That's the conclusion of a study published in May 2020 in medRxiv. Called a DMB protocol, the study had a control group which didn't receive DMB and one which received one daily dose of vitamin D3 1000 IU, magnesium 150mg, and vitamin B12 500mcg for up to 14 days. The researchers found that only 3/17 patients in the study arm required supplemental oxygen, compared to 16/26 in the control group.
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(source: https://www.news-medical.net/news/20200604/Study-says-vitamin-magnesium-combo-may-reduce-severity-of-COVID-19-in-seniors.aspx)