key: cord-0829942-tuckrqwv
authors: Kazemi, Asma; Mohammadi, Vida; Aghababaee, Sahar Keshtkar; Golzarand, Mahdieh; Clark, Cain C T; Babajafari, Siavash
title: Association of Vitamin D Status with SARS-CoV-2 Infection or COVID-19 Severity: A Systematic Review and Meta-analysis
date: 2021-03-05
journal: Adv Nutr
DOI: 10.1093/advances/nmab012
sha: 7467706d523657ed83985a719c40edfb12d37392
doc_id: 829942
cord_uid: tuckrqwv

This systematic review was conducted to summarize and clarify the evidence on the association between 25-hydroxyvitamin-D [25(OH)D] concentrations and coronavirus disease 2019 (COVID-19) risk and outcomes. PubMed, Scopus, and Web of Science databases and Google Scholar were searched up to 26 November 2020. All retrospective and prospective cohort, cross-sectional, case-control, and randomized controlled trial studies that investigated the relation between 25(OH)D and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and COVID-19 severity were included. Thirty-nine studies were included in the current systematic review. In studies that were adjusted (OR: 1.77; 95% CI: 1.24, 2.53; I(2): 44.2%) and nonadjusted for confounders (OR: 1.75; 95% CI: 1.44, 2.13; I(2): 33.0%) there was a higher risk of SARS-CoV-2 infection in the vitamin D deficiency (VDD) group. Fifteen studies evaluated associations between VDD and composite severity. In the studies that were adjusted (OR: 2.57; 95% CI: 1.65, 4.01; I(2) = 0.0%) and nonadjusted for confounders (OR: 10.61; 95% CI: 2.07, 54.23; I(2) = 90.8%) there was a higher severity in the VDD group. Analysis of studies with crude OR (OR: 2.62; 95% CI: 1.13, 6.05; I(2): 47.9%), and adjusted studies that used the Cox survival method (HR: 2.35; 95% CI: 1.22, 4.52; I(2): 84%) indicated a significant association of VDD with mortality, while in adjusted studies that used logistic regression, no relation was observed (OR: 1.05; 95% CI: 0.63, 1.75; I(2): 76.6%). The results of studies that examined relations between VDD and intensive care unit (ICU) admission, pulmonary complications, hospitalization, and inflammation were inconsistent. In conclusion, although studies were heterogeneous in methodological and statistical approach, most of them indicated a significant relation between 25(OH)D and SARS-CoV-2 infection, COVID-19 composite severity, and mortality. With regard to infection, caution should be taken in interpreting the results, due to inherent study limitations. For ICU admission, inflammation, hospitalization, and pulmonary involvement, the evidence is currently inconsistent and insufficient.

Vitamin D deficiency (VDD) and insufficiency in adults and children, as a global problem, is associated with several disorders, including metabolic disorders, autoimmune diseases, cardiovascular disease, diabetes, and infections, and has been widely considered by researchers and clinicians (1) . In particular, several studies have investigated the link between the risk of respiratory tract infections and VDD (2) . For instance, Mamani et al. (3) reported an association between incidence of community-acquired pneumonia and low serum concentrations of 25-hydroxyvitamin D [25(OH)D], and adverse outcomes were observed in acute respiratory distress syndrome (ARDS) patients with VDD (4) .

Vitamin D is a fat-soluble vitamin that plays an important role in several physiological processes, such as bone metabolism, calcium and phosphorus absorption, and immune system function (5) . It may reduce the risk of microbial infections through stimulating innate cellular immunity, inhibiting the cytokine storm, decreasing proinflammatory cytokine production, and modulating the adaptive immune response (6) . Vitamin D3 and vitamin D2 are 2 primary metabolites of vitamin D (7) . Unstable 7-dehydrocholesterol in the skin is transformed to pre-vitamin D3 and stable vitamin D3, respectively, when exposed to UV-B radiation (8) . Vitamin D3, or cholecalciferol, can also be found in foods, such as dairy products, eggs, and fish (9) . Vitamin D3 is subsequently converted to 25-hydroxyvitamin D3 (25(OH)D 3) through 25-hydroxylase enzyme activity during the hydroxylation process in the liver. The 25(OH)D 3 form then transfers to the kidney and converts to 1α,25dihydroxyvitamin D3 via 1α-hydroxylase, otherwise known as calcitriol, the active form of vitamin D (8, 10) .

