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.