregnant women	NR	NR	NR	27.9	12.52 ± 8.28
Seven et al., 2021 [30]	November 2021	Turkey	Cross-sectional	292 mild COVID-19 cases	83	102	107	28	15.5 ± 7.6	Above 30	Severity	Serum vitamin D levels were significantly higher in mild patients compared to severe cases (p = 0.01)
				111 severe COVID-19 cases	7	25	79	29.5	13 ± 8.9
Moreno-Fernandez et al., 2022 [28]	January 2022	Spain	Case-control	63 COVID-19 cases	0	0	63	31.9	21.28 ± 9.52	NR	Incidence	There is no significant difference between cases and controls (p > 0.05)
				61 healthy pregnant women	0	0	61	31.5	18.54 ± 8.04
Schmitt et al., 2022 [29]	January 2022	France	Retrospective cohort	15 COVID-19 cases	0	0	15	30	10.4 ± 9.1	Above 30	Incidence	Serum vitamin D levels were significantly lower in cases (p < 0.05)
				19 healthy pregnant women	0	0	19	31	19.1 ± 6.2
				7 asymptomatic COVID-19 cases	0	0	7	30.7	13.04 ± 7.95	Above 30	Severity	Serum vitamin D levels were significantly lower in symptomatic cases (p < 0.05)
				8 symptomatic COVID-19 cases	0	0	8	29.5	10.35 ± 6.12
Ferrer-Sánchez et al., 2022 [22]	March 2022	Spain	Case-control	82 COVID-19 cases	0	0	82	31	10.15 ± 7	Above 30	Incidence	Serum vitamin D levels were significantly lower in cases (p = 0.005)
				174 healthy pregnant women	0	0	174	32	13.8 ± 8.5
				75 mild COVID-19 cases	0	0	75	NR	10.5 ± 7.26	Above 30	Severity	There is no significant difference (p = 0.25)
				7 moderate & severe COVID-19 cases	0	0	7	NR	8.7 ± 2.15
				78 COVID-19 cases, did not admit to ICU	0	0	78	NR	10.15 ± 7.1	Above 30	Severity	There is no significant difference (p = 0.41)
				4 COVID-19 cases, admitted to ICU	0	0	4	NR	9.3 ± 4.24

^aValues are mean for age, and mean ± SD for serum vitamin D level.