Currently, the global community is involved in a novel pandemic named coronavirus disease , a respiratory tract infection caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) (11) . The WHO reported the total global cases of SARS-CoV-2 infection and death as >61.8 and 1.4 million, respectively (weekly epidemiological update, 1 December 2020) (12) . This novel coronavirus (SARS-CoV-2), like the other viruses of the βcoronavirus family, is extremely contagious, and COVID-19 symptoms vary from initially mild symptoms such as dry cough, fever, fatigue, and gastrointestinal symptoms, to severe situations requiring admission to an intensive care unit (ICU) or death in severe cases (13, 14) . In some cases, inflammation can increase following both local and systemic immune responses generated by this virus and an increased number of leukocyte and concentrations of plasma proinflammatory cytokines have been reported in patients infected with SARS-CoV-2 (15) .

Several studies have investigated the association of 25(OH)D 3 concentrations and supplementation with the risk and severity of respiratory virus infections (16, 17) . Indeed, Martineau et al. (18) conducted a meta-analysis that included 25 placebo-controlled clinical trials (total of 10,933 people) and concluded that vitamin D supplementation reduces the risk of acute respiratory infections, especially in people with the lowest 25(OH)D concentrations.

Recently, a growing body of evidence has emerged regarding potential factors affecting the incidence and severity of COVID-19 (19) (20) (21) . Recent reports highlight that certain factors may be effective in controlling this pandemic or reducing the damage caused by it. Indeed, based on the global prevalence of VDD (22) , it has attracted considerable attention as a potential factor associated with the risk or severity of COVID-19, and several studies have reported on this possible association (6, (23) (24) (25) . However, results currently preclude a clear consensus. Thus, we conducted this systematic review to summarize and clarify the evidence on the association between 25(OH)D concentrations and COVID-19 risk and outcomes. 

The protocol of this study has been registered in PROS-PERO International Prospective Register of Systematic Reviews (www.crd.york.ac.uk/prospero/index.asp, identifier CRD42020203903). The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement was used in developing and conducting this systematic review (26) .

PubMed, Scopus, and Web of Science databases and the first 500 Google Scholar search results were searched up to 26 November 2020, with no restriction in language. Reference lists of included studies and relevant review articles were also scanned for additional relevant studies. The following search strategy was used for our search: (Coronavirus or COVID-19 or SARS-CoV-2) AND (vitamin D or 25-OH-D or cholecalciferol or 25-hydroxycholecalciferol or calcitriol or 25-hydroxyvitamin D or hydroxycholecalciferols or 25hydroxyvitamin D3).

Two reviewers independently assessed the eligibility of studies. Studies that met the following criteria were included: 1) study design as retrospective, prospective, or crosssectional, or case-control studies reporting serum/plasma concentrations of 25(OH)D; 2) participants as patients diagnosed with COVID-19 with no restriction on age; 3) exposure/intervention as serum/plasma concentrations of vitamin D either reported as a continuous or categorical variable (deficiency vs. sufficiency); and 4) outcome as SARS-CoV-2 infection or COVID-19 severity, with severity defined as at least 1 of the following outcomes-ARDS and/or mechanical ventilation, ICU admission, length of hospitalization, and death. The exclusion criteria were as follows: 1) case reports, abstracts, and summaries of discussion; 2) insufficient data on vitamin D measurement or COVID-19 outcomes; 3) preprint studies without peer review; and 4) studies that were not individual based (compared countries or regions).

