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.
Healthcare workers in Britain and Thailand have started taking part in a trial to determine whether two anti-malarial drugs can prevent COVID-19, including one that US President Donald Trump says he has been taking.
The study, involving more than 40,000 healthcare workers across Europe, Africa, Asia and South America, seeks to determine whether chloroquine and hydroxychloroquine could play a role in the fight against the novel coronavirus.
Demand for hydroxychloroquine surged after Trump touted it in early April. He said this week he was now taking it as a preventive medicine against the virus despite medical warnings about its use.
The US Food and Drug Administration warned against use of the medication outside hospitals, where the agency has granted temporary authorisation for its use in some cases, or clinical trials.
While the University of Oxford trial is taking place in a controlled clinical environment, the World Health Organization has warned that some individuals were self-medicating and risked causing themselves serious harm.
It has not yet been shown to be safe and effective in the prevention or treatment of coronavirus and can cause dangerous heart arrhythmias.
The trial also involves researchers from the UK, Thailand, Vietnam, Laos, Cambodia and Italy.