Could “challenge trials” speed coronavirus vaccine development?

Scientists have warned it could be many months, before we know if any of the coronavirus vaccines currently undergoing trials are effective. One way to speed up the process, would be if so-called 'challenge trials' are allowed, where volunteers are given the vaccine and then deliberately infected with coronavirus, to see if they're protected. But some experts aren’t convinced such trials are safe or necessary. Our Medical Editor Fergus Walsh reports.