The following data were extracted independently by 2 reviewers: first author, study design, start and completion date, geographical location, age and gender composition of patients, objective of the study [if the aim of the study was to assess association of 25(OH)D status with risk of SARS-CoV-2 infection or to assess the association with severity of disease], definition of VDD, time of serum 25(OH)D measurement, prevalence of VDD and insufficiency, definition of disease severity, the number of events and nonevents in the case and control groups, relative risk and 95% CIs for SARS-CoV-2 infection and disease severity, and adjustment factors.

Quality assessment of observational studies was assessed using the Newcastle-Ottawa Scale, which included 3 items: selection, comparability, and outcome (27) . Studies with a score of ≥7 were defined as high quality. The Cochrane risk-of-bias tool was used to evaluate quality assessment Google Scholar and other of randomized trials. This tool included selection bias, performance and detection bias, attrition bias, reporting bias, and the other biases (28) .

Wherever it was probable, we pooled data and conducted meta-analysis (SARS-CoV-2 infection, disease severity, ICU admission, and mortality). We used ORs to estimate the association between VDD and SARS-CoV-2 infection and COVID-19 severity. ORs with 95% CIs were obtained using a random-effects model. In studies that did not report relative risk, the OR was calculated by the number of events and nonevents in the case and control groups; these studies together with studies with crude ORs were analyzed separately from the studies that reported adjusted relative risk. To compare concentrations of 25(OH)D 3 between groups, we used the weighted mean difference (WMD) and its 95% CI. Heterogeneity was evaluated using Cochran's Q test, deriving its magnitude from the I 2 . If at least 10 studies were available, we explored potential small-study effects, such as publication bias, using visual examination of the funnel plot and Egger's test (29) . All analyses were conducted using Stata version 13 software (StataCorp).

As described in Figure 1 , 1518 records were obtained by the literature search. Of these, 57 articles met the inclusion criteria; however, 3 studies were excluded because they used old 25(OH)D data, and 15 papers were preprints (Supplemental Table 1 ). Finally, 39 studies were included, with different geographical locations and ethnic backgrounds, including Europe (n = 17 studies), North America (United States) (n = 2), South America (n = 2), West Asia (n = 9), South Asia (n = 4), East Asia (n = 4), and Africa (n = 1). Ten studies were of a case-control design, 19 cross-sectional, 2 retrospective cohorts, 2 randomized controlled trials (RCTs), 2 quasi-experimental design, and 4 studies were only descriptive. All studies were conducted in adults, except for 1 study in children and 1 study in pregnant women. All studies, except for 2, included both male and female participants; in 1 study, participants were only male (30) , and in another, only females were included (31) . Nine studies were not included in the analysis because 4 of them were only descriptive [only reported concentration of 25(OH)D in patients; Supplemental Table 2 ] (31-34), 1 study was in children (35) , and 4 were different in design from other studies [they assessed the effect of 25(OH)D 3 supplementation instead of 25(OH)D measurement] (14, (36) (37) (38) .

Twenty-one studies examined the association of 25(OH)D concentrations with the severity, 14 studies with SARS-CoV-2 infection, whereas 10 of them assessed severity as a secondary outcome. Characteristics of studies that examined the association of vitamin D with SARS-CoV-2 infection are summarized in Table 1 , and those examining COVID-19 severity are summarized in Table 2 .

Nine studies evaluated the relation between VDD and SARS-CoV-2 infection. Studies that were adjusted (n = 3) (39- Two studies were not included in the analysis (35, 50) ; both studies indicated that 25(OH)D concentrations were significantly lower in cases compared with controls. In 1 study, the participants were children (35) ; the other study only reported that COVID-19 patients had a significantly lower 25(OH)D concentration compared with healthy counterparts; however, the mean ± SD values of 25(OH)D were not provided (50) . Results of studies are summarized in Supplemental Table 3 .

Twenty-one studies assessed the association of VDD with severity (composite severity or 1 feature of severity) as a primary outcome, and 10 studies as a secondary outcome.

