Coronavirus vaccine: What are human challenge trials?

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.