UK first in world to start using Oxford-AstraZeneca vaccine

While two doses are required to fully protect against COVID-19, both provide high levels of protection after the first dose, the committee said. Making the first dose the priority will "maximize benefits from the vaccination program in the short term," it said.

Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said policymakers are being forced to balance the potential risks of this change against the benefits in the middle of a deadly pandemic.

"We have a crisis situation in the UK with a new variant spreading rapidly, and as has become clear to everyone during 2020, delays cost lives," Evans said. "When resources of doses and people to vaccinate are limited, then vaccinating more people with potentially less efficacy is demonstrably better than a fuller efficacy in only half."

In England alone, 23,557 people were hospitalized with COVID-19 on Saturday. While figures for Northern Ireland, Scotland and Wales haven't been updated in recent days, that's higher than the U.K.-wide peak during the first wave of the pandemic.

The government closed non-essential shops across London and parts of southeast England before Christmas to try to contain the new variant, but health officials say tougher measures are now needed.

Johnson said there were "tough, tough" weeks to come in the fight against COVID-19. More school closures, curfews and the total banning of household mixing could be on the agenda.

While schools in London are already closed due to high infection rates in the capital, students in many parts of the country were returning to in-person classes Monday after the Christmas holidays. Unions representing teachers, however, have called for schools throughout England to remain closed for at least two weeks, with classes shifted to remote learning.

Professor Andrew Pollard, one of the scientists who led development of the Oxford-AstraZeneca vaccine, received his shot on Monday.

"It was an incredibly proud moment for me, to have received the actual vaccine that the University of Oxford and the AstraZeneca teams have worked so hard to make available to the U.K. and the world," he said.