Vitamin D is an important component in the prevention and treatment of influenza23 and upper respiratory tract infections.24 While vitamin D does not appear to have a direct effect on the virus itself, it strengthens immune function, thus allowing the host body to combat the virus more effectively.25
As detailed in “Vitamin D Prevents Infections,” research shows high-dose vitamin D supplementation lowers the risk of respiratory illnesses and lung infections in the elderly by 40%. As noted by an author of that study, “Vitamin D can improve the immune system’s ability to fight infections because it bolsters the first line of defense of the immune system.”
As mentioned earlier, vitamin D also suppresses inflammatory processes and inhibits excessive production of proinflammatory cytokines that give rise to a cytokine storm.26 Taken together, this might make vitamin D quite useful against COVID-19, because while robust immune function is required for your body to combat the virus, an overactivated immune system is also responsible for the cytokine storm we see in COVID-19 infection that can lead to death.
Vitamin D upregulates production of human cathelicidin, LL-37, which has antimicrobial and antiendotoxin activities.27
Vitamin D supplementation has been shown to protect against acute respiratory infections.28 Daily or weekly supplementation (opposed to infrequent bolus doses) of vitamin D had the greatest protective effect in those with the lowest vitamin D levels.29
In one study,30 those with severe vitamin D deficiency who took a daily or weekly supplement cut their respiratory infection risk in half, whereas the acute administration of high bolus doses of vitamin D had no significant impact on infection risk.
Data analysis31 by GrassrootsHealth shows people with a vitamin D level of at least 40 ng/mL reduced their risk of colds by 15% and flu by 41%, compared to those with a level below 20 ng/mL.

In a study34 that looked at data from 780 COVID-19 patients in Indonesia, those with a vitamin D level between 20 ng/mL and 30 ng/mL had a sevenfold higher risk of death than those with a level above 30 ng/mL. Having a level below 20 ng/mL was associated with a 12 times higher risk of death.
Research35,36 posted on the preprint server MedRxiv June 10, 2020, reports a combination of vitamin D3, B12 and magnesium inhibited the progression of COVID-19 in patients over the age of 50, resulting in “a significant reduction in proportion of patients with clinical deterioration requiring oxygen support and/or intensive care support.”
“The Role of Vitamin D in the Prevention of Coronavirus Disease 2019 Infection and Mortality”37 — which looked at the average vitamin D levels and the number of COVID-19 cases and death rates in 20 European countries — found lower vitamin D levels correlated with higher caseloads and mortality. The authors concluded, “We believe that we can advise vitamin D supplementation to protect against SARS-CoV2 infection.”
Northwestern University researchers report finding an inverse relationship between vitamin D and CRP, a marker for inflammation. Those with higher CRP had lower vitamin D and vice versa. According to the authors:38
“COVID-19 patient-level data shows a notable OR of 3.4 … for high CRP in severe COVID-19 patients.

https://articles.mercola.com/sites/articles/archive/2020/07/04/coronavirus-prevention-vitamin-d.aspx