Background: The recently identified SARS-CoV-2 coronavirus has resulted in the
Covid-19 pandemic with severe morbidity and high mortality, particularly in certain sections of the
population. The co-morbidity patterns associated with adverse outcomes are multiple and complex
and there is emerging epidemiological, nutritional and molecular biological evidence that an inadequate
vitamin D status is a contributing factor.
Objective: The aim was to review the role of vitamin D in immune function with particular reference
to the mechanisms whereby it supports immune efficiency, host protection and immune modulation.
The evidence for the possible benefit of vitamin D supplementation to ameliorate the severity
of respiratory infection by SARS-CoV-2 and other pathogens was also reviewed with a view to
making a recommendation.
Methods: PubMed, MEDLINE and Google Scholar were searched using the terms: Covid-19, coronavirus,
SARS-CoV-2, vitamin D, calcitriol, deficiency, adaptive immunity, innate immunity, ventilation,
critical care, intensive care, acute respiratory distress syndrome, cytokine storm, respiratory
viruses, respiratory tract infection, respiratory syncytial virus, influenza, supplementation. Papers
for inclusion were selected on the basis of relevance and quality.
Findings: Vitamin D insufficiency is widespread in many parts of the world. Vitamin D is needed
for normal protective and surveillance immune function and there is evidence that deficiency increases
the risk of some respiratory infections, probably including Covid-19. By binding with dedicated
receptors on immune cells vitamin D influences several strands of immune function, including
the production of anti-microbial peptides and several cytokines that promote an appropriate immune
response. Vitamin D supplementation probably reduces the risk of respiratory infection, with
persuasive biological, epidemiological and observational evidence for possible benefit against
Conclusion: Despite the lack of direct evidence specific to Covid-19 a cogent theoretical case can
be made for giving adults from selected groups, and arguably all adults, routine supplementation
with vitamin D to improve immune efficiency and reduce the incidence and severity of respiratory
infections. This could be particularly important in sections of the population with a high prevalence
of vitamin D insufficiency. Targeted research is required to provide firm evidence to guide practice.