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.
The impact on transmission is critical.
If a vaccine only stops you getting severely ill, but you can still catch and pass on the virus, then everyone will need to be immunised to be protected.
But if it also stops you spreading the virus then it would have a far greater impact on the pandemic as each person who is vaccinated indirectly protects other people too.
The study by the University of Oxford swabbed participants every week to test them for the presence of the virus.