A trial to see whether two anti-malarial drugs could prevent Covid-19 has begun in Brighton and Oxford.
Chloroquine, hydroxychloroquine or a placebo will be given to more than 40,000 healthcare workers from Europe, Africa, Asia and South America.
All the participants are staff who are in contact with Covid-19 patients.
US President Donald Trump was criticised this week after he said he had been taking hydroxychloroquine, despite warnings it might be unsafe.
The first UK participants in the global trial are being enrolled at the Brighton and Sussex University Hospitals and the John Radcliffe Hospital in Oxford.