The COVID-19 vaccine developed by AstraZeneca and the University of Oxford appears to significantly slow coronavirus transmission, making it the first coronavirus shot to show evidence of doing so.
Oxford researchers gave PCR swabs to study participants every week to detect any presence of COVID-19. The method did not directly measure transmission — that would have required tracing the contacts of the study volunteers, according to CNN. But the researchers found the vaccine reduced the rate of positive swabs by 67%.
If the vaccine was merely making infections milder, the PCR positivity rate would not have changed, the researchers wrote in the preprint study, which is being reviewed by The Lancet. They argued that measuring PCR positivity rates was an "appropriate" way to determine whether the vaccine reduces infection.
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Reducing the transmission rate has proven among the most challenging aspects of the pandemic response. The coronavirus has caused more than 26 million infections and 400,000 deaths in the U.S., according to the Centers for Disease Control and Prevention.
Reducing the transmission rate would help get the virus under control as countries around the world scramble to vaccinate their populations as quickly and safely as possible.
British Health Secretary Matt Hancock praised the findings on Twitter as "absolutely superb."
"We now know that the Oxford vaccine also reduces transmission and that will help us all get out of this pandemic," he said in an interview with the BBC, according to the New York Times.
The European Union, United Kingdom and India have authorized the AstraZeneca vaccine. Clinical trials are still ongoing in the U.S., with results expected to be released later this month. AstraZeneca could then seek an emergency use authorization from the U.S. Food and Drug Administration.
Though this marks the first study analyzing whether a COVID-19 vaccine can prevent transmission, health experts previously have said it would be surprising if COVID-19 shots did not mitigate the spread of infections.
Vaccines generally reduce transmission risk in addition to protecting the recipient from illness, Dr. Paul Sax, of Harvard Medical School, has written in The New England Journal of Medicine.
"After all, this is the primary reason why we have policies on mandatory school immunizations, influenza vaccines for hospital employees and country-specific entry requirements for the yellow fever vaccine."
Dr. Paul Offit, director of the Children's Hospital of Philadelphia's Vaccine Education Center, said many vaccines don't prevent asymptomatic infection, but he said asymptomatic spread may not be as worrisome going forward.
"Usually, if you're vaccinated, you tend to shed virus less," Offit said on the SiriusXM show "Doctor Radio Reports." "So I think, although we focus on that a lot, I don't think it's going to be as critical, frankly, as we move to more and more people, either being naturally infected or immunized."
Some scientists cautioned that more information is needed before any changes are made to social distancing guidelines in places where the AstraZeneca vaccine has been administered.
"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," Dr. Doug Brown, chief executive of the British Society for Immunology, told the New York Times.
The Oxford study also found that delaying the vaccine's second shot boosted its efficacy rating to 82.4% when the shots are administered at least 12 weeks apart. When they are administered with less than six weeks of separation, the efficacy rating was only 54.9%.
A single dose of the vaccine was found to be 76% effective at preventing COVID-19 between day 22 — the time frame the immune systems needs to develop protection — and 90 days after vaccination.
The findings support the U.K.'s decision to delay the second shot of the AstraZeneca vaccine, NBC News reported. Reaching a higher quantity of people could reduce the overall spread of the virus while vaccine supply is low.
But that doesn't mean the Pfizer and Moderna vaccines — the two that have been authorized in the U.S. — would be effective if spaced out in a similar way. Additional research is be needed on those vaccines specifically.
The U.S. has agreed to buy 300 million doses of AstraZeneca's vaccine. Oxford said data on the vaccine's ability to neutralize the new coronavirus variants will be released in the coming days.
As of Tuesday, the U.S. has vaccinated nearly 33 million people. A third vaccine could be authorized within the next several weeks, increasing the nation's supply.
Johnson & Johnson has completed a clinical trial on its COVID-19 vaccine candidate and plans to apply for emergency use authorization this month. Data showed its vaccine was 66% effective at preventing moderate to severe illness.