In this edition of the Explained series, find out what are human challenge trials and what's the controversy around it.
The World Health Organisation (WHO) has announced that eight vaccine candidates for COVID-19 have entered human trials while another 110 are in various stages of development.
But, scientists are sceptical about human challenge trials to advance vaccine development.
In this edition of the Explained series, find out what human challenge trials are and the controversy around it.
Watch the video for more.