The vaccine could help control the pandemic, a scientist leading the study has said, as phase three data was published.
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.
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.
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.
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.
“We are hugely grateful to our trial volunteers for working with us over the past eight months to bring us to this milestone.”
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.
Study author Dr Merryn Voysey, from the University of Oxford, said: “The results presented in this report provide the key findings from our first interim analysis.
“In future analyses, with more data included as it becomes available, we will investigate differences in key subgroups such as older adults, various ethnicities, doses, timing of booster vaccines, and we will determine which immune responses equate to protection from infection or disease.”
Prof Pollard added: “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.”
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.
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