Introduction

The outbreak of COVID-19 started at Wuhan City of China in December 2019. It has spread at a remarkable speed to countries all around the world. There is now a race to find an effective vaccine and therapy. COVID-19 is an enveloped, single-stranded, positive-sense RNA virus, which belongs to the coronavirus family. The crown-like appearance of COVID-19 is due to the presence of spike glycoproteins on the surface envelope(1). It is the seventh member of the coronavirus family that is able to infect humans(2). This virus is phylogenetically related to previously known severe acute respiratory syndrome coronavirus (SARS-CoV) and the Middle East respiratory syndrome coronavirus (MERS-CoV)(3). Symptoms associated with COVID-19 are fever, persistent dry cough, breathlessness and lethargy; with a minority of patients suffering from headache, haemoptysis, diarrhoea, erythematous rash and urticaria(4-6). Severe cases of COVID-19 may progress to pneumonia, acute respiratory distress syndrome (ARDS) and multi organ failure(4). In terms of mortality rate, COVID-19 ranked the third in the family of coronavirus related death, compared to MERS-CoV with a reported mortality rate of 37% and SARS-CoV of 10%(5). To date, there are over four millions confirmed cases of COVID-19, with a fatality rate of just over 300,000(7). At present, there is no definitive therapy for COVID-19. Most of the therapeutic options come from previous experience in managing SARS and MERS epidemics. Different regimes have been proposed, including the use of type 1 interferon (IFN-I), hydroxychloroquine, Ritonavir/lopinavir, Remdesivir, and neutralising antibodies (Nabs)(8).While waiting for the development of an effective vaccine, there has been ongoing research looking at alternatives that may help in the management of COVID-19 patients. Vitamin C, vitamin D, and Zinc have all been investigated as potential adjuvant therapies(9, 10).
Vitamin D in particular, has been in the spotlight recently. The prevalence of vitamin D deficiency is increasing, with some studies labelling it as a pandemic(11, 12). There is now an abundance of evidence in the literature highlighting the consequences of vitamin D deficiency, and its association with a variety of acute and chronic illnesses including autoimmune disease, cardiovascular disease, cancers, diabetes mellitus and neurological disorders(13, 14). Traditionally, vitamin D is known as the “sunshine vitamin”, sunlight being the main source. 7-dehydrocholesterol in the skin is converted to cholecalciferol. It then undergoes further hydroxylation in the liver and kidney before becoming the biological active form, 1,25-dihydroxyvitaminD (1,25(OH)2D), also known as calcitriol(15). 1,25(OH)2D activates the vitamin D receptor (VDR), which is a nuclear receptor, to exert its functions. It is now known that VDR is highly expressed in most cell types, suggesting the myriad of regulatory roles vitamin D plays in maintaining healthy functioning of our body(16) (Figure 1). In particular, there is strong evidence supporting the role of vitamin D in regulating the immune system(17).
Unfortunately, sunlight alone is a not a reliable source of vitamin D, especially among those living at high latitudes, ethnicity with darker skin colour and the institutionalised population(18). In high human density habitations, a lack of open spaces and high pollution has led to a decrease in direct sunlight exposure(18). To counteract this problem, vitamin D supplementation has been recommended. Research studies have shown that vitamin D supplement is effective in raising serum level of vitamin D(19, 20). Vitamin D supplement helps to strengthen the immune response and plays a vital role in respiratory illness prevention, among its many other health benefits(17, 21). This paper discusses the roles of vitamin D in reducing the risk of viral infections and mitigating the severity of disease progression. More importantly this paper investigates the feasibility of using vitamin D as an adjuvant therapy in alleviating some of the symptoms associated with pneumonia and acute respiratory distress syndrome (ARDS) that lead to a poorer outcome among COVID-19 patients. This paper also discusses whether vitamin D could play any role in reversing disease progression and the prevention of COVID-19 in the general population.