New data also shows the vaccine was 76% effective during the three months after the first dose.
The Oxford-AstraZeneca vaccine could lead to a “substantial” fall in the spread of the virus, say scientists.
The impact of Covid vaccines on transmission has been a crucial unknown that will dramatically shape the future of the pandemic.
The study, which has not been formally published, also showed the vaccine remained effective while people waited for a second dose.
It was 76% effective during the three months after the first shot.