People are putting their lives on the line to help test a Covid-19 vaccine.
Tens of thousands have signed up for human challenge trials, with the hope of developing a vaccine quickly, and saving thousands of lives.
Oxford University medical ethics researcher Dominic Wilkinson told Kate Hawkesby the participants stay at a special facility where they're exposed to the virus, a few weeks after being given the jab.
He says there's a chance they'll catch Covid-19 and become seriously ill.
However, Wilkinson says the risks are low because only healthy people take part.
"They're compensated for their time, but money isn't their main motivator.
"They think they're at high risk of getting the virus this winter in places like the UK, and are at low risk of getting sick so they might as well contribute to getting a vaccine sooner."