Fifteen studies evaluated the association between VDD and composite severity. Studies that were adjusted (38, 41, 44, 46, 51, 52) (OR: 2.57; 95% CI: 1.65, 4.01; I 2 = 0.0%; Figure 3A ) and nonadjusted for confounders (42, 45, (53) (54) (55) Figure 3B ) revealed a higher severity in the VDD group. Four studies were not included in the analysis; one of these studies was conducted in children and found a negative correlation between fever symptom and 25(OH)D concentration (P = 0.02), while no significant correlations were found between other clinical parameters and 25(OH)D concentration (35) . The other study had a quasi-experimental design and indicated that vitamin D3 supplementation was inversely associated with Ordinal Scale for Clinical Improvement (OSCI) score for COVID-19 (β = −3.84; 95% CI: −6.07, −1.62; P = 0.001) (56) . The third study, which assessed vitamin D supplementation in patients with a past history of COVID-19, found that it reduces the risk of exacerbation and worsening of the disease (OR: 0.29; 95% CI: 0.10, 0.083; P = 0.02) (57) . The last study did not provide sufficient data, and only reported that VDD was significantly associated with severity; however, no data were available to indicate this (58) . Results of studies have been summarized in Supplemental Table 4 .

Four studies examined the relation between VDD and ICU admission and 1 study between VDD and ICU stay duration. Pooled analysis of 3 studies (38, 44, 59) with unadjusted ORs indicated no significant relation between VDD and ICU admission (OR: 1.17; 95% CI: 0.67, 2.03; I 2 = 69.3%), while an RCT that was not pooled with these studies revealed a lower risk of ICU admission in the intervention group compared with the control group (OR: 0.03; 95% CI: 0.003, 0.25; P = < 0.001) (36). Carpagnano et al. (59) verified the association of VDD with ICU stay, highlighting that 10 patients with severe VDD had a median ICU stay of 8 d with the interquartile range (IQR) of 6 to 11.25., while 32 patients without VDD had a median stay of 12.5 d (IQ25 8, IQ75 20.5) (Supplemental Table 5 ). Commission and State Administration of Traditional Chinese Medicine: 1) mild: mild symptoms with no signs of pneumonia on imaging; 2) moderate: fever, respiratory symptoms with radiological evidence of pneumonia; 3) severe [i.e., meeting any of the following: respiratory distress, respiratory rate ≥30 breaths/min, hypoxemia, SpO 2 ≤93% (at rest), or lung infiltrates of >50% within 24-48 h]; and 4) critical (i.e., meeting any of the following criteria: respiratory failure requiring mechanical ventilation, shock, or multiple organ dysfunction requiring ICU monitoring and treatment). 4 CDC criteria were used for the disease severity and prognosis, which includes mild-moderate (mild respiratory symptoms and fever on an average of 5-6 d after infection), severe disease (dyspnea, respiratory frequency ≥30 breaths/min, blood oxygen saturation ≤93%, and/or lung infiltrates >50% of the lung field within 24-48 h) and critical (respiratory failure, septic shock, and/or multiple-organ dysfunction/failure). 5 Per Guidelines of the National Health Commission of China severe cases met at least 1 of the following criteria: 1) respiratory rate Table 6 .

Three studies investigated the relation between 25(OH)D and hospital admission and 2 with hospital stay. A significant association between VDD and risk of hospitalization was observed in Radujkovic et al. Table 7 ).

Thirteen studies compared the serum concentration of 25(OH)D between patients with severe and nonsevere status of COVID-19 (either composite or 1 feature of severity). Analysis of 12 studies (13, 30, 41, 42, 43, 46, 50, 53-55, 59, 61) , with 806 cases and 1024 controls, indicated that serum concentrations of 25(OH)D in patients with severe status of disease was lower (WMD = −7.17 ng/mL; 95% CI: −9.99, −4.34; I 2 = 87.6%) compared with less-severe counterparts (Supplemental Figure 2) . In all of the studies except for one (43) , 25(OH)D was measured after SARS-CoV-2 testing. One study was not included in the analysis, since the sample size according to hospitalization was not reported. Indeed, in this retrospective study, mean concentrations of 25 Table 8 ).

