This, and as someone who recently found out that they have a severe vitamin D deficiency, I was surprised to know that it's not something normally checked for in yearly blood tests.
Check the videos on Dr John Campbell's (PhD nurse educator in the UK) web site. The UK national health service has good medical records that can be cross-referenced and cross-linked every which way. Someone examined the historical records, found a reasonably large number of persons who had vitamin D levels checked before the pandemic, and checked those statistics vs the Covid-19 experience of the same people. (Imagine living in a country where that could happen.) IIRC, this did find some benefits wrt COVID-19 of not having a very low vitamin D level.
