Here's the paper mentioned in (a) above that's the source of the claim:
[1]
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3585561Here is another paper with similarly stark data:
[2] https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3571484
Table 1 in each of those papers show Vitamin D status vs Outcomes. The correlation between Vitamin D status, where Normal is >30 ng/ml (or 75 nmol/L), and death rates is stark.
From [1] (which had n=780 cases) here is the punchline:
"98.9% of Vitamin D deficient cases died while only 1.1% of them were active cases. 87.8% of Vitamin D insufficient cases died while only 12.2% of them were active cases. Only 4.1% of cases with normal Vitamin D levels died while 95.9% of them were active cases."
From [2] (which had n=212 cases) here is the punchline:
"Of the 212 (100.0%) cases of Covid-2019, 49 (23.1%) were identified mild, 59 (27.8%) were ordinary, 56 (26.4%) were severe, and 48 (22.6%) were critical (Table 1). Mean serum 25(OH)D level was 23.8 ng/ml. Serum 25(OH)D level of cases with mild outcome was 31.2 ng/ml, 27.4 ng/ml for ordinary, 21.2 ng/ml for severe, and 17.1 ng/ml for critical."
Note: the classification for outcomes was "(1) mild – mild clinical features without pneumonia diagnosis, (2) ordinary – confirmed pneumonia in chest computer tomography with fever and other respiratory symptoms, (3) severe – hypoxia (at most 93% oxygen saturation) and respiratory distress or abnormal blood gas analysis results (PaCO2 >50 mm Hg or PaO2 < 0 mm Hg), and (4) critical – respiratory failure requiring intensive case monitoring."
I want to see a dozen more studies like [1] and [2] to see if this holds up to replication.
The show notes for the parent video are quite comprehensive with links to references. The summary above leaves out the detailed discussion of the interplay between Vitamin D, the renin-angiotensin-system, the ACE2 receptor, and SARS-CoV-2:
[3] https://www.foundmyfitness.com/episodes/vitamin-d-covid-19