According to this study, it makes a lot of sense to supplement with Vitamin D to reduce the risk and impact of Korona. But when supplementing with Vitamin D recommendation is usually to also include Vitamin K2 (to avoid artery calcification).

My guess is that demand for vitamin K2 from NattoPharma is likely to grow because of Korona.
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Vitamin D supplements protect against acute respiratory infections including colds and flu, according to a study led by Queen Mary University of London
Date: February 16, 2017
Source: University of Queen Mary London

Daily or weekly supplementation halved the risk of acute respiratory infection in people with the lowest baseline vitamin D levels, below 25 nanomoles per litre (nmol/L). However, people with higher baseline vitamin D levels also benefited, although the effect was more modest (10 per cent risk reduction). Overall, the reduction in risk of acute respiratory infection induced by vitamin D was on a par with the protective effect of injectable 'flu vaccine against 'flu-like illnesses.

https://www.sciencedaily.com/releases/2017/02/170216110002.htm