We already know but
Vitamin D Linked to Low Virus Death Rate Says New Studyhttps://www.newsmax.com/health…-19/2020/05/07/id/966554/
Vitamin D has been shown to protect against acute respiratory infections, and older adults, the group most deficient in vitamin D, are also the ones most seriously affected by the virusscientists have found people in countries that have the highest mortality rates from COVID-19 like Italy and Spain, also had the lowest levels of vitamin D. Conversely, the highest levels of vitamin D were found in northern European countries, like Scandinavia, which was among the countries with the lowest mortality rates
Well, I am not sure we already know it. The link you cited is a correlation. Correlation is not causation. It could be that people who eat more vitamin D or get out in the sun more are the healthy ones. Then what? As always, what is needed is a prospective, randomized, placebo controlled trial. I have looked for some but didn't find them. I've read of claimed success for treating bad influenza cases with vitamin D but there are conflicting data about that. Caveat emptor. Vitamin D is pretty harmless but not entirely so in very large quantities. One thing it may cause is calcifications where you don't want them. That evidence is wobbly too. It's difficult to know.