Isn't it that when someone is sick or their health is detriorating, the first thing that happens is that their body stops producing Vitamin D? And since they are not able to go out from that point it further reduces vitamin D?
Because they're not measuring post-COVID-exposure Vitamin D levels.
No, he's making an observation unrelated to covid. At any time you're sick your vitamin D production stops so you're deficient and then on top of that you don't feel good enough to go outside so your further deprived of vitamin D.