Interim results from pooled studies show the vaccine was 70.4 per cent effective, on average, in preventing coronavirus after two doses were given.
For people given two full doses of the jab in one study, the vaccine was 62.1 per cent effective.
In a study where people received a half dose followed by a full dose, the vaccine was 90 per cent effective.
The overall efficacy of 70.4 per cent is based on 11,636 volunteers across the United Kingdom and Brazil, and combined across three groups of people vaccinated.
This comes after England’s deputy chief medical officer Jonathan Van-Tam suggested people who cannot leave their homes may need to wait for the Oxford/AstraZeneca vaccine.
Data for the vaccine, which has been developed with pharmaceutical giant AstraZeneca, has been submitted to the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) for approval.
Writing in The Lancet medical journal, the researchers said there were no admissions to hospital or severe disease in people receiving the vaccine.
However, they said more detail is needed on how effective the vaccine is in older adults – those at most risk of severe Covid-19.
The data in The Lancet analysis was mostly restricted to people aged 55 and under, with work in older age groups still ongoing.
Prof Andrew Pollard said: “Control of the pandemic will only be achieved if the licensing, manufacturing and distribution of these vaccines can be achieved at an unprecedented scale and vaccination is rolled out to those who are vulnerable.
“Our findings indicate that our vaccine’s efficacy exceeds the thresholds set by health authorities and may have a potential public health impact.”
Professor Sarah Gilbert, professor of vaccinology at the University of Oxford, said: “Following the demonstration of vaccine efficacy in many preclinical studies, we now have clear evidence of efficacy in the trial results presented in a peer-reviewed publication today.
“Now under regulatory review, we hope that this vaccine will shortly be in use to start saving lives.”
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Commenting on the efficacy results, Prof Pollard told a press briefing: “I think what we need are vaccines in people’s arms.
“I think there’s a really important point that the only way that we get the pandemic behind us is to have doses of vaccines out there in populations here and around the world.
“And personally I think we need to be completely agnostic about which vaccine is being used because we need multiple vaccines, as soon as possible.
“Otherwise we’ll still be in this position in six months time.
“Whereas now we are starting to get to a point with three vaccines showing efficacy of starting to get the pandemic behind us.
“This is really can’t be a competition between developers, this has to be a competition against the virus.”
In the study, there were 131 cases of Covid-19 in the 11,636 people taking part.
In the group where people received a half dose followed by a full dose, vaccine efficacy was 90 per cent, based on three out of 1,367 cases in the vaccine group, and 30 out of 1,374 cases in the control group.
The researchers concluded that, irrespective of age or time between doses, the people given the half dose followed by the full dose had better odds of protection against Covid-19.
However, they said this data “provides a suggestion” and more research is needed as further cases are identified.
Prof Gilbert said there are some promising signs about the vaccine potentially stopping transmission of Covid-19.
She said: “The really good news is that from three weeks after the first dose of the vaccine – and that’s the first dose not the second dose – there were no hospitalisations or severe cases of Covid in the participants in this trial and that is the kind of effect that will have a really huge impact on health systems, and then the next level of protection that we’re looking at is against symptomatic PCR confirmed infection.”
She added: “We are getting weekly swabs from people – it’s possible that people could have had a very transient infection in between the weekly swabs although it’s probably quite unlikely – and we are seeing an effect on reducing the asymptomatic infections as well.”