Vitamin D3 is produced by the action of sunlight on the skin but in the northern hemisphere we don't get constant sunshine. When we are in the sun many of us use sunscreens that can minimise the production of vitamin D.
Vitamin D is available as a supplement in two forms - D2 and D3 (Cholecalciferol) D3 supplements are of the same type as created in our body by sunlight and raise serum levels of the nutrient 2 to 3 times higher than the same dose of D2.
Without sufficient levels of D3 in the blood, proper bone development and muscle contraction cannot take place.
A Vitamin D3 deficiency can lead to adverse medical conditions. Areas of our body, including breast, brain, prostate and colon tissue have receptor cells that are activated by theprecursor form of Vitamin D
There have been a number of reports and articles written in relation to the need for Vitamin D3 to help control corvid-19 including in todays Express newspaper https://www.express.co.uk/life-style/health/1340769/coronavirus-uk-latest-news-update-vitamin-d-symptoms 29th October 2020