My first reaction was what the f*ck are these guys trying to show. Its all over the place. They are using lots of fancy terms to hide the fact that its a badly constructed piece of research. Its basically a meta paper masquerading as a primary research paper. There are two things that immediately set off my BS detector. Very small numbers. < 300. And very big numbers. > 10K. This paper has weird combination of magnitudes. Plus other problems.
Then there are lines like this
For all outcomes, a COVID-19 infection defined as a positive SARS-CoV-2 infection (e.g., RNA RT-PCR or serloogy test), electronic health record evidence of SARS-CoV-2 infection (using International Classification of Diseases or physician notes), or self-reported infections from the patients.
Which means that when combined with the non screening of subjects for initial Vitamin D deficiency state the paper is basically junk. Mathematically speaking. Like so many of these papers.
Its basically this guy…
Who is a doctor who has a BSc in Math from McGill. Just had a look at the undergrad syllabus. Not impressed. Very lightweigh compared to 30 / 40 years ago. Or what Mathematics under-grads have to do in France, Germany or Russia.