Abstract

Coronavirus disease 2019 (COVID-19), has triggered an unprecedented pandemic in the world. Public health measures that can decrease the risk of infection and death in addition to quarantines are urgently needed. The covid-19 pandemic has led to numerous claims about possible treatments. One of them is the role of vitamin D in the prevention and management of covid-19. Vitamin D is an immunomodulator hormone with established effectiveness against various upper respiratory infections. Vitamin D can stall hyper-inflammatory responses and expedite healing processes of the affected areas, primarily in the lung tissue. Thus, there are ecological and mechanistic reasons to promote exploration of vitamin D action in COVID-19 patients.

Several studies examined the potential of vitamin D to reduce the effects of SARS-CoV-2 infection by modulating the immune system. Vitamin D has been found to boost the innate immune system and stimulate the adaptive immune response against SARS-CoV-2 infection. In this review, we deliver a complete update of the immunological mechanisms underlying the positive effects of vitamin D in reducing SARS-CoV-2 infection as well as a detailed survey of the current epidemiological studies and clinical trials that tested vitamin D as a potential beneficial agent against COVID-19 infection. We believe that a better understanding of the histopathology and immunopathology of the Covid-19 as well as the mechanism of vitamin D effects on Covid-19 severity will finally pave the way for a more effective prevention and control of this worldwide pandemic.

Introduction

As the pandemic of a novel coronavirus recognized as a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remains spreading worldwide, research on risk factors for intensive care unit admission and mortality are emerging. 

Initial studies from China and Italy showed that that older age, male sex, existing comorbidities and compromised immune system increase the risk of developing severe respiratory disease associated with COVID-19 (1). Mortality rates from COVID-19 differ significantly among countries. Countries that lie below 35 degrees North have comparatively low mortality rates (e.g., 14.6% of patients in northern latitudes to 0.6% of patients in southern latitudes) (2). At above 35 degrees North latitude, people do not get sufficient sunlight to retain adequate vitamin D levels during winter which suggest a probable role for vitamin D in determining outcomes from COVID-19 (3).

Vitamin D has a significant role in keeping healthy teeth, bones and immunity. Sunlight is the most important source of vitamin D in nature (4,5). Although vitamin D is available in some foods like oily fish, eggs etc. the amount of accessible vitamin D is not sufficient to meet the daily vitamin D requirement. Vitamin D is biologically inert and undergoes two consecutive hydroxylations in the liver and kidney to become the biologically active 1,25 -dihydroxy vitamin D (6). The two most significant forms are vitamin D3 (cholecalciferol)and vitamin D2 (ergocalciferol). Both are bound to the vitamin D binding protein (VDBP) and transported to the liver and converted to 25-hydroxyvitamin D. 

The half-life of 25-hydroxyvitamin D is 2–3 weeks. 25-Hydroxyvitamin D is transformed to 24,25-dihydroxyvitaminD, which is the utmost abundant product of 25-hydroxyvitaminD catabolism. Most of the 25-hydroxyvitamin D measured in the serum is 25-hydroxyvitaminD3(7). It has been related to diabetes, Various forms of cancers, cardiovascular disease and innate immunity and expression of around 200 different genes (8). The lockdowns and self-isolation applied by the governments around the world have elevated concerns about additional deficiency in vitamin D levels. Vitamin D deficiency affects nearly 50% of the population globally (9). The important role of vitamin D is its effect in modulating the innate and adaptive immune systems, and deficiency of vitamin D led to an increased susceptibility to respiratory infections (9;10).

The prognosis of SARS-COV-2 is still poorly understood. Some studies have suggested association of vitamin D deficiency with COVID-19-related mortality and morbidity. 

This review emphases on the potential effects of vitamin D on the risk and severity of COVID-19 related respiratory dysfunction. We hypothesize that deficiency of vitamin D may be related with an increased morbidity/mortality of COVID-19 respiratory infection.

