By Joseph Walker and Jenny Strasburg 

LONDON -- The U.K. authorized a Covid-19 vaccine developed by the University of Oxford and AstraZeneca PLC, opening the door for the rollout of millions of doses in a country where infections have surged amid a more infectious variant of the virus.

The green light represents the third emergency-use approval of a Western-developed vaccine this month and comes as cases rise sharply in the U.S. and Europe. A shot developed by Pfizer Inc. and Germany's BioNTech SE and one by Moderna Inc. have both been cleared in the U.S. and are being distributed there.

AstraZeneca's shot -- less effective in clinical trials than its rivals' injections -- won't be available in the U.S. until the Food and Drug Administration reviews large-scale trials still being conducted there and decides to authorize its use.

The U.K. authorization comes as the country battles a new, potentially more contagious variant of the coronavirus. The mutated virus triggered travel bans recently on visitors and goods from Britain, ratcheting up the political urgency for a speedy vaccine rollout here. Pfizer's shot is already available in the U.K., where Prime Minister Boris Johnson said on Dec. 21 that a half million of the first of a two-dose regimen have been administered.

Scientists have said that the three Covid-19 vaccines authorized by Western countries so far should be similarly effective against the new variant as they have been in clinical trials, a view AstraZeneca executives share, according to a person familiar with the matter. The new variant could be as much as 70% more transmissible than more established forms of Covid-19, government officials say.

The U.K. medicines regulator said the vaccine can be administered to people 18 years and older. The authorization clears the use of two standard vaccine doses, a regimen that was 62% effective in preventing symptomatic Covid-19 in an interim analysis of trials involving about 11,600 people in the U.K. and Brazil.

In the same study, a small subset of volunteers of age 55 and younger were given a half-dose shot followed by a standard dose. That subset showed a higher protection rate of 90%. The U.K. government hasn't authorized the use of the half-dose regimen.

"We felt the results were not borne out by the full analysis," Munir Pirmohamed, a pharmacologist who chairs the government's advisory group that advised on the regulator's recommendations, said in a media briefing Wednesday.

U.K. officials said they authorized the vaccine Tuesday and released the first batches that night.

They advised that no preference generally should be given between the two vaccines so far authorized by the U.K.: the Oxford-AstraZeneca vaccine and the Pfizer-BioNTech vaccine. The higher efficacy shown by the Pfizer vaccine isn't directly comparable with the efficacy of the Oxford-AstraZeneca vaccine, officials said, because of scientific and clinical-trial differences.

AstraZeneca and U.K. health officials said the two-dose regimen is flexible in terms of when the second dose should be given, anywhere from four weeks to 12 weeks after the first dose.

Delaying the second dose would help stretch supplies of the vaccine so more Britons get a first shot sooner. But the vaccine's full protection -- benefits shown in clinical trials to prevent hospitalization -- only kicks in after the second dose.

U.K. health officials said Wednesday the updated second-dose guidance also applies to the Pfizer-BioNTech shot already being administered in the country. Previously, the second dose was recommended 21 days after the second dose. U.K. officials on Wednesday changed that to "within 12 weeks," saying the change was based on the latest scientific advice. The vaccine's makers have said full protection doesn't start until at least seven days after the second dose.

Pfizer has promised the U.K. millions more doses by the end of the year, but the AstraZeneca and Oxford shot -- made at facilities in the country and elsewhere -- promises to quicken the rate of inoculations by the country's state-run health-care system. The National Health Service will determine which shot to provide people depending on supply.

AstraZeneca said Wednesday it will supply millions of doses in the first quarter of next year, without specifying an exact number. The U.K. has ordered up to 100 million doses of the vaccine, enough for 50 million people.

The vaccine provided between 62% and 90% effectiveness against symptomatic Covid-19, depending on dosage size. Two full doses about a month apart led to the 62% reading, while a small subset of trial volunteers who received a lower initial dose followed by a second standard dose showed a 90% efficacy reading. That subset of trial participants were all 55 or younger. Oxford and AstraZeneca scientists have said they are unsure why that regimen would be more effective.

Government advisers behind the authorization said the 90% efficacy shown in the half-dose regimen might have been caused by the longer period of time between the first and second doses in that trial subgroup, rather than the half-dose itself. They said the analysis of the half-dose subgroup helped lead to the U.K. government's decision to recommend up to 12 weeks between doses.

The advisers said for the full-dose regimen, immunity kicks in roughly three weeks after the first dose. They estimated 70% effectiveness in the vaccine during the period from three weeks after the first dose, through the next three months. They said it isn't clear whether that number will hold up in fuller clinical trials, which will provide more data.

It is unclear how quickly other nations might authorize the vaccine. A large clinical trial is under way in the U.S., where AstraZeneca executives expect to have full trial data to submit to U.S. regulators by February, according to a person familiar with the matter. They have been submitting batches of data to European regulators and are poised for potential authorization across the European Union by February, the person said.

Oxford and AstraZeneca emerged early this year as front-runners in the race to develop a Covid-19 vaccine before falling behind competitors that were able to complete clinical trials faster. Developed by scientists at Oxford's Jenner Institute, the vaccine uses a weakened cold virus to courier genetic material from the coronavirus into cells and trigger immunity with two shots spaced one month apart.

AstraZeneca agreed in April to co-develop Oxford's technology and distribute it around the world at no profit through the pandemic. By November, the company had agreements to supply three billion doses to every region in the world.

The partners have faced setbacks along the way to Wednesday's milestone. In May, they launched a combined Phase 2 and 3 clinical trial in the U.K. aimed at enrolling more than 10,000 volunteers and obtaining initial results by August. But the study progressed slower than hoped for initially, partly because of relatively low infection rates in the U.K. for much of the summer and fall.

By the time AstraZeneca launched a planned Phase 3 U.S. study of 30,000 volunteers in late August, the company was trailing behind rivals Pfizer and Moderna, which had begun similarly sized studies a month earlier. Within two weeks, AstraZeneca paused all its studies globally after a U.K. volunteer had an unexplained illness. U.K. regulators allowed studies to resume quickly, but the U.S. trial remained grounded for more than six weeks before U.S. regulators lifted the hold in October.

Write to Joseph Walker at joseph.walker@wsj.com and Jenny Strasburg at jenny.strasburg@wsj.com

 

(END) Dow Jones Newswires

December 30, 2020 06:55 ET (11:55 GMT)

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