There is an article on NIH by a Canadian physician (Dr. Gerry Schwalfenberg) who works in nursing homes. He has developed a protocol for reducing flu like illness among his patients. This should be tried against the corona virus in US hospitals:
"A colleague of mine and I have introduced vitamin D at doses that have achieved greater than 100 nmol/L in most of our patients for the past number of years, and we now see very few patients in our clinics with the flu or influenzalike illness. In those patients who do have influenza, we have treated them with the vitamin D hammer, as coined by my colleague. This is a 1-time 50 000 IU dose of vitamin D3 or 10 000 IU 3 times daily for 2 to 3 days. The results are dramatic, with complete resolution of symptoms in 48 to 72 hours. One-time doses of vitamin D at this level have been used safely and have never been shown to be toxic. We urgently need a study of this intervention. The cost of vitamin D is about a penny for 1000 IU, so this treatment costs less than a dollar."
Vitamin D for influenza
There is a presentation by Dr. Robert P Heaney on the healthful effects of adequate vitamin D levels:
Vitamin D Sunshine Optimal Health: Putting it all Together