COVID-19 studies:  C19 studies: C19:  IvermectinIVM Vitamin DV.D HC QHC Q Vitamin CV.C ZincZn PVP-IPVP-I FLVFLV REGNR2 LY-CoVLY RemdesivirRMD
Vitamin D study #43 of 71
12/4 Analysis of outcomes based on serum levels
Katz el al., Nutrition, doi:10.1016/j.nut.2020.111106 (Peer Reviewed)
Increased risk for Covid-19 in patients with Vitamin D deficiency
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Retrospective database analysis showing patients with vitamin D deficiency were 4.6 times more likely to be COVID-19 positive, p<0.001.

Katz et al., 12/4/2020, retrospective, USA, North America, peer-reviewed, 3 authors.
risk of COVID-19 case, 78.4% lower, RR 0.22, p < 0.001, adjusted per study.

Effect extraction follows pre-specified rules prioritizing more serious outcomes. For an individual study the most serious outcome may have a smaller number of events and lower statistical signficance, however this provides the strongest evidence for the most serious outcomes when combining the results of many trials.
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