The jab may also reduce transmission by 67%.
The Oxford/AstraZeneca vaccine may have a "substantial effect" on transmission of the virus, and a three-month gap between doses does not lower protection, according to a report.
Tests results by University of Oxford, published in a pre-print report with The Lancet, showed a 67% reduction in positive swabs among those vaccinated.
In the report, the university also said that a single standard dose of the Oxford/AstraZeneca vaccine is 76% effective from day 22 to day 90 after the jab - it means that protection is not reduced in the three months between the first and second dose.
The scientists who developed the vaccine at the Jenner Institute have analysed more data from the UK clinical trial, including a further 201 cases of COVID.
They say it shows that one dose of the vaccine prevented 76% of infections and there was no drop in protection over the following 90 days.
The study has been submitted to The Lancet medical journal and supports the UK government's decision to extend the interval between two doses of the Oxford-AstraZeneca coronavirus vaccine for up to 12 weeks.
The long gap between doses also seems to boost the effectiveness of the vaccine.
The vaccine is also likely to reduce transmission of the virus, according to the study - which hasn't yet been peer reviewed.
PCR tests showed there was a 67% reduction in positive swabs in those who had been vaccinated. In other words there were fewer people with asymptomatic infections that could pass the virus on.