Vitamin D supplementation of 10-25 μg a day has a protective effect against acute respiratory infections (11) but research on a direct effect in covid-19 is sparse. Experimental evidence is also varying. Some research studies have shown an association between vitamin deficiency and greater incidence or severity of SARS-CoV-2 infection (12;13). In this review we summaries all the research studies related to COVID-19 and vitamin D and Vitamin D as a treatment for Covid-19. 

REVIEW OF LITERATURE

Vitamin D 

Vitamin D is a steroid hormone, which comes mainly from the synthesis at the level of the skin, of a precursor that is 7-dehydrocholesterol, which due to the action of ultraviolet light (UVL) B (280–315 nm) exposure is converted into vitamin D3 (cholecalciferol). In countries at far latitudes, this origin is seasonal only (14). 

25(OH) D can be gained from a diet like oily fish, eggs and liver but few commonly eaten foods contain sufficient amounts so some countries have a compulsory vitamin D food fortification policy (15). The recommended amount of vitamin D is 10 μg or 400 IU of vitamin D daily during winter. Vitamin D levels can be affected by obesity, sunscreen, clothing, genetics, gender and socio-economic status (16). Vitamin D2 is found from mushrooms or some vegetables and D3 is found from sun exposure or diet, are hydroxylated in the liver and kidneys where the active form of this vitamin is generated as, 1,25 Dihydroxycholecalciferol (1,25(OH)2D). Macrophages cells have the capability to convert 25 (OH) D to 1,25(OH)2D via CYP27B1. The biomarker of a patient’s Vitamin D status is the concentration of total 25(OH)D (25-hydroxyvitamin D) concentration in serum, since vitamin D deficiency correlates better with 25(OH)D than with 1,25(OH)2D (17,18). The effects of Vitamin D on calcium and phosphate absorption, osteoclast activation and `1on bone calcification and muscle strength are broadly recognized (19).

Vitamin D and Covid-19

The pathogenic mechanisms of COVID-19 illness have not been totally clarified yet. Various dysregulated pathways are recognized as directly linked to COVID-19 morbidity and mortality (20;21;22). These comprise the role of renin-angiotensin pathways in viral entry into alveolar cells and following acute lung injury, dysregulated immune responses and activation of coagulation pathways (23).

Research Studies recommend that entry of SARS- CoV-2 infection into alveolar epithelial cells which triggered by binding of the virus surface spike (S)-protein to the angiotensin converting enzyme 2 (ACE2) receptor might lead to dysregulation of the renin-angiotensin system (RAS) resultant acute lung injury due to over- accumulation of angiotensin II (Ang II) in alveolar cells (24;25;26;27;28).

Various Studies suggested that vitamin D deficiency leads to over-activation of the pulmonary RAS which provide a possible clarification for the detected inverse relationship between blood pressure and serum 25(OH)D levels (29;30). Vitamin D may have a protective role on acute lung injury by regulating the balance amid the expression of members of the RAS (31). It is therefore conceivable that vitamin D deficiency may aggravate dysregulation of the pulmonary RAS induced by SARS-CoV-2 infection. 

What does research say?

Presently, Research studies designs to hypothesize a relationship between hypovitaminosis D and the severity of SARS-CoV-2 infection are either ecological, demographic with risk groups for Vitamin D deficiency, or on the association of 25(OH) D levels with the peril of having a positive test for the virus (32). Ecological   research studies use databases with information on 25(OH)D concentration of populations and countries and relate it to mortality, severity and susceptibility to SARS-CoV-2 infection.

Considering latitude, an inverse relationship was seen between mortality and 25(OH)D status in Asia, Middle East and Oceania and astonishingly, in the USA and South America, the relationship was direct (33-34). Ilie et al. have shown that the Pearson correlation coefficients between mean 25(OH)D levels and COVID-19 cases, and mean 25(OH)D levels and COVID-19 deaths per million population were negative and statistically significant based on data from 20 European countries (35); though, this study was re analyzed by Kumar et al., by addition of life expectancy factor to the model, and the result was the loss of the significance of 25(OH)D levels as a predictor of mortality from COVID-19 (36).

