There were a few soft recommendations. Specifically I remember an ER doctor in 2019 saying vitamin D seemed to be a differentiator in the sample of cases he was seeing in the ER (everyone was starting to panic) and the CDC walking it back as unsubstantiated. I mentioned it at work, then 10 days later my boss's boss asked me where I had heard it, because he had heard the same thing.

There have been a number of people on HN who have attributed any measurable COVID benefit of Vitamin D, to a confounding variable, as recently as 3 months ago - https://news.ycombinator.com/item?id=44705486 The Big Vitamin D Mistake

One of the best treatments is interferon. It's something you will also produce yourself, with therapeutic effect, if exposited to sunlight or the infrared light used in red light therapy. Here's a video about it, from a continuing education provider: https://www.youtube.com/watch?v=oRkxH56LqCo

Vitamin D therapy doesn't have such an effect.

Because the right people couldn't make money off of it - same reason that a lot of beneficial treatments were not recommended or flat out defamed.

Would do...? Zinc taken the first day of symptoms may reduce the duration of a cold by a day on average. It's probably useful, but not certain.

Be sure to take zinc with meals, or your stomach may hate you.

That's Doctor Roger Seheult, MD, who hosts videos for continuing education provider MedCram (https://www.medcram.com/). They post lots of free videos on their YouTUbe channel (https://www.youtube.com/channel/UCG-iSMVtWbbwDDXgXXypARQ) and several of them cover the research behind heliotherapy.

Yeah, this is a crucial point. This is studying "adults with suboptimal baseline." If you have a vitamin deficiency and get sick, I would expect supplementation with that vitamin to provide some relief, regardless of which vitamin it is.

This does not mean that the same will happen for people who did not have a deficiency.

Having said that, there is good evidence that Vitamin D deficiency is widespread, and supplementation of Vitamin D is relatively safe unless you take excessive amounts.