The author's repeatedly demonstrated inability to discern valid information from not makes them fundamentally incapable of conducting such a review. Whether that's out of malice or incompetence, the result is the same.
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They've repeatedly promoted debunked information and fraudulent studies on their Twitter, and a quick skim of the sources for this paper indicates that they've cited such "studies" in this paper.
For example, the "Dr. Mercola" previously exposed as a fraud is...cited here, citation #84. And what a shock, his "study" was also published in this same journal and remains unretracted by it.
"Results: One population study and seven clinical studies were identified, which reported D3 blood levels preinfection or on the day of hospital admission. The two independent datasets showed a negative Pearson correlation of D3 levels and mortality risk (r(17) = −0.4154, p = 0.0770/r(13) = −0.4886, p = 0.0646). For the combined data, median (IQR) D3 levels were 23.2 ng/mL (17.4–26.8), and a significant Pearson correlation was observed (r(32) = −0.3989, p = 0.0194). Regression suggested a theoretical point of zero mortality at approximately 50 ng/mL D3. Conclusions: The datasets provide strong evidence that low D3 is a predictor rather than just a side effect of the infection."
A review of studies on subjects.
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In addition I see one of the authors's Twitter promoting other discredited research and retracted/removed papers over the past year, such as the now infamous "Dr. Mercola".
https://mobile.twitter.com/bernd_glauner/status/127549493530...
https://www.nytimes.com/2021/07/24/technology/joseph-mercola...
I don't see any reason to think this author is capable of actually conducting a review of this topic, given that they are clearly easily misled by misinformation on the topic (to be charitable).