The Oxford-AstraZeneca vaccine has been approved for use in the UK, with the first doses due to be given on Monday amid rising coronavirus cases.
The UK has ordered 100 million doses - enough to vaccinate 50 million people.
This will cover the entire population, when combined with the full order of the Pfizer-BioNTech jab, Health Secretary Matt Hancock said.
It comes as millions more people in England are expected to be placed under the toughest tier four restrictions.
Prime Minister Boris Johnson called the development "a triumph" for British science, adding: "We will now move to vaccinate as many people as quickly as possible."
And England's chief medical officer Chris Whitty praised the "considerable collective effort that has brought us to this point".
Speaking on BBC Breakfast, Mr Hancock said the development was a "significant moment" in the fight against the virus.
On Tuesday, 53,135 new Covid cases were recorded in the UK - the highest single day rise since mass testing began - as well as 414 more deaths within 28 days of a positive test.
The Oxford-AstraZeneca vaccine was designed in the first months of 2020, tested on the first volunteer in April, and has since been through large-scale clinical trials involving thousands of people.
It has been developed at a pace that would have been unthinkable before the pandemic.
It is the second jab to be approved in the UK after the Pfizer-BioNTech vaccine was given the go-ahead in December.