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.