ROLE OF VITAMIN D SUPPLEMENTATION IN THE PREVENTION OF INFECTION AND SEVERE COURSE IN COVID-19: TESTING THE HYPOTHESIS

The coronavirus disease 2019 (COVID-19) pandemic has disrupted the normal activities of various settings, including clinics, laboratories, and libraries. As the world deals with the fast-mutating causative virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), apart from the search for the best vaccine candidate, efforts towards repurposing existing molecules to save lives must continue. Considerable interest has centered around the implications of vitamin D deficiency and its supplementation on the outcomes in patients with COVID-19. We hypothesize that vitamin D supplementation has the potential to confer protection against SARS-CoV-2 infection and a severe COVID-19 course. Various animal, human observational as well as interventional studies have shown a protective role of vitamin D in COVID19. More robustly designed studies where vitamin D is supplemented prophylactically and administered to those already infected are needed to determine the precise contribution of this supplementation in preventing SARS-CoV-2 infection and modifying the course of COVID-19.


INTRODUCTION
As the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutates, we now know of variants that are more transmissible than their predecessors [1,2]. Most vaccines have shown impressive efficacy in various phases of the trials, but their efficacy against the mutant strains remains to be established [2][3][4]. While efforts towards vaccination must continue, an eye should remain on repurposing existing drugs towards minimizing the damage caused by coronavirus disease 2019 (COVID-19).
There have historically been studies that have implicated vitamin D deficiency (VDD) as a factor influencing the development or worsening of infections, inflammation and allergies. Studies have identified VDD as a risk factor for community-acquired pneumonia and it has also INFECTIOUS DISEASES HYPOTHESIS REVIEW been associated with worse outcomes, including higher mortality in respiratory infections and sepsis [5,6]. Researchers have observed that patients with tuberculosis have significantly lower vitamin D levels, and higher levels reduced time to sputum and culture conversions, thus vitamin D supplementation should be considered along with anti-tubercular therapy [7][8][9]. Evidence has emerged that vitamin D supplementation potentially mitigates SARS-CoV-2 infection and reduces the severity of COVID-19. Observational data have indicated a protective role for vitamin D with regard to SARS-CoV-2 infection and COVID-19 [10,11]. The pandemic restrictions and stay-at-home orders have increased the prevalence of VDD [12]. Furthermore, observational data suggest a higher rate of infections and poorer prognosis in those with VDD [13].

HYPOTHESIS
Our understanding of this vitamin, in-vitro models, and recent research suggest that vitamin D supplementation may have roles from preventing SARS-CoV-2 infection to reducing COVID-19 severity, as well as in mitigating the inflammation induced in the later phase of the disease. We hypothesize that vitamin D supplementation has the potential to confer protection against SARS-CoV-2 infection and severe COVID-19.

