The Oxford-AstraZeneca COVID-19 vaccine may be able to reduce transmission of the coronavirus, while also stopping serious disease and death from COVID-19, early data suggests.

This is the first time that a vaccine has shown it can stop spread of the virus, according to the BBC. Still, experts caution that more data is needed to confirm that trend.

To look at transmission, the researchers at the University of Oxford conducted weekly coronavirus tests of participants in the U.K. enrolled in a vaccine trial and found that the rate of positive results declined by about 67% after participants received one dose. Testing negative means no virus is present and makes it less likely a person is infected, even asymptomatically. People without detectable virus in their respiratory tract can’t spread the virus. The work has not yet been peer-reviewed but was published as a preprint with the Journal The Lancet. 

Other vaccine studies, by contrast, have mainly tested people who showed symptoms of COVID-19, not everyone enrolled in the trial, which means they couldn’t tell how many vaccinated people were asymptomatic, but still infectious to others.

“If there was no impact of a vaccine on asymptomatic infection, it would be expected that an efficacious vaccine would simply convert severe cases to mild cases and mild cases to asymptomatic, with overall PCR positivity unchanged,” the authors wrote in the study. But that’s not what happened — the total number of positives declined. 

Experts exercised caution, saying that more data is needed before this reduction in transmission is confirmed, according to The New York Times. 

And data from this vaccine can’t be applied to others for COVID-19. However, there are hints that the Moderna vaccine may also cut transmission; when participants came in for their second dose of that shot, they were also tested for SARS-CoV-2 and the rate of asymptomatic cases dropped by 60%, according to the Boston Herald.

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The AstraZeneca trial was studying whether the vaccine prevented  severe disease and death. They found that a single dose of the vaccine is 76% effective in protecting people from symptomatic disease 22 days after vaccination. They also found that the timing of the second dose greatly impacted the efficacy; the vaccine efficacy rose from 54.9% when the second dose was given less than six weeks after the first shot to 82.4% when the two doses were given 12 or more weeks apart.

This finding suggests that the dosing interval, and not the dosing level, has the biggest  impact on the efficacy of the vaccine, according to a statement. The U.K. has taken a different approach than other nations by trying to vaccinate as many people as possible with a single dose and delaying the second dose by about 12 weeks, according to the BBC. Some experts have cautioned that delaying the time between doses, however, could create space for new variants to emerge, according to a JAMA perspective. 

The Oxford-AstraZeneca vaccine has emergency approval in the U.K. but has yet to be given approval in the U.S. One in six people in the U.K., or about 10 million, have been vaccinated in the U.K. with either the Pfizer or the Oxford-AstraZeneca vaccine. But the U.K. has also approved the Moderna COVID-19 vaccine and will likely receive doses in the spring, according to the BBC.

Originally published on Live Science.

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