The Oxford vaccine is cheaper to produce, and easier to store and transport compared to the Pfizer-BioNTech vaccine
Britain on Wednesday became the first country in the world to approve AstraZeneca and Oxford University's low-cost Covid-19 vaccine, raising hopes it will help tackle rising cases and ease pressure on creaking health services.
The independent Medicines and Healthcare products and Regulatory Agency (MHRA) said the vaccine "met its strict standards of safety, quality and effectiveness", and a roll-out was set for January 4.
Prime Minister Boris Johnson, who spent several days in intensive care with Covid-19 earlier this year, called it "truly fantastic news" and "a triumph for British science".
"We will now move to vaccinate as many people as quickly as possible," he tweeted.
Britain has already approved the Pfizer/BioNTech vaccine for general use, and some 800,000 people have received a first dose in the country's biggest ever vaccination drive.
But as daily Covid-19 infection rates hit record highs, the government is pinning its hopes on the Oxford/AstraZeneca jab, which is cheaper to produce, and easier to store and transport.
Unlike the Pfizer-BioNTech vaccine, it does not require ultra-low freezing temperatures and can use normal refrigerated supply chains, making it a more attractive proposition globally.
The partners, who have billed it as a "vaccine for the world", have also promised to provide it not-for-profit to developing nations, and hope to make up to three billion doses in 2021.
Britain is struggling with another surge in the virus, with a record 53,135 daily cases reported on Tuesday, heaping fresh pressure on health services during their busiest winter months.
More than 71,000 people testing positive for the disease have now died — one of the worst tolls in the world.
But doctors say many frontline healthcare workers have been struck down with a new, potentially more contagious variant of the virus, which is thought to be behind the surge.
More than 24 million people, or 43% of England alone, are already living under strict stay-at-home measures, with bars, restaurants, pubs and other entertainment closed.
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The government is under pressure to follow other European countries and introduce even tougher restrictions, including delaying the return to school next week.
Health Secretary Matt Hancock is due to update parliament on Wednesday about whether to tighten the screw further.
But he said approval of the new vaccine was a "way out of the pandemic".
"Now we need to hold our nerve while we get through this together," he added.
Britain has ordered 100 million doses and the Department of Health expects four million doses to be ready by the end of the year, and 40 million by the end of March.
AstraZeneca Chief Executive Officer Pascal Soriot called the approval "an important day for millions of people in the UK".
"It's exactly what we need right now," Andrew Hayward, professor of infectious diseases epidemiology at University College London, told BBC television.
A "step change" in transmissibility has created "a race between us and the virus, and we need to slow the virus down as much as we can whilst we get as many people vaccinated as possible", he added.
The Oxford/AstraZeneca vaccine is based on a weakened version of a common cold virus (adenovirus) in chimpanzees which has been genetically changed to stop Covid-19 replicating in humans.
It delivers genetic cargo into cells, giving them instructions how to fight SARS-CoV-2.
The vaccine costs about £2.50 ($3.40, 2.75 euros) per dose — a fraction of the price of the Moderna and Pfizer/BioNTech drugs — which Hancock said provided hope for the rest of the world.
"This vaccine will be made available to some of the poorest regions of the world at a low cost, helping protect countless people from this awful disease," he added.
On Sunday, Soriot said the vaccine provides "100% protection" against severe Covid-19 disease requiring hospitalization.
He predicted trials would show his firm had achieved a vaccine efficacy equal to Pfizer/BioNTech at 95% and Moderna at 94.5%.
Earlier trials had shown varying outcomes in the AstraZeneca shot's efficacy.
Initial large-scale trials in which volunteers in the UK and Brazil were given two full doses showed 62% effectiveness.
For volunteers who received a half-dose first and then a full dose one month later, however, the vaccine was found to have 90% efficacy.
The values were averaged to give the 70% figure reported by the developers in an interim study.