Mendelian randomization studies use genetic variants as markers to assess relationship in observational data (37), and have been used in studies of the association between 25(OH)D and severity of COVID-19 infection which is based on the fact that the polymorphism of the Vitamin D receptor has influence on the response to 25(OH)D. Mendelian randomization studies use the genetic variant as a surrogate variable for 25(OH)D deficiency, to conclude the effect of an exposure [25(OH)D concentration] to an outcome (COVID-19 susceptibility, severity or mortality) (38). 

Presently, 3 studies have been available using Mendelian randomization on the connection of 25(OH)D concentration with the risk or severity of COVID-19. According to the studies, there is indication that vitamin D deficiency can increase the seasonal increase of virus infections of the respiratory tract, from the common cold to influenza, and now also COVID-19. 

According to the study of Sardu et al. (39), the hospitalized patients with COVID-19 disease with severity which   include ICU admissions, could be due to endothelial dysfunction which is higher in patients with comorbidities. Research studies by. Guan et al. reported that 23.7% of hypertensive patients had disease severity (vs. 13.4% of normotensive subjects), and that 35.8% (vs. 13.7%) reached mechanical ventilation and death (40). Unexpectedly, many of the treatments for pre-existing cardiovascular diseases can increase the ACE2 levels which result in a negative outcome of COVID−19. Remarkably, vitamin D treatment has been shown to hinder ACE2 expression in the kidneys (41;42), and thereby could stop COVID-19 entry into the cell in patients with pre-existing comorbidities. Moreover, Zhou et al. (43) lately showed a significant association between levels of D-dimer > 1 µg/mL and COVID-related death with coagulation dysfunction (44;45). Vitamin D and its related metabolites regulate thrombosis-related pathways by modulation of genes such as those linked to angiogenesis (46).  These studies provide evidence for the increased risk of thrombotic events linked with a vitamin D deficiency (47). This research strongly supports investigation of    vitamin    D    replacement    in    patients    with   co- morbidities.

Genetic research studies have shown a polymorphism in the vitamin D receptor gene linked with upper respiratory tract infections in children and adults (48). Dysregulated immune responses related with extreme and prolonged cytokine/chemokine responses recognized as a ‘cytokine storm’, appear to be directly connected to worse consequences in COVID-19 (49;50;51). All cells of the immune system have been shown to express the Vitamin D receptor and research studies have shown that 1,25(OH)2D and vitamin D are important immune system regulators (52;53;54;55;56;57). Vitamin D is associated with innate immunity through cationic antimicrobial peptide cathelicidin as a vitamin D target gene and Vitamin D receptor upregulation in monocytes. Earlier studies have shown that cathelicidin activation as a possible mechanism driving vitamin D defensive effects on respiratory tract infections (58;59;60;61;62).

Research studies in a mostly African- American population showed that lung function abatements linked with low cathelicidin were highest in individuals with low vitamin D levels (63). Moreover, vitamin D is involved in downregulating the production of pro-inflammatory cytokines [tumor necrosis factor α (TNF α), interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-12 (IL- 12), and interferon-gamma (IFN-γ)] which contribute to the cytokine storm (64;65;66;67;68;69;70).

Downregulation of IFNγ and IL-6 inflammatory responses have been described as negative prognostic indicators in ventilated patients with COVID- 19 (71). Additional vitamin D roles such as the musculoskeletal properties can be also possibly beneficial in COVID-19 (72). Though the information is far from decisive in attributing a role for vitamin D in the risk and severity of COVID-19, together these studies deliver a strong basis for more pre-clinical and clinical research for more investigation.

Vitamin D Deficiency and COVID-19 Incidence/Disease Severity

Human requirement for vitamin D is evaluated by measuring 25(OH)D levels in blood plasma or serum, and is obtained primarily through the synthesis of this pre-hormone in the skin during exposure to ultraviolet B (UVB) radiation, with minor contribution from diet. A level of 20–50 nanograms/milliliter is considered adequate for healthy people. A level less than 12 ng/mL indicates vitamin D deficiency (73). 

