Although I quite agree that nothing is proven, the Vitamin D issue potentially explains another interesting characteristic, that being when populations move inside (away from the sun), rates of hospitalization due to COVID seem to increase.
When the Northeast US and Europe was stuck inside during winter, the virus ran rampant; when summer came, and those areas started to spend more time outside (producing more Vitamin D), their infections dropped off. At the same time, when the heat hit the south and the southerners moved inside to their air conditioning, they had a significant uptick in cases while the problem appeared solved in the Northeast and Europe.
Now that Europe is starting to move inside again, their cases are rising again.
I had thought at the time that air conditioning and heating systems were showing themselves to be a primary conduction mechanism for the virus. But that may not be the only factor at work here.
I certainly wouldn't claim correlation in this respect to be perfect, because it's not, and nor would you expect it to be, because individuals have such differing behaviors when weather starts to change.
However, if the winter brings a second wave in Europe and the Northeast US.....lack of sunlight may well be one of the variables involved in that equation.