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Here is the study: https://www.bmj.com/content/356/bmj.i6583
The study is a tough read, so here is a video explaining the results: https://www.youtube.com/watch?v=W5yVGmfivAk
Since coronavirus is a respiratory illness, this could be a tool in slowing it's spread. Especially since most people get their D from UV light, which is in short supply in northern regions right now.