Vitamin D deficiency have described to be related with PCR-positivity for SARS-CoV-2 individuals in two different studies. D’Avolio(74) et al. detected significantly lower 25(OH)D levels (p = 0.004) in PCR-positive patients for SARS-CoV-2 (median value 11.1 ng/mL) linked with negative patients (24.6 ng/mL) in Study in Switzerland which suggest a possible role of the vitamin D in the COVID-19 infection (75). Similar results were detected in a healthy workers study in Chicago where the vitamin D deficient group had a higher risk for COVID-19 compared with the control group (76).  

Positive correlation was also detected in a large US cohort study (n = 191,779) where SARS-CoV-2 positivity was powerfully and inversely related with circulating 25(OH)D levels (77). Furthermore, Faul(78) et al. (2020) presented a small study of patients with PCR-positive SARS- CoV-2 associated pneumonia, where a serum 25OHD level was less than 30 nmol which suggest  low 25OHD level which contributes to severe disease and development to ARDS in some patients infected with SARS-CoV-2 (79).

Similar results were found where, low serum 25[OH]D levels in patients hospitalized with COVID-19 was related with greater disease severity but not with mortality in a United Kingdom (80).

Serum calcium levels are well documented to be significantly positively connected with vitamin D levels. and higher occurrences of organ injury, septic shock, and higher mortality have been detected in COVID-19 patients with lesser calcium levels (81). Together these results deliver a strong basis for further exploration of the potential role of vitamin D and its supplementation as well as its role as a prognostic biomarker or COVID-19 severity (82).

Study by Whittemore (83) stated a statistically significant association between lower COVID-19 death rates and a country’s latitude which suggest an association between sunlight exposure and reduced mortality. 

What do other studies show?

In support of this, two other studies have described the correlation between mean vitamin D levels and incidence and COVID-19 related mortality across European countries (84;85). Both studies have shown a strong opposite association between mean vitamin D level and COVID-19 mortality (86;87) or COVID-19 occurrence (88). These results were also supported by the results of the systematic review by Laird et al. (89). 

Moreover, none of the above- stated studies report the duration of time amid the identification of first cases in a country and the reported death rates are unknown. It is likely that death rates in a country where cases began to appear newly will appear lower than those in countries fighting the virus for a lengthier time. For example, Portugal (39 nmol/L) and Sweden (73.5 nmol/L) were reported as having the lowest or highest mean serum vitamin D levels for the populations respectively. Though, the number of mortalities   in Sweden are higher than Portugal which contradict the assumptions drawn by the authors. Interestingly, two research studies showing the potential association of vitamin D deficiency and the high incidence of COVID-19 have not described positive correlations between the two (90;91). 

Raisi-Estabragh et al. (92) specifically explore whether cardiometabolic exposures which include vitamin D, socio- economic, lifestyle could clarify the differential patterns of COVID-19 incidence observed in diverse populations.  He could not conclude that sex and ethnicity differential pattern of COVID-19 were clarified sufficiently by differences in cardiometabolic factors, 25(OH)-vitamin D levels or socio-economic factors. Research studies by Hastie et al. (93) could not find an association between plasma 25(OH)D concentrations and COVID-19 infection in the UK (94).

The National Institute for Health and Care Excellence (NICE) review

The National Institute for Health and Care Excellence (NICE), review (95) included one randomised controlled study of vitamin D as treatment and 12 observational studies examining associations between serum vitamin D concentrations and incidence of covid-19. The one small (n=76) study from Spain described significant reduction of Covid-19 severity among patients given high dose vitamin D during their hospital admission (96).

Two further trials reported contradictory findings. A single oral dose of 5000 μg of vitamin D3 did not affect span of stay among Brazilian patients with severe covid-19 (n=240) (97). In a study from India, though, (n=40) patients with mild or asymptomatic covid-19 were more likely to test negative at 21 days following daily vitamin supplementation starting at 1500 μg (98). 

