A single dose of the Oxford vaccine may reduce transmission of coronavirus by two thirds, according to a new study.
Researchers said that the first dose of the Oxford/AstraZeneca jab offers protection of 76% up to three months and may reduce transmission by 67% - with efficacy rising to 82.4% after the second dose 12 weeks later.
The data from the study by the University of Oxford, which has not yet been peer reviewed, supports the four to 12-week prime-boost dosing interval that many global regulators, including the UK's, have recommended.
Before these results, little was known about how effective the Covid-19 vaccines were at preventing transmission of the disease.
But these results show that people who have been vaccinated are protected from the disease and are unlikely to pass the virus on.
It also suggests the vaccine eliminates severe illness, as none of those in the study were admitted to hospital with coronavirus.
Health Secretary Matt Hancock said: "This is a hugely encouraging study and further reinforces our confidence that vaccines are capable of reducing transmission and protecting people from this awful disease.
"This report shows the Oxford vaccine works and works well."
Dr Gillies O'Bryan-Tear said the results were the first definitive estimate of the impact of vaccination on transmission rates.
He said: "If the effect on transmission is confirmed for the Pfizer vaccine too, this would be a very positive.
"If these vaccines reduce transmission to the extent reported, it will mean that the easing of social restrictions will be enabled sooner, than if we have to wait for herd immunity - which may never in fact be achieved because of insufficient vaccine population coverage."
He added: "That would be the holy grail of the global vaccine rollout, and these data bring us one step closer."
Prime Minister Boris Johnson said the data from the study was "really encouraging".
Professor Andrew Pollard, chief investigator of the Oxford Vaccine Trial - and study co-author, said: "These new data provide an important verification of the interim data that was used by more than 25 regulators including the MHRA (Medicines and Healthcare products Regulatory Agency) and EMA (European Medicines Agency) to grant the vaccine emergency use authorisation.
"It also supports the policy recommendation made by the Joint Committee on Vaccination and Immunisation (JCVI) for a 12-week prime-boost interval, as they look for the optimal approach to roll out, and reassures us that people are protected from 22 days after a single dose of the vaccine."
The study came as Public Health England (PHE) said it was investigating strains of coronavirus in the UK which have developed a mutation that has been worrying scientists.
Eleven cases in the Bristol area have been identified as the variant that originally arose in Kent but are now showing the E484K mutation.
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A cluster of 32 cases in Liverpool also have the same mutation but relate to the original strain of coronavirus that has been around since the start of the pandemic.
The South African variant - which also shows the mutation - is under investigation in at least eight postcode areas of England where cases not linked to travel have been found.
A PHE spokesman said: "PHE is monitoring the situation closely and all necessary public health interventions are being undertaken, including enhanced contact tracing and control measures."
The E484K mutation has been shown to reduce the effectiveness of vaccines in preventing people contracting Covid-19.
However, public health experts believe current vaccines will still be effective against strains with the mutation, although at a lower level, and are good at preventing severe disease.
