A Covid vaccine could be given to some of the most vulnerable people "this side of Christmas", according to the chairwoman of the UK Vaccine Taskforce.
But limited supplies would mean the government would have to decide who should get it, and when.
Kate Bingham also said a vaccine will not be "a silver bullet" that would allow life to get back to normal overnight.
And she warned that it was unlikely to protect everyone from infection.
Ms Bingham said she was optimistic that a vaccine would be found that would "protect some people from infection and can reduce the severity of symptoms".
But she said it was "very unlikely" to be a single jab and that ongoing revaccination would be needed - probably every few years.