Interim results from the pooled studies show that the vaccine was 70.4% effective, on average, in preventing coronavirus after two doses were given.
For people who received two full doses of the vaccine in one study, the vaccine was 62.1% effective.
In a study where people were given 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 UK and Brazil, and combined across three groups of people vaccinated.
Data for the vaccine, which was developed with pharmaceutical giant AstraZeneca, has been submitted to the UK 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 published the interim review of the phase three trial and show that this new vaccine has a good record of safety and efficacy against the coronavirus. .
“We are extremely grateful to our trial volunteers for working with us over the past eight months to bring us to this important milestone.”
Writing in the medical journal The Lancet, the researchers said there was no hospital admission or serious illness in those receiving the vaccine.
However, they said more details were needed on the vaccine’s effectiveness in older adults – those who are most at risk of developing severe Covid-19.
Data from The Lancet analysis was mostly limited to people aged 55 and under, with work in older groups still ongoing.
Study author Dr Merryn Voysey, University of Oxford, said: “The results presented in this report provide the main conclusions of our first interim analysis.
“In future analyzes, with more data included as they become available, we will study differences in key subgroups such as the elderly, various ethnicities, doses, timing of booster shots. , and we will determine which immune responses correspond to protection against infection or disease. . ”
Professor Pollard added: “Control of the pandemic will only be achieved if the licensing, manufacture and distribution of these vaccines can be achieved on an unprecedented scale and if vaccination is rolled out to vulnerable people.
“Our results indicate that the efficacy of our vaccine exceeds thresholds set by health authorities and may have a potential impact on public health.”
Professor Sarah Gilbert, Professor of Vaccinology at the University of Oxford, said: “Following the demonstration of the vaccine’s effectiveness in numerous preclinical studies, we now have clear evidence of the effectiveness of the trial results. featured in a peer-reviewed publication today.
“Now under regulatory review, we hope this vaccine will soon be used to start saving lives.”