British drugs group AstraZeneca and the University of Oxford said their jointly-developed vaccine against Covid-19 has shown "an average efficacy of 70%" in trials.
"This vaccine's efficacy and safety confirm that it will be highly effective against Covid-19 and will have an immediate impact on this public health emergency," AstraZeneca chief executive Pascal Soriot said in a statement.
The results ranged between 62 and 90% efficacy depending on the vaccine dosage.
The 70% average is lower compared with the efficacy of coronavirus vaccines trialed by rivals Pfizer/BioNTech and Moderna, which have come in above 90%.
Today's statement said "positive high-level results from an interim analysis of clinical trials of AZD1222 in the UK and Brazil showed the vaccine was highly effective in preventing Covid-19... and no hospitalisations or severe cases of the disease were reported in participants".
It added: "One dosing regimen (n=2,741) showed vaccine efficacy of 90% when AZD1222 was given as a half dose, followed by a full dose at least one month apart."
The pair said that regimen n=8,895 showed 62% efficacy when given as two full doses at least one month apart.
"The combined analysis from both dosing regimens (n=11,636) resulted in an average efficacy of 70%."
AstraZeneca said it would "immediately prepare regulatory submission of the data to authorities around the world that have a framework in place for conditional or early approval".
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It added that it would seek emergency-use listing from the World Health Organization to accelerate vaccine availability in low-income countries.
AstraZeneca said it is looking at a capacity of up to three billion doses of the vaccine in 2021 pending regulatory approval.
It said the vaccine can be stored, transported and handled "at normal refrigerated conditions" of between 2 and 8C (36-46F) for at least six months.
More than 23,000 adults are currently being assessed in the trials, with the number expected to rise to up to 60,000, the statement said.
"Clinical trials are also being conducted in the US, Japan, Russia, South Africa, Kenya and Latin America with planned trials in other European and Asian countries," it added.
Oxford professor Andrew Pollard said the latest findings show "an effective vaccine that will save many lives".
"Excitingly, we've found that one of our dosing regimens may be around 90% effective and if this dosing regime is used, more people could be vaccinated with planned vaccine supply.
"Today's announcement is only possible thanks to the many volunteers in our trial, and the hard working and talented team of researchers based around the world," added Prof Pollard.
His colleague, Professor Sarah Gilbert, Professor of Vaccinology at the University of Oxford, said: "The announcement today takes us another step closer to the time when we can use vaccines to bring an end to the devastation caused by SARS-CoV-2.
"We will continue to work to provide the detailed information to regulators. It has been a privilege to be part of this multi-national effort which will reap benefits for the whole world."
AstraZeneca executive vice president Mene Panglos has said immunisations could begin next month if the regulators give the new vaccine the go-ahead.
"We are extremely excited because what the data has shown us is that the vaccine is clearly effective in terms of reducing infections, it is clearly effective in terms of reducing people getting sick and going to hospital, it is potentially reducing transmission, and I think has got every chance of being a very successful, very effective vaccine that can get us back to normal," he told the BBC Radio 4 Today programme.
"One of the real benefits of this vaccine is the fact that we can manufacture it at scale. It is a relatively easy vaccine to distribute around the world.
"I hope that if the regulators deem the vaccine to be safe and effective - which I hope they will - we should be able to start immunising people in December."
The UK has placed orders for 100 million doses of the Oxford vaccine - enough to vaccinate most of the population - with rollout expected in the coming weeks if the jab is approved.
British Prime Minister Boris Johnson described the news as "incredibly exciting".
Incredibly exciting news the Oxford vaccine has proved so effective in trials. There are still further safety checks ahead, but these are fantastic results.— Boris Johnson (@BorisJohnson) November 23, 2020
Well done to our brilliant scientists at @UniofOxford & @AstraZeneca, and all who volunteered in the trials. https://t.co/84o8TKhQga
British health minister Matt Hancock said manufacturing of the AstraZeneca and Oxford University vaccine had already begun in anticipation of positive results.
Asked if the plan was to start vaccinating before Christmas, he told BBC Breakfast: "That's right, the NHS is on stand-by to be ready as soon as this can be manufactured.
"The manufacturing process for vaccines is also hard, this isn't a chemical that you make, this is a biological compound, a biological product, so it has to be manufactured of course.
"That manufacturing has already started in anticipation of these results coming through positively.
"The NHS is on stand-by to start delivering the vaccine from next month. It's going to be a huge effort, I think everybody knows, but the NHS will be ready."
Scientists have welcomed the news from the Oxford/AstraZeneca Covid vaccine trial.
Peter Horby, professor of emerging infectious diseases and global health at the University of Oxford tweeted: "Oxford jab is far cheaper, and is easier to store and get to every corner of the world than the other two."
Dr Michael Tildesley, associate professor in infectious disease modelling at the University of Warwick, said: "It's absolutely excellent news about the Oxford vaccine because this is really the vaccine that the government has pinned a lot of their hopes on in terms of resources - we've ordered 100 million doses of the Oxford vaccine."
Meanwhile, Speaking to RTÉ's Morning Ireland, Dr Anne Moore, a senior lecturer in biochemistry at UCC who has worked as an immunologist at Oxford University, says AstraZenica will now seek regulatory approval for their vaccine to be used under emergency use authorisation.
However, she warned that it is important that the safety data and immunity data is seen - with the key question being "how long does protection last?"
Dr Moore said the elderly have had a good response to the AstraZenica Covid vaccine, which is very good news.
In addition, she said, this vaccine does not have to be held at minus 80 degree temperatures and has been produced for not for profit, which means it will be sold to low and middle income countries at cost price.
Additional reporting Reuters, PA