The University of Oxford research is the first to document a vaccine substantially reducing the virus’s spread. British officials said the results underscored how mass inoculations were key to ending the pandemic. The vaccine developed by the University of Oxford and AstraZeneca not only protects people from serious illness and death but also substantially slows the transmission of the virus, according to a new study — a finding that underscores the importance of mass vaccination as a path out of the pandemic.

The study by researchers at the University of Oxford is the first to document evidence that any coronavirus vaccine can reduce transmission of the virus. Researchers measured the impact on transmission by swabbing participants every week seeking to detect signs of the virus. If there is no virus present, even if someone is infected, it cannot be spread. And they found a 67 percent reduction in positive swabs among those vaccinated.

The results, detailed by Oxford and AstraZeneca researchers in a manuscript that has not been peer-reviewed, found that the vaccine could cut transmission by nearly two-thirds.

Matt Hancock, the British health secretary, hailed the results on Wednesday as “absolutely superb.” “We now know that the Oxford vaccine also reduces transmission and that will help us all get out of this pandemic,” Mr. Hancock said in an interview Wednesday morning with the BBC.

The results, he said, “should give everyone confidence that this jab works not only to keep you safe but to keep you from passing on the virus to others.” Some scientists looking at the limited information released cautioned that more analysis of the data was needed before such broad conclusions could be firmly stated.

“While this would be extremely welcome news, we do need more data before this can be confirmed and so it’s important that we all still continue to follow social distancing guidance after we have been vaccinated,” said Dr. Doug Brown, chief executive of the British Society for Immunology.

The Oxford and AstraZeneca researchers also found that a single dose of the vaccine was 76 percent effective at preventing Covid-19. The data measured the three months after the first shot was given, not including an initial three-week period needed for protection to take effect.