The AstraZeneca vaccine cuts COVID transmission—and delaying the second dose is a good idea, study finds

February 3, 2021, 11:22 AM UTC

New data from clinical trials of the coronavirus vaccine developed by pharmaceutical company AstraZeneca and scientists at the University of Oxford shows a single dose of the vaccine reduces transmission of the virus by about two-thirds.

It is the first clinical evidence so far that one of the approved coronavirus vaccines doesn’t just help the person receiving the jab avoid COVID-19 symptoms, but also provides “sterilizing immunity” that prevents the inoculated person from being able to infect others.

That is significant because if the vaccines prevent transmission, as well as preventing illness, it will enable countries to lift social distancing measures and lockdowns more rapidly than would otherwise be the case.

The Oxford-AstraZeneca vaccine is also seen as crucial to helping end the pandemic globally because it costs less than other vaccines, and can be stored at normal refrigerator temperatures, unlike the vaccines produced by Pfizer and Moderna, which require special equipment to keep them in ultra-freezing conditions.

Deborah Dunn-Walters, professor of immunology at the University of Surrey and chair of the British Society for Immunology’s COVID-19 task force, said it was “very encouraging” that the study provided “indication that virus carriage is reduced.”

The new data, which has not yet been peer-reviewed, was published by Oxford researchers who pioneered the vaccine. It shows that, in clinical trials in the U.K., the number of volunteers who tested positive for coronavirus using polymerase chain reaction (PCR) tests after a single dose was reduced by 67%. This suggests these people were not only asymptomatic, but had such low viral loads they would be unable to infect others.

Why wait

The same study, looking at a broader pool of volunteers from clinical trials in the U.K., Brazil, and South Africa, also showed that a single dose of the vaccine was about 76% effective at preventing symptomatic COVID-19, starting 22 days after the first dose, as it took the body time to develop an immune response. This immunity then lasted for at least 90 days.

The analysis found that waiting for 12 weeks to give a second dose conferred better overall immunity than giving a second dose sooner. The vaccine’s efficacy rose from 54.9% when the second dose was given at less than six weeks to 82.4% when given at 12 weeks.

The vaccine was shown to be 100% effective at preventing severe COVID-19, with no hospitalizations or deaths reported among those who received the jab.

The data will provide much-needed scientific support to the U.K. government’s controversial decision to use its existing vaccine supplies to give as many people as possible a first shot, while delaying their second shots for 12 weeks.

“This is excellent news and very much what was expected from what we know about other vaccines,” David Matthews, a virologist at the University of Bristol, said.


Most epidemiologists agreed that vaccinating more people with an initial dose sooner would likely save more lives. And ever since the U.K. announced its decision, several other countries have announced similar plans.

But some virologists, as well as Pfizer and Moderna, makers of two of the vaccines the U.K. has authorized, have questioned the decision. That’s because for these vaccines, which use a different technology than the Oxford-AstraZeneca vaccine, there is no clinical trial data yet to indicate how such a delay in second doses affects how well the vaccines work. Those two vaccines, which use messenger RNA, were only tested with booster shots given three to four weeks after the initial dose.

Matt Hancock, the U.K. health secretary, hailed the new analysis as a “hugely encouraging study” that showed “the Oxford vaccine works and works well.”

In recent weeks, government officials in Europe had raised questions about how well the Oxford-AstraZeneca vaccine worked in older people, with an advisory body to the German health regulator recommending against giving the inoculation to adults over the age of 65 because there wasn’t enough clinical data from this age cohort to prove the vaccine worked for them. French President Emmanuel Macron then gave an interview in which he said the vaccine was “quasi-ineffective” in people over 65. Nonetheless, the European Medicines Agency, last week authorized the Oxford-AstraZeneca vaccine for use in all adults.

Some scientists, though, warned against reading too much into the latest study results. Azra Ghani, professor of infectious disease epidemiology at Imperial College London, said the Oxford-run clinical trials were not designed to assess different dosing gaps and that participants across groups with different timings for the second dose were not randomized.

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