Table 1.

Proposed mechanisms whereby vitamin D reduces risk of COVID-19 (note, order of mechanisms should be carefully considered, perhaps placing more important ones near the beginning).

Effect	Mechanism	Reference
Inactivates viruses	Induction of cathelicidin	[47]
Reduces risk of cytokine storm	Reduces concentration of proinflammatory cytokines and increases concentration of anti-inflammatory cytokines	[24]
Reduces risk of cytokine storm	Induces T regulatory cell production	[27]
Reduces risk of pneumonia	Reduces risk of endothelial dysfunction	[48]
Increases the metabolic tolerance of the host to damage inflicted by the pathogen infection	Reduces matrix metalloproteinase-9 concentrations	[49]
Reduces free SARS-CoV-2 concentrations	Increases soluble ACE2 concentrations that can bind to SARS-CoV-2	[50]
Anti-viral effects	Balanced differentiation of effector CD8 and CD4 T cells	[51]
Reduces risk of myocarditis	Reduces concentration of catecholamines	[52]
Reduces risk of myocarditis	Inhibits RAS	[53]
Reduces risk of vascular dilation and permeability and hypotensin	Inhibits RAS-mediated bradykinin storm	[46]
Protects against the effects of histamines such as acute immune-mediated reactions [54], lung dysregulation [55], increase in Th2 and decrease in Th1 cytokines [56], and thus susceptibility to respiratory tract infections [57]	Preserves stability of mast cells, which can release histamine when activated.	[58]
Promotes adaptive immunity	Regulations of T cell proliferation	[27]
Neuroprotection	Reduces inflammation and oxidative stress	[59]
Protection against exacerbation by other viruses	Reduces risk of Epstein–Barr virus infection	[60]

Abbreviations: SARS-CoV-2 = severe acute respiratory syndrome coronavirus 2; ACE2 = angiotensin-converting enzyme 2; RAS = renin angiotensin system.