The transmission is reduced by 67 percent after the first dose of the vaccine, according to an analysis of swabs obtained from UK volunteers.
Oxford scientists also found that a booster dose given 12 weeks after the first jab increases the efficacy to 82.4 percent, with indications that leaving a longer gap between doses increases effectiveness.
Prime Minister Boris Johnson hailed the results as “really encouraging.”
“I’m incredibly grateful to all our scientists, NHS staff and volunteers working on our vaccination programme,” he wrote on Twitter.
The Oxford/AstraZeneca vaccine, along with the Pfizer/BioNTech jab, are already being rolled out across the UK.
The vaccine developed by U.S. firm Moderna has also been approved but is not yet in use.
By Tuesday, more than 9.6 million people had received a first dose of a COVID-19 vaccine, with 496,796 people having also had a booster shot.
The European Medicines Agency has also authorised the Oxford/AstraZeneca vaccine for use in all adults throughout the European Union.
However, health authorities in Germany and other countries have raised concerns that the firm didn’t test the vaccine in enough older people to prove it works for them, and indicated they would not recommend it for people over 65.
French President Emmanuel Macron said Tuesday that his country will only administer the jab to people under the age of 65.