Patients with severe COVID-19 infection develop dyspnea and hypoxemia within 1 week after beginning of the disease, which may rapidly develop into ARDS or end-organ failure. 20% of the patients with severe COVID-19 developed ARDS (99). There are many research studies that suggest that higher serum 25-hydroxyvitamin D [25(OH)D] concentrations reduce the risk of respiratory tract infections within all age groups (100;101;102;103;104;105). Various research studies specifically associate vitamin D deficiency as an independent risk factor for ARI/ARDS (106;107;108;109).

In summary, there is robust evidence for the immunomodulatory properties of vitamin D in influencing ALRI disease severity (110;111) hereby providing proof-of-principle for its involvement in progression of COVID- 19 pathogenesis.

Vitamin D   Supplementation   for   COVID-19 Prevention/Treatment

For treatment of COVID-19 patients, higher vitamin D3 doses were recommended by Scientists (112;113;114;115). A recent publication on 4 COVID-19 patients with vitamin D deficiency described that all 4 patients treated with cholecalciferol 1000 IU/d or ergocalciferol 50,000 IU/d had a good result and were discharged within 14 days; with biomarker changes from day 0–6 detected (116). 

Caccialanza et al. (117) have described a protocol for early nutritional supplementation of non-critically ill Covid 19 hospitalized patients in Italy. In this protocol, cholecalciferol is prescribed according to blood tests results [50,000 IU/wk. if 25(OH)D < 20 ng/mL; 25,000 IU/wk. if 25(OH)D 20 to <30 ng/mL]. 

Research studies conducted by Beigmohammadi et al. (118) for patients with COVID-19 admitted to intensive care given a supplement of a number of vitamins including a single dose of Vitamin D 600,000 IU to evaluate severity and mortality rate. 

Quesada-Gomez et al. (119) recommended that the stimulation of Vitamin D Receptor (VDR) in patients with SARS-CoV-2 infection, may reduce ARDS as a result of VDR’s capacity as a negative regulator of renin-angiotensin system in alveolar cells and suggested oral calcitriol supplementation as a treatment approach. 

However, according to our literature review the exact efficacy of the above-mentioned measures for prevention of, or as an adjuvant treatment for COVID-19 remains to be determined.

The results of 9 ongoing research studies (26 registered) on vitamin D are anticipated as they will help to explain the role of vitamin D in COVID-19 infection and its potential benefit in prevention and treatment of COVID-19 disease (120). These include: 4 studies describing survival rate on COVID-19 patients (single dose of 25,000 UI of vitamin D), ZnD3-CoVici (2000 IU per day for 2 months), CoVitTrial (single dose of cholecalciferol 400,000 IU) (121).

Furthermore, Martineau (122) has started a Research study which aims to investigate how diet and lifestyle might affect transmission of SARS-CoV-2, severity of symptoms and speed of recovery in 12000 individuals.

Conclusion

Immune dysregulation is an important characteristic of severe COVID-19. So, the repair of immune balance to stop the hyper-inflammatory cytokine storm is a practical strategy to fight disease severity in COVID-19. Though, conventional immunomodulatory therapies may be a two-edged sword as they can accidentally suppress protective immune responses. In this situation, vitamin D’s double roles of firstly controlling viral replication and later reducing the hyper-inflammation are tantalizing. 

In summary, in spite of controversial data, vitamin D intake and adequate 25OHD status may be linked with a lower risk of COVID-19 and an improved clinical result in infected patients. As there are no therapeutic drugs presently available for the treatment of the COVID-19, vitamin D is a low-cost and safe option which can be used as an accessory with the accepted therapeutic plan. 

Based on the present suggestion, it is not conceivable to establish an ideal serum 25OHD level and a quantity of daily intake that could possibly help with the prevention and treatment of COVID-19. Randomized control trial data are less, with diverse forms of intervention (cholecalciferol and calcifediol) and different assumptions. Though, maintaining an acceptable 25OHD status may be a good method to help to prevent COVID-19 and to lower the risk of severe consequences in infected patients. Further research on this subject is still needed.

Research by Dr. Disha Trivedi is a Medical Writer in MVS Pharma, Germany.