We assessed the association of VDD with C-reactive protein (CRP), IL-6, D-dimer, and ferritin in COVID-19 patients. Nine studies examined the association of at least 1 of these markers with VDD. In an RCT in 40 COVID-19 patients, cholecalciferol supplementation did not significantly reduce CRP and D-dimer (14) . A retrospective study in 42 patients with acute respiratory failure due to COVID-19 (64) revealed no statistically significant differences in inflammation indices among the 4 vitamin D groups (normal, insufficiency, deficiency, severe deficiency). Another retrospective study in 197 COVID-19 patients revealed that only ferritin, but not CRP, IL -6, and D-dimer, was significantly higher in VDD compared with non-VDD (44) . In a prospective multicenter observational study in 109 patients, the correlation between 25(OH)D concentrations at follow-up and CRP, IL-6, ferritin, and D-dimer was not significant. The same was true for 25(OH)D concentrations measured at disease onset and CRP Table 9 .

Among 15 studies that assessed the relation between mortality and VDD, 13 studies were included in the analysis. Pooled analysis of 4 adjusted studies that used the Cox survival method (13, 51, 56, 60) (HR: 2.35; 95% CI: 1.22, 4.52; I 2 : 84%; Figure 4A ) and 5 studies (44, 47, 53, 55, 62) with crude OR (OR: 2.62; 95% CI: 1.13, 6.05; I 2 : 47.8%; Figure 4B ) indicated a significant association of VDD with mortality, while in adjusted studies that used logistic regression (54, 59, 65) , no relation was observed (OR: 1.05; 95% CI: 0.63, 1.75; I 2 : 76.6%). Two studies were not included in the analysis since 1 study had an RCT design (36) and another one used different statistical methods (64) . In the RCT, 2 deaths in the control group versus no deaths in the intervention group were observed (36) . In the other study, which had a retrospective design, patients with serum 25(OH)D <10 ng/mL had a 50% probability of mortality, while those with 25(OH)D ≥10 ng/mL had a 5% mortality risk after 10 d of hospitalization (P = 0.02) (64) .

Moreover, 6 studies compared serum concentrations of 25(OH)D between deceased patients and those who survived (50, 54, 55, 60, 63, 66) Table 10 .

Publication bias and quality assessment Assessment of publication bias was conducted for 25(OH)D concentration between SARS-CoV-2-positive and -negative subjects as well as between severe and less-severe COVID-19 groups. Based on Egger's test, publication bias was evident in comparison of SARS-CoV-2-positive withnegative subjects (P = 0.002) and the funnel plot was asymmetric (Supplemental Figure 4A) . The probable reason for publication bias may be that the studies with 25(OH)D data collected before SARS-CoV-2 testing had larger sample sizes and detected smaller differences compared with the studies that measured 25(OH)D after SARS-CoV-2 testing. There was no publication bias in the comparison of severe and less-severe COVID-19 patients (P = 0.60); however, a small deviation towards an WMD ∼ −5 and an SE ≈2 was observed in a funnel plot (Supplemental Figure 4B) ; this implies that studies with a smaller SE (more precision) indicate less difference in 25(OH)D concentration compared with the pooled WMD. Therefore, it should be considered that a small overestimation is probable. The quality of most of the studies was classified as poor (Supplemental Tables  11-14) . Moreover, the strength and limitations of studies are summarized in Supplemental Table 15 . 

In this systematic review, we investigated the relation between 25(OH)D concentrations and risk of SARS-CoV-2 infection and COVID-19 severity. For this purpose, we systematically reviewed and, where appropriate, metaanalyzed the related retrospective, cohort, cross-sectional, and clinical trial studies that assessed the association of 25(OH)D concentrations and the risk of SARS-CoV-2 infection, composite severity, or 1 feature of severity.

