LONDON — Britain on Wednesday became the first country to authorize a coronavirus vaccine from the University of Oxford and the British-Swedish pharmaceutical firm AstraZeneca, adding a second shot to the fight against a surging outbreak here driven by a highly infectious variant of the virus.

The Oxford-AstraZeneca doses are cheaper than other coronavirus vaccines and easier to transport, as they can be stored at normal fridge temperatures.

Health Minister Matt Hancock praised the homegrown vaccine as a "game-changer" and said clinical trials have proved it to be safe and effective — but he did not put to rest questions about how effective.

Researchers from the Oxford-AstraZeneca team earlier this month published interim results that showed their vaccine was 62 percent effective for volunteers given two full doses and 90 percent effective for a smaller subgroup who received a half dose followed by a full dose. The scientists said they were studying why the different regimens produced such different results.

Britain’s Medicines and Healthcare products Regulatory Agency authorized the vaccine at two full doses for people 18 and older, concluding the results of the half-dose regimen could not yet be supported.

British regulators also noted data is limited on how well the vaccine works in those over 65, who are among the most vulnerable to the virus but who represented less than 6 percent of the clinical trial participants. Still, the regulators okayed use in older populations. Health officials stressed that the vaccine has been shown to greatly reduce severe illness from covid-19, the disease the virus causes, which will cut the number of hospitalizations and deaths.

Moncef Slaoui, the head of the White House’s Operation Warp Speed, said at a Wednesday briefing that he anticipates the AstraZeneca vaccine gaining U.S. authorization in April, based on successful completion of Phase 3 clinical trials in the United States.

He said that he has “no doubt the vaccine is very effective against severe disease” but that the “biggest question mark is efficacy in the elderly population that needs to be further documented.”

The Pfizer-BioNTech and Moderna vaccines already rolled out in the United States use a different technology and had simpler trial designs, showing them to be 95 percent effective.

Britain has injected some 600,000 people with the Pfizer vaccine since becoming the first Western country to roll out mass inoculations. But experts say the government must ramp up quickly, to inoculate 2 million people a week — 10 times the current rate — to beat back the pandemic.

The National Health Service will start inoculating with the AstraZeneca vaccine on Monday, with nursing home residents, health-care workers and people over 80 at the front of the line. There are plans to deliver the vaccine in mass immunization centers, such as sporting arenas and convention halls.

The urgency is especially great with a new variant of the virus spreading rapidly. Researchers have found no indication the variant is deadlier, but it appears to be 50 percent more transmissible.

Britain recorded more than 53,000 cases on Tuesday — the highest in a single day. British hospitals have more coronavirus patients than when the first wave gripped the country in April.

As of Thursday, 8 in 10 people in England will be under the highest tier of restrictions. Wales, Scotland and Northern Ireland have enacted tight restrictions, as well.

Hancock said having both the Pfizer and AstraZeneca vaccines means the government now has “a very high degree of confidence that we can be out of this by spring.”

Hancock said that in the interest of injecting as many people as possible, as quickly as possible, it would be sufficient to give a first dose of the AstraZeneca vaccine and allow more than the usual 21 days between shots.

“In the data, the scientists and the regulators have found the immunity comes from around two weeks after the first dose, and then the second dose should be taken up to 12 weeks later to give you that long-term protection,” Hancock said.

“This means we can spend the first three months vaccinating people with the first doses, getting them that immunity, getting people protection quicker than we possibly could have done otherwise,” he said.

Britain bet the house on the AstraZeneca vaccine, ordering 100 million doses, compared with 40 million doses of the Pfizer vaccine and 7 million doses of Moderna, which the United Kingdom has not yet reviewed for emergency authorization.

AstraZeneca says it can deliver about 2 million doses a week initially. It has pledged 40 million doses by the end of March. The company has manufacturing facilities in Britain.

June Raine, chief executive of the British regulator, said “no corners were cut” in the rapid review of the vaccine, even though it was developed at Oxford and backed by the British government. Raine said it has “the potential to save many lives.”

Prime Minister Boris Johnson said the approval of the new vaccine was “truly fantastic news — and a triumph for British science.”

British scientists noted that the vaccine could help protect not only Britons but also many in the developing world.

Regulators in India will meet Friday to consider whether to grant emergency approval. Serum Institute of India, one of the world’s biggest vaccine manufacturers, has nearly 50 million doses ready and plans to produce 300 million by July. Half of the production will be used in India.

The vaccine uses a harmless cold virus that typically infects chimpanzees to deliver to the body’s cells the genetic code of the spike protein that dots the outside of the coronavirus. That teaches the body’s immune system to block the real virus.

At a briefing announcing the approval of the vaccine, Wei Shen Lim, a respiratory physician and chair of the Joint Committee on Vaccination and Immunization, said, “The data shared with us, and I am not sure it is entirely in the public domain, calculated the vaccine efficacy between Day 22 of Dose 1 to the time of Dose 2 being given, the figure is around 70 percent.”

Lim added, “But I don’t think I should be revealing any more than that at this point in time.”

In an AstraZeneca news release on Wednesday, the company said that “the primary efficacy endpoint based on a pooled analysis showed that the vaccine was 70.4 percent.”

“Today is an important day for millions of people in the U.K. who will get access to this new vaccine,” said Pascal Soriot, chief executive of AstraZeneca. “It has been shown to be effective, well-tolerated, simple to administer and is supplied by AstraZeneca at no profit.”

He said scientists believe the AstraZeneca vaccine will be effective against the new variant detected in Britain.

Public health officials say there is much to recommend the new vaccine, as it costs less than $4 a dose, is relatively easy to manufacture at huge scale and does not require special handling.

The British government has not released how much it is paying for vaccines. A spokesman for the Department for Business, Energy and Industrial Strategy said the prices are “commercially sensitive so can’t be disclosed by us.” But U.S. figures suggest that the Pfizer vaccine may be five times as expensive as AstraZeneca’s.

The Pfizer vaccine must also be kept in special freezers and dry ice at extremely low temperatures.

The British health secretary conceded that the Pfizer requirements “made it more challenging to get out, especially to some of the smaller care homes, and those limitations aren’t there for this Oxford-AstraZeneca vaccine.”

Approval of the vaccine provides “a ray of hope at the end of a very challenging year,” said Lawrence Young, a professor of molecular oncology at Warwick Medical School. “It also comes at a time when we have just recorded the largest daily increase in the number of infections in the U.K. and when the NHS is struggling with the number of hospitalizations. And all this on the backdrop of a more infectious variant of the virus.”

Andrew Pollard, director of the Oxford Vaccine Group, told the BBC that the pandemic this year was “like being in a blizzard.”

“We’ve been really struggling uphill through snow drifts with this icy wind in our faces, and I think this morning we do have some respite with this good news and the warmth that that brings,” he said.

Isaac Stanley-Becker in Washington contributed to this report.