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 #42 of 71
11/30 Pre-Exposure Prophylaxis study (treated before exposed to the virus)
Louca et al., medRxiv, doi:10.1101/2020.11.27.20239087 (Preprint)
Dietary supplements during the COVID-19 pandemic: insights from 1.4M users of the COVID Symptom Study app - a longitudinal app-based community survey
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Survey analysis of dietary supplements showing vitamin D usage associated with lower incidence of COVID-19.
These results are for PCR+ cases only, they do not reflect potential benefits of vitamin D for reducing the severity of cases.
A number of biases could affect the results. Users of the app are likely to be quite different to the general population. People experiencing symptoms may be more likely to install and use the app. The real effect could be significantly larger, for example some participants may have been motivatated to start using supplements after experiencing symptoms.

Louca et al., 11/30/2020, retrospective, United Kingdom, Europe, preprint, 26 authors, dosage not specified.
risk of COVID-19 case, 8.0% lower, RR 0.92, p < 0.001, United Kingdom.
risk of COVID-19 case, 24.0% lower, RR 0.76, p < 0.001, treatment 19444, control 26313, United States.
risk of COVID-19 case, 19.0% lower, RR 0.81, p < 0.001, treatment 6722, control 20651, Sweden.

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.
Details of all 71 studies
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