Higher risk of SARS-CoV-2 infection was observed in VDD and serum concentrations of 25(OH)D were lower in COVID-19 patients compared with healthy counterparts, as indicated by pooled results of both adjusted and nonadjusted studies. Among the 3 adjusted studies, 2 measured 25(OH)D in the preceding year before SARS-CoV-2 infection (39, 40) ; the sample sizes in one of these studies were sufficiently powered (case/control: 782/7025) (39) . The nonadjusted studies measured 25(OH)D at admission and the sample sizes were sufficient in 4 studies (186/2700, 197/197, 128/219, 335/560) (39, 43, 39, 39) . Moreover, concentrations of 25(OH)D were lower in COVID-19 patients compared with healthy subjects. Based on the findings, VDD is associated with increased risk of SARS-CoV-2 infection; however, caution should be made in interpreting these results, since the studies have inherent limitations.

All of the studies indicated a lower concentration of 25(OH)D with more severe status (composite severity) of disease. Furthermore, VDD was associated with composite severity in studies that were both adjusted and not adjusted for confounders. The significant relation between VDD and composite severity was evident in all of the primary studies, except for the Hernández et al. (44) and De Smet et al. (42) studies, where De Smet et al. revealed such a relation only in males but not in females. Zero heterogeneity was estimated for adjusted studies based on the I 2 statistic. It should be noted that the heterogeneity I 2 statistic can be biased in small meta-analyses and so an I 2 of 0.0% does not necessarily reflect perfect homogeneity (67) .

Pooled results from the studies that were unadjusted and adjusted studies using Cox survival analysis indicated a higher risk of mortality in VDD; however, the adjusted studies that used logistic regression failed to find a significant relation. The Cox model estimates the instantaneous probability of death at a particular time, while logistic regression estimates the cumulative probability; instantaneous risk could be important as the cumulative probability can be conditioned by a complex clinical outcome. Moreover, it is noteworthy to mention that the Cox model tends to have greater statistical power to detect a significant exposure effect than logistic regression (68) . Among the 4 adjusted studies that used logistic regression, 1 study indicated higher risk of mortality in VDD, 2 revealed no significant relation, and 1 study unexpectedly found a lower risk of mortality in VDD. In this study, the prevalence of ≥2 comorbidities was higher in the non-VDD (46.7%) versus the VDD group (30.3%). Although this difference between groups was not statistically significant, it could be important because of the small sample size (n = 30 in non-VDD and n = 99 in VDD). The authors adjusted for some confounders (age, sex, CRP, ischemic heart disease, and severe pneumonia), but the effects of other chronic diseases that were more prevalent in the non-VDD versus VDD groups (albeit nonsignificant) were not adjusted. Moreover, the population in this study was old (mean age of 77 y) and so at high risk for other nutrient deficiencies. The 2 studies that were not included in the analysis also indicated a significant relation, in which 1 study was an RCT (36, 64) . Consistently, pooled results indicated a higher concentration of 25(OH)D in patients who survived versus those who died. Overall, evidence indicates that VDD greatly increases the risk of mortality.

Pooled analysis of unadjusted studies failed to detect any significant relation between 25(OH)D concentration and ICU admission, although an RCT indicated a significant association (36) .

For pulmonary complications, results of studies were inconsistent; 4 studies found a significant relation between 25(OH)D concentration and an increased risk of pulmonary involvement, while 4 studies failed to find any relation. Among them, only Radujkovic et al. (13) and Abrishimi et al. (60) were adjusted for confounders, and both found a significant association between VDD and risk of pulmonary involvement. Radujkovic et al. had some other strengths, such as a cohort design and larger sample size, as compared with the other studies. Although this study indicated a very large risk in VDD, the HR in this study was for the combination of both ventilator requirement and death. In Abrishami et al., increases in 25(OH)D concentration led to only a 4% reduction in severe lung involvement. Therefore, it seems pragmatic to suggest that no conclusion can be drawn regarding the relation between 25(OH)D and pulmonary complications.

