Irish say boosting Vitamin D is cheap and may help lower COVID-19 deaths:

https://medicalxpress.com/news/2020-05-vitamin-d-severity-covid-advice.html

Sample quote

The authors propose that, whereas optimising vitamin D levels will certainly benefit bone and muscle health, the data suggests that it is also likely to reduce serious COVID-19 complications. This may be because vitamin D is important in regulation and suppression of the inflammatory cytokine response, which causes the severe consequences of COVID-19 and 'acute respiratory distress syndrome' associated with ventilation and death.

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