Scientists are looking for volunteers to take part in a trial to see if taking vitamin D can give the immune system a boost against COVID.
People who join would be sent pills in the post to take daily for six months if a finger-prick test shows they are deficient in the "sunshine vitamin".
UK residents are already advised to consider taking supplements over winter when Vitamin D levels can dip.
That is to improve general health, not specifically to stop infections.
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The trial, led by researchers from Queen Mary University of London and funded by Barts Charity, will use higher doses of Vitamin D than regular supplements.
Principal investigator David Jolliffe said the trial "has the potential to give a definitive answer" to the question of whether Vitamin D offers protection against COVID.
"Vitamin D supplements are low in cost, low in risk and widely accessible; if proven effective, they could significantly aid in our global fight against the virus," he said.
Although Vitamin D supplements are very safe, taking more than the recommended amount every day can be dangerous in the long run.
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