Rather stunning preprint out just a few days ago.

Vitamin D Treatment in Covid-19

Note: Calcifediol is 25-hydroxy Vitamin D, the substance your liver turns Vitamin D3 and D2 into and is measured in your blood.

Quote
All hospitalized patients received as best available therapy the same standard care, (per hospital protocol), of a combination of hydroxychloroquine (400 mg every 12 hours on the first day, and 200 mg every 12 hours for the following 5 days), azithromycin (500 mg orally for 5 days. Eligible patients were allocated at a 2 calcifediol:1 no calcifediol ratio through electronic randomization on the day of admission to take oral calcifediol (0.532 mg), or not. Patients in the calcifediol treatment group continued with oral calcifediol (0.266 mg) on day 3 and 7, and then weekly until discharge or ICU admission. Outcomes of effectiveness included rate of ICU admission and deaths.

Results
Of 50 patients treated with calcifediol, one required admission to the ICU (2%), while of 26 untreated patients, 13 required admission (50%) p value X2 Fischer test p < 0.001. Univariate Risk Estimate Odds Ratio for ICU in patients with Calcifediol treatment versus without Calcifediol treatment: 0.02 (95%CI 0.002-0.17). Multivariate Risk Estimate Odds Ratio for ICU in patients with Calcifediol treatment vs Without Calcifediol treatment ICU (adjusting by Hypertension and T2DM): 0.03 (95%CI: 0.003-0.25). Of the patients treated with calcifediol, none died, and all were discharged, without complications. The 13 patients not treated with calcifediol, who were not admitted to the ICU, were discharged. Of the 13 patients admitted to the ICU, two died and the remaining 11 were discharged.

Those spectacular results were from a large dose of Vitamin D in the form it circulates in the blood, but given orally. It looks like they did not measure the 25-hydroxy Vitamin D levels in the blood (though they measured a LOT of other things). Perhaps they thought that would break their "blindness" in the trial. But just to give you an idea, that's about 106 ng/ml. I used to take about that much D3 every day. I take a bit less now. But that was just the first dose. Their follow up doses are less than I take and given days apart!

Everybody got hydroxychloroquine and azithromycin, though these patients were actually too late to benefit from it, and they have now stopped using it.

But these results mean that high-dose (not over-dose) Vitamin D therapy is by far the most effective treatment we have for serious Covid-19.