All 3 studies that examined the association between VDD and hospitalization indicated a significant relation (13, 40, 57) . One study adjusted for confounders and had a good quality design (13) ; another study adjusted for confounders and had a large sample size but the authors used vitamin D data that were measured in the past (40) , while the third study did not adjust for confounders and had a poor design (57) . With regard to the relation between 25(OH)D concentration and hospital length of stay, 1 study found a significant relation (44) , while the other failed to find any relation (46) . In total, the evidence is not adequate to draw a conclusion with regard to the association of vitamin D with hospitalization admission and length of stay.

We assessed CRP, D-dimer, ferritin, and IL-6 as the inflammatory markers. Five studies indicated a positive association between 25(OH)D concentration and inflammation. In 2 studies peak CRP and CRP >40 mg/L were evaluated in related to VDD (38, 47) . In 1 study, only IL-6 was measured, and in the other 2 studies, the relation was examined using Pearson correlation coefficients (50, 54) . Four studies failed to detect a significant relation (14, 44, 64, 61) ; among them, the highest-quality study was a clinical trial that failed to discern the effect of cholecalciferol supplementation on CRP and D-dimer (14) , although it does not appear that 25(OH)D concentration is correlated with inflammation in nonacute phases, given that the evidence is currently not sufficient.

Several mechanisms are involved in elucidating the relation between VDD and SARS-CoV-2 infection risk and outcomes. Vitamin D improves cellular immunity and can decrease the plasma concentrations of proinflammatory cytokines, such as TNF-α and IFN-γ , that have been produced as part of the cytokine storm by the innate immune system in viral infections such as COVID-19, in addition to increasing concentrations of anti-inflammatory markers (69) . Furthermore, vitamin D can regulate adaptive immunity response by stopping the T-helper (Th) cell type 1 (Th1) reaction, elevating production of cytokine by Th2, and increasing the induction of T-regulatory cells (70) (71) (72) .

In addition, due to the highly expressed concentrations of vitamin D receptors (VDRs) in B-and T-lymphocytes (73) , vitamin D can affect immune system function. VDR is a member of the nuclear hormone receptor (NHR) family, which is a known transcription factor (74) ; indeed, VDR is present in both T and B immune cells and regulates a variety of metabolic pathways, such as those involved in the immune response and cancer (75) . High concentrations of transforming growth factor β (TGF-β) have been reported in the acute phase of COVID-19, where TGF-β signaling is closely related to SARS-CoV-2 and is suppressed by VDR via genomic competition with Mothers against decapentaplegic homolog 3 (Smad3) occupancy on proinflammatory (e.g., IL-6) genes and therefore creating a stable physiologic situation (76) .

Another probable mechanism is that vitamin D can induce cathelicidin, IL-37, and defensins as antimicrobial peptides, and promote cellular innate immunity and reduce virus replication (77) (78) (79) .

It has been posited that vitamin D can enhance the expression of some genes related to antioxidant systems, such as the glutathione reductase gene (80) ; accordingly, some studies have reported that vitamin D metabolites have vascular-related functions including anticoagulant effects through modifying the expression of thrombomodulin and tissue factor in monocyte and aortic cells (81, 82) .

Because of the worldwide increasing prevalence of COVID-19 as a novel pandemic, it is important to research potential antiviral treatments or preventions. Therefore, we conducted this systematic review to investigate the association of vitamin D concentration with SARS-CoV-2 infection and various clinical outcomes.

Some systematic reviews have investigated the association between vitamin D3 and COVID-19 risk and severity (83, 84) , in addition to a meta-analysis by Pereira et al. (85) , which included 27 studies. The priority of the present study was to include a higher number of studies and exclude preprint articles that had not been peer reviewed and studies with high risk of bias. Moreover, problematically, studies that did and did not adjust for confounding variables were pooled together in the Pereira et al. study, while we analyzed these studies separately.

The main limitation of the present systematic review is the inclusion of studies that were heterogeneous in design, methodology, and statistical approach, and since most of the studies were observational, causality cannot be inferred. Sex and age are important factors that have been shown to be related to both COVID-19 and 25(OH)D concentrations independently. Thus, it is of high importance that the relation between COVID-19 and vitamin D be verified in different subgroups of age and sex. Indeed, we were unable to do so due to the results not being reported separately in the included studies.

