The role of vitamin D in reducing risk of COVID-19: a brief survey of the literature
by Dr. William B. Grant, PhD in Orthomolecular medicine vol 16 n 33 June 9 2020
(OMNS June 9, 2020) The evidence that higher vitamin D status is causally linked to lower risk of COVID-19 incidence, severity, and death continues to increase. This brief report outlines what has been learned through early June 2020 and provides links to some of the key references.
It should be noted that acceptance of the role of vitamin D supplementation will probably not be achieved before reports are published that demonstrate randomized controlled trials of vitamin D supplementation significantly reduced COVID-19 incidence or death. Several RCTs and observational studies regarding vitamin D supplementation and COVID-19 incidence and outcomes are either in the planning stage or in progress. The obvious groups to study are those at highest risk: dark-skinned people living at high latitudes, people in nursing homes or health care facilities; prisoners; factory workers such as in meat-packing facilities in the U.S.; health care workers. A major problem is that the powers that be see vitamin D as a threat to income and profit, so use the Disinformation Playbook to suppress positive information on vitamin D. 
In a review published in early April, it was proposed that vitamin D supplementation could reduce the risk of COVID-19. Two mechanisms were identified: 1, reduced survival and replication of viruses through vitamin D-stimulated release of cathelicidin and defensins, and 2, reduced risk of the cytokine storm by reducing production of pro-inflammatory cytokines. 
Reference was also made to the finding that vitamin D supplementation reduces risk of acute respiratory tract infections as demonstrated by randomized controlled trials.  It was recommended that vitamin D supplementation be aimed at increasing serum 25-hydroxyvitamin D [25(OH)D] levels to 40-60 ng/ml (100-150 nmol/l), which would require daily doses up to 4000 to 5000 IU/d vitamin D3. Magnesium should also be supplemented, perhaps 400 mg/d, since the conversion of vitamin D to different metabolites requires the presence of magnesium. This recommendation was based on findings in observational studies such as one conducted by Grassrootshealth.net on influenza-like illness. 
More recently, it was suggested that for those who have not been supplementing with vitamin D that they start supplementing with a large bolus dose of vitamin D of several hundred thousand IU within one-to-two weeks. The rationale is that without the bolus the body would otherwise take several months to achieve the optimum level.  It was also suggested that while vitamin D supplementation could stop COVID-19 from developing at the beginning of symptoms, it probably would not be very useful after lung and organ damage occurs in the acute stage. Most recently, evidence was outlined to show that vitamin D deficiency could explain much of the reason for higher case and mortality rates for Black, Asian, and Minority Ethnic (BAME) residents in England.