Human challenge trials are regarded by some as a faster way to develop vaccines – but they come with big risks.
Challenge trials have been used for hundreds of years but a lot of people question how ethical is to deliberately expose people to an infectious disease.
More than 140 different organisations around the world are currently trying to discover a vaccine for Covid-19.
But could challenge trials be used to speed up the process?
BBC Health Correspondent
Laura Foster explains how they work.
Video by Terry Saunders, Megan Fisher and Laura Foster.