I disagree.  I believe that the key is zinc deficiency.

Elderly are proven to be zinc deficient (NIH study) and are hit the hardest.
Black people are also proven to be zinc deficient (NIH study) and are hit harder than whites.

Red meat (beef) is the highest source of zinc.  Chinese don't get a lot of red meat, nor do Italians.
 
I don't believe that the emerging beef shortage is just a coincidence either.
 
All the dots lead to Zinc.

Zinc is probably essential in this, but Vit D acts as a hormone to modulate the immune system so it doesn't overreact as well. It can prevent the cytokine storm reaction that supposedly occurs in the lungs by this infection. Most of us are Vit D deficient anyway. The USDA is laughable.(400iu daily?) I take at least 10,000 IU of good quality ( non-soy sourced) Vit D a day with K2. I can't recall where I read it ,but  not one patient with a Vit D level above 30 mg/dl was hospitalized in those tested. The sweet spot is about 60-80 mg /dl.
If you are out in the sun with significant skin exposure at prime time ( 12pm-2pm) for 15 minutes, Your body will make 25,000 IU of Vit D itself. (You shouldn't burn at 15 minutes)

Sure, Vitamin D and Vitamin C are also key to fighting off any disease.