HYPOTHESIS TESTING
Experimental models have revealed that 1,25dihydroxycholecalciferol (calcitriol), the active form of vitamin D, downregulates the expression of angiotensinconverting enzyme 2 [14]. Therefore, it can reduce the entry of SARS-CoV-2 into the cells and is thus likely to be of enhanced benefit in those with diabetes. This means that vitamin D is likely to have a role before a person contracts SARS-CoV-2 infection, and also after one contracts the infection and before they develop COVID-19. Cathelicidin antimicrobial peptide (CAMP) has an antioxidant activity, besides its role in destroying microbial membranes. It has been shown to reduce the severity of lung injury. In infections, vitamin D is converted to the active form in the alveolar epithelial cells, which then upregulates the production of CAMP [15]. It also reduces vascular permeability and cell apoptosis [16]. Calcitriol downregulates Th1 cells, tumor necrosis factor-alpha, nuclear factor-κB, and interferongamma [17][18][19][20]. It also reduces the production of interleukins, thus it has a potential role in dampening the cytokine release syndrome [21]. The various possible mechanisms through which vitamin D can provide protection in COVID-19 are shown in figure 1.
Ecological studies found a higher incidence of SARS-CoV-2 positivity in countries with a lower national average 25-hydroxyvitamin D (25-OHD) levels [10]. Researchers also attempted to find the relation between the latitudinal position of countries and the number of SARS-COV-2 cases [22]. A Swiss study published as early as May 2020 revealed that subjects who had tested positive for SARS-CoV-2 had comparatively lower 25-OHD levels [13]. A few months later, a larger study from the US found that SARS-CoV-2 positivity was inversely related to circulating 25-OHD levels [11]. A study of over 80,000 subjects in the North West of England who had a documented 25-OHD level in the preceding 12 months found that VDD was associated with an increased risk of hospitalization [23]. The initial human studies were mostly retrospective, which by their very nature have certain limitations [24].
In late 2020, a randomized, placebo-controlled study was conducted on SARS-CoV-2 positive, asymptomatic or mildly symptomatic individuals in India. The authors found that a significantly higher percentage of those who received 60,000 IU of cholecalciferol daily for 7 days (versus those who received placebo) tested negative on day 14 and also had lower fibrinogen levels [25]. A pilot randomized trial in Spain on patients hospitalized with COVID-19 compared those who received the best available treatment with those who additionally received calcifediol on days 0, 3, and 7, and then weekly. Two percent (n = 1) in the calcifediol group versus 50% ( n = 13) required transfer to the intensive care unit (ICU) [26]. A recently published study on 103 COVID-19 in-patients in North Italy found that the severely ill had a mean 25-OHD level of 18.2 ± 11.4 ng/mL versus 30.3 ± 8.5 ng/mL in those who remained mildly symptomatic. The levels correlated inversely with interleukin-6 levels, need for transfer to the ICU, and mortality [27]. Multivariate analysis of an Israeli cohort found that 25-OHD levels were independent risk factors for COVID-19 and hospitalization [28]. A quasi-experimental study on the frail elderly in France found that bolus doses of vitamin D given regularly resulted in less severe disease and improved survival [29]. At the same time, there have been some studies, including a randomized trial that failed to show benefit with vitamin D supplementation in COVID-19 [30].
Ongoing trials and studies in the coming days are likely to shed more light on the role of vitamin D in the context of COVID-19. COVIDENCE UK is a large prospective study that plans to recruit over 12,000 subjects. Those who join the study would fill a vitamin D status and other risks assessment questionnaire, and will then be followed up to document the development of SARS-CoV-2 infection and COVID-19 [31]. There are ongoing trials on asymptomatic, mildly symptomatic, and nonhospitalized patients who have tested positive for SARS-CoV-2. While one such trial is studying the role of a single bolus dose in reducing the time taken to test negative after a positive test, another trial is testing the efficacy of smaller doses given daily for 28 days in reducing hospitalisations and mortality [32,33]. Community-based, as well as hospital-based studies, are needed to precisely delineate the place and impact of vitamin D supplementation on SARS-CoV-2 infection and COVID-19. Studies for determining the prevalence of vitamin D deficiency, estimation of serum levels, and meticulous registries of SARS-CoV-2 infections will help determine the precise nature of the association of vitamin D levels and risk of infection. A negative correlation would imply a role of vitamin D supplementation in SARS-CoV-2 prophylaxis. Casecontrol studies comparing COVID-19 hospitalized patients with and without VDD will help determine its influence on the course of the disease. Similarly, randomized trials recruiting patients admitted with COVID-19 will help determine the impact of vitamin D supplementation on outcomes in COVID-19.
The role of VDD in COVID-19 is of particular interest to the countries of the region as studies have shown that VDD is far more prevalent in Central Asia [39,40].
Researchers have also noted a dearth of studies on the subject from this region, highlighting the relative lack of concern and awareness [41].

ETHICAL CONSIDERATIONS
There has been a lack of awareness and due attention to vitamin D levels, and their correction has not been accorded. The role of vitamin D in health outside of the musculoskeletal system remains a not-so-widely known subject. While the stay-at-home orders are being enforced to limit the spread of infection, they lead to a decline in the serum levels of 25-OHD, and necessary awareness needs to be raised and steps taken to prevent VDD. Further studies may also provide the rationale for checking the serum levels of 25-OHD in those admitted for COVID-19, and for making corrections.

CONCLUSION
While the jury is still out on the role of vitamin D supplementation in preventing SARS-CoV-2 infection and a severe COVID-19 course, it might be pragmatic to supplement this vitamin as per the national guidelines. At such doses, there does not appear to be a significant risk of adverse effects or toxicity and are outweighed by the potential benefits.