UK Prime Minister Boris Johnson has hailed the success of Britain's vaccine program after data showed 15 million first doses had been delivered, but he added there was still a long way to go.

"Today we have reached a significant milestone," he said.

"No one is resting on their laurels... We've still got a long way to go and there will undoubtedly be bumps in the road, but after all we've achieved I know we can go forward with great confidence."