Best article on vitamin D 

I cannot emphasize enough that using enough vitamin D supplementation is absolutely critical for fighting COVID and many other infections and medical problems. Not only should you read the following article, which is long and complex, but you also need to convince others about the importance of vitamin D. This article provides many details on how to use vitamin D and how it protects your health. 

Rapidly Increasing Serum 25(OH)D Boosts the Immune System, against Infections—Sepsis and COVID-19

Conclusions 

A robust immune system is essential to overcome infections without complications. It depends on the adequate entry of vitamin D3 and 25(OH)D into immune cells for generating calcitriol. The latter required maintaining a serum 25(OH)D concentration of over 50 ng/mL. Therefore, to successfully manage and overcome an infectious epidemic or a pandemic, it is crucial to maintain the population's serum 25(OH)D concentration above the mentioned therapeutic level. 

In acutely ill persons, especially those with vitamin D deficiency having infections, raising serum D3 and 25(OH)D concentrations quickly is paramount and life-saving. In these urgent situations, 0.5 to 1.0 mg of calcifediol can raise serum 25(OH)D concentrations above the minimum therapeutic levels of 50 ng/mL in four hours and boosts the immune system within a day that facilitates to overcome infections. 

While calcifediol raises serum 25(OH)D within hours, the oral administration of even high doses of vitamin D takes three to five days to raise serum 25(OH)D concentrations. This delay is due to its less efficient absorption than calcifediol and the need for vitamin D to undergo 25-hydroxylation in the liver, a rate-limiting step. In acutely ill patients, as in those in the ICU, administering even high doses of oral D3 may take a week to increase serum 25(OH)D concentration. Therefore, it is unhelpful in emergencies like SARS-CoV-2 infections.