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.
Vitamin D deficiency is more common in older people, in people who are overweight, and in black and Asian people - all of the groups who are at increased risk of becoming very ill with Covid.
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.