It doesn’t seem like they controlled for any other variable at all - even time. From the paper: “Various literature databases like PubMed and Google Scholar were searched to find the mean of 25-hydroxyvitamin D [25(OH)D] levels in different states and union territories of India”. I would have to assume not all those studies were done at the same time.

This would maybe be a great elevator pitch to get some resources. Not sure if that’s the intent.

Relatedly, have there been proper double-blind studies for vitamin d injections & COVID outcomes? I would guess there’s not a lot of interest given other substances that have better “method of action” hypotheses. Or maybe it’s just lack of funding (hence we see an article like this every month or so)?