In conclusion, although studies were heterogeneous in methodological and statistical approach, and some inherent limitations were present, the findings of the present study indicated a significant relation between 25(OH)D concentration and SARS-CoV-2 infection, COVID-19 composite severity, and mortality. For infection, caution should be taken in interpreting the results due to inherent limitations of studies. For ICU admission, inflammation, hospitalization, and pulmonary involvement, the evidence is currently inconsistent and insufficient. Moreover, future studies should investigate the association of COVID-19 with vitamin D in subgroups of age and sex.

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Vitamin D supplementation associated to better survival in hospitalized frail elderly COVID-19 patients: the GERIA-COVID quasi-experimental study

Interaction between age and vitamin D deficiency in severe Covid-19 infection

Analysis of vitamin D level among asymptomatic and critically ill COVID-19 patients and its correlation with inflammatory markers

Impact of serum 25(OH) vitamin D level on mortality in patients with COVID-19 in Turkey

Serum 25(OH)D level in patients with CoVID-19

Vitamin D and survival in COVID-19 patients: a quasi-experimental study

Supplement usage pattern in a group of COVID-19 patients in Tehran

Role of vitamin D in pathogenesis and severity of coronavirus disease 2019 (COVID-19) infection

Low serum 25-hydroxyvitamin D (25[OH]D) levels in patients hospitalized with COVID-19 are associated with greater disease severity

Possible association of vitamin D status with lung involvement and outcome in patients with COVID-19: a retrospective study

Impact of vitamin D deficiency on COVID-19-a prospective analysis from the CovILD Registry

Examine the association between severe vitamin D deficiency and mortality in patients with Covid-19

Serum levels of vitamin C and vitamin D in a cohort of critically ill COVID-19 patients of a North American community hospital intensive care unit in May 2020: a pilot study

Vitamin D deficiency as a predictor of poor prognosis in patients with acute respiratory failure due to COVID-19

Vitamin D 25OH deficiency in COVID-19 patients admitted to a tertiary referral hospital

Deficiency of vitamin D is a risk factor of mortality in patients with COVID-19

The heterogeneity statistic I(2) can be biased in small meta-analyses

Survival analysis and regression models

Effect of single-dose injection of vitamin D on immune cytokines in ulcerative colitis patients: a randomized placebo-controlled trial

25-Dihydroxyvitamin D3 suppresses human T helper/inducer lymphocyte activity in vitro

Vitamin D and 1,25 (OH) 2D regulation of T cells

1,25-Dihydroxyvitamin D3 and IL-2 combine to inhibit T cell production of inflammatory cytokines and promote development of regulatory T cells expressing CTLA-4 and FoxP3

Vitamin D and influenza-prevention or therapy?

Shining light on the COVID-19 pandemic: a vitamin D receptor checkpoint in defense of unregulated wound healing

Vitamin D receptor agonists, cancer and the immune system: an intricate relationship

A vitamin D receptor/SMAD genomic circuit gates hepatic fibrotic response

Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response

Vitamin D-directed rheostatic regulation of monocyte antibacterial responses

Antiviral activity and increased host defense against influenza infection elicited by the human cathelicidin LL-37

Mechanisms of action of vitamin D as supplemental therapy for Pneumocystis pneumonia

Are vitamin D receptor activators useful for the treatment of thrombosis?

Effects of vitamin D analogs on gene expression profiling in human coronary artery smooth muscle cells

Vitamin D insufficiency as a potential culprit in critical COVID-19 patients

Evidence regarding vitamin D and risk of COVID-19 and its severity

Vitamin D deficiency aggravates COVID-19: systematic review and meta-analysis

The authors' responsibilities were as follows-AK: designed the study; AK, VM, and SKA: performed the literature search and screening and wrote the initial draft, which was modified after feedback from all coauthors; AK, VM, and MG: performed data extraction; MG and AK: performed quality assessment; CCTC and SB: critical revising of the manuscript; AK: had primary responsibility for content; and all authors: read and approved the final manuscript.