Well, the bone health ones…
As somebody who is involved with this professionally I can tell you vast majority cannot be repeated , Most Vit D papers are published because it’s very easy research to do especially for medical docs and students . You take the Vit D biomarkers easily available and then just try to correlate it whichever way you fancy. It’s also easy to take a bunch of Viit D stats somebody else collected , go into a database for something else and try to find some correlation there (so you don’t even need to get your blood samples ).
The other reason is the supplementation industry.
Supplementation of vitamin D has not even been proven to increase health or life spans for people who were marked as Vit D deficient. There is a lot of controversy about the setting of cut off levels for Vit D deficiency. Some countries use different levels such as China. Some think D2 is important, others don’t.
What’s important to remember is that Vitamin D deficiency has been shown to be a marker for early mortality but only if you already have a serious chronic disease ie it’s an outcome not a cause. Are there some instances where supplementation might help ? I guess so but across the population there’s just no good evidence for that.
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An example of how somebody could link ‘Vit D deficiency’ with Covid mortality .
'Low vitamin D levels have been observed in a high number of covid deaths ’ screams the headline.
If you didn’t know that Vit D levels drop as you age you might as a casual reader jump to conclusions right?
But you could just as well say
'Wrinkles associated with a high number of covid deaths ’ and it would mean the same thing…
Because wrinkles and low Vit D are just thousands of things that come with the aging process.
We could link watching daytime TV to higher covid mortality too.
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Yes, they have tried to connect vitamin D to a host of health conditions. The company I work for has, in the past, asked my team to write up documents supporting associations with cancer, heart disease, and other conditions, but we never found consistent supporting evidence once you control for confounding variables such as race, income, obesity, smoking, diet, and underlying conditions to list a few. The exception is bone health, given that vitamin D is needed for calcium absorption.
There is some biologic plausibility for associations of vitamin D and Covid risk, and many observational studies have supported associations, which explains why there are so many clinical trials underway. I don’t think the associations are compelling at this point, but I understand that some could and do.
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Yes, but most epidemiologic studies at least attempt to control for confounding variables like age, sex, income, race/ethnicity.
It ravaged there because Italy is fully of old people who live in family units or crowded housing .
Not because of some spurious idea about low Vitamin D.
Old people generally have lower Vitamin D!
But guess what…It’s their shitty immune systems , their slower neutralising antibody production (amongst other issues ) that’s the real problem.
Oh yeah do they repeat the study?
Without repeating the study independently multiple times the results are often worse than garbage they are misleading.
Scientists and medical docs and students need to publish due to publication pressure. How many of these studies have you read ? Most of them are garbage and in not very good journals. I guess you know this already from above .
Literally you can go into pubmed and search Vitamin D and any disease ever and find any number of crappy papers trying to correlate them.
Pun/play on words intended? 
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I meant in the general sense. But that is how science progresses. One group does their best to carefully evaluate associations and then have their findings vetted through an imperfect peer-review system. Then they or others try to reproduce the findings in other populations. Most times the findings are not completely reproduced, sometimes even when real. And while waiting for definitive results, I go ahead and pop a supplement most days given the low risk of toxicity and the fact that I get them for free 
https://ijponline.biomedcentral.com/articles/10.1186/s13052-018-0590-x
That’s simply super low, children supposedly have higher vitamin D Levels than adults…
And it’s central/southern Italy where living conditions are really bad. Northern Italy is kinda the only region of Italy where some things still work not like 3rd world countries. Only public administration resembles 3rd world a lot…
Are red heads less susceptible to Covid 19? If yes, then that would point to a further link.
Many (but only a few related to Covid-19), and I agree with your general assessment of quality. I do think the link between vitamin D will turn out to be more correlation than causation, or maybe even reverse causation, but not convinced either way for now.
I did a search for Covid-19 and red hair, but this is all I got:
https://pubmed.ncbi.nlm.nih.gov/32720862/
Experimental Measurement of the Diameter of a Human Hair via Two-Color Light Diffraction
Cecilia R Dichtel et al. Psychiatry. 2020.
The width of a human hair sourced from a female elementary school student was measured by light diffraction using red and blue laser pointers. The two laser sources both provided consistent estimates of the hair diameter of approximately 50 μm. The overall experiment and writing process provided a temporary respite from COVID-19 shelter-in-place requirements and deteriorating spring weather that precluded outdoor activities.
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…but “world’s most clever scientific mind” is called for? Interesting 
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Red heads require far less sunlight to produce adequate amounts of vitamin D. An adaption, but of course are more vulnerable to skin cancer. That is why I asked. 
Rickets !
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I wasn’t thinking of studies such as the following - which are nearly entirely positive -
As much as randomized controlled studies which, of course, are going to be harder to come from. A study from Spain showed the Vitamin D group performed far better than control, an Indian study was promising but didn’t include anybody who really was sick and a study from Brazil showed…nothing. I don’t really know the reason and am not qualified to figure it out but it was a larger group of people and, thus, I said that studies had been mixed.
Might as well eat some fish and get a bit of sunshine regardless of Covid anyway.
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Well I just guess we gonna keep on waiting for results of Vitamin-D likely till the crisis is over this May/June. Same for Ivermectin btw…
On the other hand Sinovac got approved in many countries with super dubious studies/results… There seem so to be an consensus no vaccine, no matter how bad should be banned, but on the other hand downtalk any medication or anything that could actually improve public health…
Favipiravir same case (though I think the data is not good enough compared to Ivermectin/Vitamin D) - it works well in mild to moderate cases / low risk patients in quite a few double blind studies (significant earlier discharge from hospital) but outside of India and some other countries not much to be heard of.
Maybe many doctors just did it like the British health secretary. Lie about Vitamin D having no effect - no bribes or lobbying = no decisions…?: