Astra Shot Better at Stopping Covid Than Slowing Transmission
A pedestrian walks past signage outside an AstraZeneca Plc research and development facility in Shanghai. (Photographer: Qilai Shen/Bloomberg)

Astra Shot Better at Stopping Covid Than Slowing Transmission

A vaccine from the University of Oxford and AstraZeneca Plc shows only a limited ability to stop transmission of the coronavirus despite preventing Covid-19 illness in a majority of those who are infected.

As promising vaccine data keeps rolling in, one of the central unanswered questions has been whether inoculations can not only stop people from getting sick but also slow the spread of the virus, a key element for reopening economies.

Oxford and Astra are the first vaccine developers to unveil data on asymptomatic infection rates in people who received their shot. Overall, it reduced such transmissions by 27% in a large study, according to peer-reviewed results published in the Lancet medical journal on Tuesday.

That’s well below the vaccine’s 70% effectiveness at preventing symptomatic Covid-19 cases overall, though even those results are clouded by questions over its benefits in older recipients.

Researchers tested more than 6,000 study participants in the U.K. for Covid-19 infections on a weekly basis and found 29 asymptomatic infections in the vaccinated group. That compared with 40 cases in the control group, which was roughly the same size.

Smaller Group

Vaccine efficacy against asymptomatic transmission was higher, at 59%, in a smaller group that received a half dose, followed by a full second dose, rather than two full shots. The data support Oxford’s preliminary finding that the low-dose, full-dose regimen appears to elicit a stronger immune response, preventing 90% of infections. Two full doses were only 4% effective against asymptomatic transmission.

Although vaccines that prevent illness but not transmission could be widely used and help lower the disease’s death toll, they could also lead to complacency within populations, experts say. Those who are immunized with them could still spread the virus to other, more vulnerable people, including those who’ve yet to receive a shot or can’t get one for medical reasons.

Andrew Pollard, the Oxford professor who led the study, called the data “tantalizing” but said it was too early to draw firm conclusions. He said researchers are trying to analyze the amount of virus in the swabs to determine whether vaccination reduced viral loads, which would help limit contagion.

Astra Shot Better at Stopping Covid Than Slowing Transmission

“What would be fantastic would be vaccines that prevent transmission so you reach the point where you not only protect the people who are vaccinated but also those who can’t be vaccinated or haven’t been vaccinated yet,” he said in an interview.

Similar questions surround other early Covid vaccines, including one from Pfizer Inc. and BioNTech SE that’s been approved in the U.K. -- where the first shots were given Tuesday. It’s possible that shot didn’t work as well at preventing asymptomatic cases as it did at preventing illness, according to a report by staff of the U.S. Food and Drug Administration.

Pfizer expects to report data on whether or not its vaccine stops transmission of the virus within the first quarter of 2021, Chief Executive Officer Albert Bourla said.

©2020 Bloomberg L.P.

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