I really like the Cochrane reviews, and love that they periodically update areas when there is more recent research. And you are correct, their articles on Vitamin D have become less enthusiastic with time. This makes sense I think, since the initial publications on a topic will all be "positive" - they have to find something new to get published (creating a publication bias). And they do try to account for publication bias, but really the best way to fix it is to have more studies. With Vitamin D getting so much attention, you can get published whether you find a correlation or not, so you have a wider range of studies to look at. I wanted to give a summary of the Cochrane reviews and the largest human trials of Vitamin D in the next blog post.

Not a physician, but still following this story closely. Can I ask you docs to weigh in on the recent results regarding the role of vitamin D in covid-19 outcomes?

This is a correlation. There are lots of conditions that are associated with low Vitamin D. Mortality overall is associated with low Vitamin D. But it doesn't mean that the low Vitamin D caused any of these problems. Having overall poor health leads to Vitamin D deficiency. The increase in mortality is more likely related to your overall health status, not your Vit D status.

Do you know anything about the link between schizophrenia and Vit. d?

Don't expect to find anything beyond a correlation between VitD levels and any serious disease. In any study that such is found, the substitution had no effect.