Vit D deficiency studies, proper studies, are probably a victim of this kind of thinking...

https://youtu.be/MGboD4KAgbM?t=3430

Why the studies have not been done yet!

Edit: 25sep2021
During the pandemic, thought it was just greed of so much money (Hundreds of BILLIONS) stopping Ivermectin approval, but because it works with Vit D and zinc and makes your immune system work better, it's going to work vs so much more, other diseases, huge hit to the "bottom line" of the pharma industry

But health should matter first!!!