Video report by ITV News Science Editor Tom Clarke
But more detail is needed on how effective the vaccine is for older adults, scientists leading the study said.
Researchers writing in The Lancet medical journal said that the interim analysis there were no admissions to hospital or severe disease in people receiving the vaccine.
Health Editor Emily Morgan and Science Editor Tom Clarke have more on the latest developments:
Data for the vaccine, developed with pharmaceutical giant AstraZeneca, has been submitted to the UK’s Medicines and Healthcare products Regulatory Agency (MHRA) for approval.
Professor Andrew Pollard, director of the Oxford Vaccine Group and chief investigator of the Oxford vaccine trial, said: “Today we have published the interim analysis of the phase three trial and show that this new vaccine has a good safety record and efficacy against the coronavirus.
Health Secretary Matt Hancock welcomes the news on the Oxford vaccine but stresses it must first be approved by the regulator:
“We are hugely grateful to our trial volunteers for working with us over the past eight months to bring us to this milestone.”
Interim results from pooled studies show the vaccine was 70.4% 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% effective.
But in a study where people received a half dose followed by a full dose, the vaccine was 90% effective.
The overall efficacy of 70.4% is based on 11,636 volunteers across the United Kingdom and Brazil, and combined across three groups of people vaccinated.
The data in The Lancet analysis was mostly restricted to people aged 55 and under, with work in older age groups still ongoing.
Professor Pollard said: “Twenty per cent of the people in the trials are older adults, both in the UK and in Brazil.
“But they were recruited later than the younger adults so they’ve had less time for cases to accrue in those age groups and for us to be able to measure an efficacy signal.
“And remember, this is an interim analysis, where the data that we have at the moment are largely from younger adults.”
He added: “And so, as we do further cuts of the data, we’ll have a lot more information about the other age groups.
“I think it is important that we do generate some more data in those older adults over time.”
Professor 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.”
On Tuesday, the NHS started rolling out the Pfizer/BioNTech jab in older people, health staff and care home workers - making Britain the first nation in the world to approve a Covid-19 vaccine.
But at a press briefing, Professor Pollard said that health services need to be "agnostic" about what vaccine to use to fight the pandemic.
“I think what we need are vaccines in people’s arms," he said.
"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.”
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.”
In the study, there were 131 cases of Covid-19 in the 11,636 people taking part.
This included 30 out of 5,807 (0.5%) cases in the vaccine group and 101 out of 5,829 (1.7%) cases in the control group, which equates to a vaccine efficacy of 70%.
When breaking this down based on vaccine dose, those who received two full doses of the vaccine saw a vaccine efficacy of 62.1%, based on 27 out of 4,440 cases in the vaccine group, and 71 out of 4,455 cases in the control group.
In the group where people received a half dose followed by a full dose, vaccine efficacy was 90%, based on three out of 1,367 cases in the vaccine group, and 30 out of 1,374 cases in the control group.
Upon request from the peer-reviewers, the researchers carried out a sub-analysis to better understand whether the difference between 62% efficacy and 90% was to do with the dose or other factors such as age and time between vaccine dose.
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.
They also said that while five cases of Covid-19 occurred in people aged over 55, vaccine efficacy in older age groups could not be assessed at the moment as there were too few cases.
Prof Pollard explained that most of the data were generated to support the two full doses, and thinks that will be the focus of the regulatory review.
But he added that it would be up to the regulators to “look at the data and decide exactly what they think their label should say at the end, and so that’s sort of in their hands at the moment”.
Professor 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."