A study has shown a possible link between higher vitamin D levels and lower virus numbers in northern Europe where vitamin D rich supplements are consumed like cod liver oil . The countries with higher mortality rates are also known to have lower vitamin D levels in southern Europe.
https://www.sciencedaily.com/releases/2020/05/200507131012.htm
https://www.sciencedaily.com/releases/2020/05/200507131012.htm