AstraZeneca Vaccine Shown to Curb Covid-19 Transmission, Preliminary Data Suggest

Researchers from the University of Oxford said Tuesday its Covid-19 vaccine could have a substantial effect on curbing virus transmission after one dose, and said that spacing doses apart by as long as three months improved effectiveness, according to data adding to previously published findings.

The data is preliminary and not peer reviewed, and some outside scientists said it wasn’t enough to draw definitive conclusions. Still, it is the first to show a vaccine might be able to slow the spread of the disease, not only prevent people from developing symptoms of it. All of the major Western vaccines now employed around the world have been shown to be effective at various levels at keeping people from getting sick. Scientists have hoped that they could reduce asymptomatic transmission, as well.

U.K. Health Secretary Matt Hancock said Wednesday it was “good news for the whole world” that the vaccine appeared to slow transmission.

Oxford said the vaccine may reduce symptomatic transmission of the virus by 67%, based on positive swab tests of vaccinated trial volunteers after a single dose, though those swab tests were only done in the U.K. The university, which developed the vaccine in partnership with AstraZeneca PLC, said the data, posted on the Lancet medical journal’s website, showed that effectiveness against symptomatic Covid-19 was well-sustained at 76% from 22 days after the first dose and until 90 days later.

But some independent researchers said it is hard to draw a complete conclusion from the results because of the limits of the trial. Larger numbers of younger volunteers receiving the two-dose vaccine at longer intervals between shots may have skewed the results, for example, said Azra Ghani, chair in infectious disease epidemiology at Imperial College London. Dosing schedules should be compared side by side, while controlling other factors such as age, Prof. Ghani said.

Similarly, scientists warned that the apparent effect on transmission left a lot of room for the virus to spread among people with no symptoms, even after vaccination. Government officials and health experts have warned vaccines won’t be a panacea for stopping the spread of the virus.

Echoing that, Oxford’s Andrew Pollard said Wednesday the evidence suggesting the vaccine might help curb transmissions is positive but cautioned against reading too much into it. For one thing, the trials couldn’t measure risks trial volunteers took or predict other factors for who would test positive, Prof. Pollard said in a press briefing with AstraZeneca. “We’re certainly not to the point anywhere in the world where physical-distancing measures could be dropped,” he said.

Another major source of unpredictability is new mutations and variants, Prof. Pollard added. A vaccine’s benefits today probably won’t last, he said: “[Viruses] are going to be able to evolve to evade human immunity, and so many of the new mutations that we’re seeing are likely to allow ongoing transmission in the future.”

“None of us are recommending anything other than to continue to follow guidelines—be socially distanced,” said Mene Pangalos, AstraZeneca’s executive vice president for biopharmaceuticals research and development.

A big AstraZeneca-run trial of the vaccine that is nearing completion in the U.S. won’t add much to the U.K. data about transmission, Dr. Pangalos said. That is because the U.S. trial isn’t running swab tests on volunteers weekly—tests the U.K. transmission evidence comes from.

The data build on results from December, and include data involving more than 17,000 volunteers in Oxford-run late-stage trials in the U.K., Brazil and South Africa.

The findings are similar to an analysis by U.K. government advisers and regulators on which they based their policy of spreading out shots of the vaccine to reach more people quickly with first doses. At the time, U.K. authorities said that after three weeks, the Oxford-AstraZeneca vaccine was around 70% effective up until the second dose, even if that was 12 weeks later.

Oxford said Tuesday that trial participants showed 82.4% effectiveness after receiving two doses spaced 12 weeks apart, compared with just 54.9% if the doses were spaced less than six weeks apart. They said protection wasn’t reduced over a three-month period. The additional data are based on results up to Dec. 7 and don’t address levels of effectiveness against newer variants common in the U.K. and South Africa. Data in coming days are expected to shed light on how well the vaccine protects against those fast-spreading versions of the virus.

The additional data don’t add significantly to findings of the vaccine’s effectiveness in the elderly. Independent researchers have criticized the lack of findings for the vaccine in adults older than 55 years. Oxford researchers have said older volunteers were recruited later for safety reasons, and have said ongoing trials should provide more information.

U.K. vaccine advisers and the British medicines regulator said in December that data from ongoing Oxford-led clinical trials supported a broad policy of prolonging the interval between vaccine doses, saying the country had to make trade-offs to curb hospital admissions and deaths.

AstraZeneca and Oxford supported the Dec. 30 policy announcement, in contrast to Pfizer Inc., which said data from the vaccine it developed with BioNTech SE didn’t support delaying the second dose past the 21 days they recommended.

The U.K. applied the policy to both vaccines. Health and government officials have continued to defend that decision against arguments from some scientists and practicing doctors that second doses of the Pfizer-BioNTech vaccine in particular shouldn’t be delayed.

Critics of the delayed second dosing say it is best to stay with proven schedules and not risk diminished protection from what has been found in carefully controlled trials.

But Oxford said the effect of longer intervals boosting protection is similar to findings for vaccines for influenza, Ebola and malaria. U.K. officials have said they are drawing from broader science than just the Covid-19 vaccine clinical trials alone. Other countries have followed suit, some recommending dosing intervals no more than six weeks apart; the U.S. has recommended sticking with the three or four weeks between doses in line with most clinical-trial results.

The U.K. has vaccinated more than 9.6 million people with at least one dose, according to government statistics as of Tuesday. Almost 500,000 people in the U.K. have received second doses. The vaccine has received a green light for use in India and elsewhere, including in Europe, but has hit manufacturing shortfalls in Europe.

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