https://academic.oup.com/jcem/article/96/7/1911/2833671
You can see that prevalence of deficiency is very high, as this other peer-reviewed study confirms:
https://pubmed.ncbi.nlm.nih.gov/21310306/
In the first article, after correcting the deficiency, the maintenance dose that they give is: 600-1000 IU/d for children; 1500–2000 IU/d for adults; 3000–6000 IU/d for obese. As you can see that's much higher than the current RDA of 400 IU/d.
And to correct the deficiency in the first place, the dose needs to be 2-3 times higher still for 8 weeks.
So around half the people in the US need to be taking a very large dose for 8 weeks, and then a maintenance dose 4-15 times higher than the RDA.
This is all from established studies done long ago that no one has disputed. But their recommendations have yet to be implemented in a lot of places, which might be why some countries fare better than others.
Indeed the first sentence talks about "corona deaths." No qualified medical professional would write that in a paper.
Study confirms vitamin D protects against colds and flu
https://news.harvard.edu/gazette/story/2017/02/study-confirm...
If there is any truth in what the authors claim, the best they could do is to work with field experts and/or submit their findings to a reputable journal for review and publishing. Someone's health might be on the line following pseudo-scientific works.