It could also be that many of us have seen studies over the past few years about the general issues with Vitamin D deficiencies, so, in addition to the recent studies; it doesn't seem like such a leap that something as simple as increasing Vitamin D could benefit covid protection/recovery.
Right. We know most people have lower levels in winter. There is reason to believe it helps. There is little reason to think it may harm - particularly in light of the seasonal deficiency. Why would anyone not think it's worth trying? I've been wondering why the CDC hasn't recommended it yet.
I would guess that it depends on what your current vit D level is. Also, depending on your genotype you might be a fast or slow metabolizer of vit D, which would also play a role in how big of a dose you would need.
I recommend the talks/interviews by Dr. Stasha Gominak a neurologist specializing in sleep.
I've been doing 4,000 iu/day of Vitamin D since March, also supplementing Zinc and C, and taking a standard multivitamin. My entire household got COVID in November and I never became symptomatic.
Both masks and Vitamin D are helpful and those at risk or in areas of great spread should consider both. This is similar to masks and face shields offering a multiplier effect. The big difference is that sufficient Vitamin D levels are strongly correlated with numerous metrics of good health including immune, metabolic, and muscle-skeletal system measures.
Just how much lower it would get cannot be discerned from available data yet. It is very hard to believe from current data that it would go all the way to zero and also very hard to believe that fixing all vitamin D deficiency would not save any lives. The confidence interval range in between is still large, but it is absolutely (past) time to act on the data because it is so unlikely to not help (or even not help a lot). And the data is so far consistent with the possibility that it could help so much that it would have reduced the pandemic to no worse than prior flu seasons had everyone's vitamin D been brought up above 30ng/ml before SARS-CoV-2 arrived.
As someone who first learned that healthy Vitamin D levels were known to decrease respiratory illness severity (regardless if they help with COVID) I have mixed feelings about the repeated front-page posts. On one hand, I think it's useful information to know. On the other hand, it starts feeling a little weird. Like that person you knew in high school who claimed that some marijuana product cured basically any illness. It feels like it makes it lose credibility.
Be careful not to conflated this letter with studies. If 142 letters were posted, I'd be right there with you. AFAIK(I haven't checked them all) the other links are mostly studies.