AstraZeneca says U.S. trial data shows 79% efficacy against symptomatic COVID-19

AstraZeneca's COVID-19 vaccine provided strong protection against disease and complete protection against hospitalization and death across all age groups in a late-stage U.S. study, the company announced Monday.

Trial also showed 100% efficacy in stopping severe disease, hospitalization

AstraZeneca says it is preparing to seek emergency use authorization in the U.S. in the weeks ahead after the latest trial data was released. 'The vaccine was well tolerated, and the independent data safety monitoring board (DSMB) identified no safety concerns related to the vaccine,' the company said in a release. (Robert Barca/Getty Images)

AstraZeneca's COVID-19 vaccine provided strong protection against disease and complete protection against hospitalization and death across all age groups in a late-stage U.S. study, the company announced Monday.

AstraZeneca said its experts also identified no safety concerns related to the vaccine, including a rare blood clot that was identified in Europe. Scientists found no increased risk of clots among the more than 20,000 people who got at least one dose of the shot, which was developed with Oxford University.

Although AstraZeneca's vaccine has been authorized for use in more than 50 countries, including Canada, it has not yet been given the green light in the U.S. The U.S. study comprised more than 30,000 volunteers, of whom two-thirds were given the vaccine while the rest got dummy shots.

In a statement, AstraZeneca said its COVID-19 vaccine had a 79 per cent efficacy rate at preventing symptomatic COVID-19 and has 100 per cent efficacy in stopping severe disease and hospitalization. Investigators said the vaccine was effective across adults of all ages, including older people — which previous studies in other countries had failed to establish.

"These findings reconfirm previous results observed," said Ann Falsey of the University of Rochester School of Medicine, who helped lead the trial. "It's exciting to see similar efficacy results in people over 65 for the first time."

Julian Tang, a virologist at the university of Leicester who was unconnected to the study, described it as "good news" for the AstraZeneca vaccine.

"The earlier U.K., Brazil, South Africa trials had a more variable and inconsistent design and it was thought that the U.S. FDA would never approve the use of the AZ vaccine on this basis, but now the U.S. clinical trial has confirmed the efficacy of this vaccine in their own clinical trials," he said.

The early findings from the U.S. study are just one set of information AstraZeneca must submit to the Food and Drug Administration. An FDA advisory committee will publicly debate the evidence behind the shots before the agency decides whether to allow emergency use of the vaccine.

Study may clear up questions about product

Scientists have been awaiting results of the U.S. study in hopes it will clear up some of the confusion about just how well the shots really work.

Britain first authorized the vaccine based on partial results from testing in the United Kingdom, Brazil and South Africa that suggested the shot had about 70 per cent efficacy. But those results were clouded by a manufacturing mistake that led some participants to get just a half dose in their first shot — an error the researchers didn't immediately acknowledge.

Then came more questions about how well the vaccine protected older adults and how long to wait before the second dose. Some European countries including Germany, France and Belgium initially withheld the shot from older adults and only reversed their decisions after new data suggested it is offering seniors protection.

AstraZeneca's vaccine development was rocky in the U.S., too. Last fall, the Food and Drug Administration suspended the company's study in 30,000 Americans for an unusual six weeks, as frustrated regulators sought information about some neurologic complaints reported in Britain; ultimately, there was no evidence the vaccine was to blame.

Last week, more than a dozen countries, mostly in Europe, temporarily suspended their use of the AstraZeneca shot after reports it was linked to blood clots. On Thursday, the European Medicines Agency concluded after an investigation that the vaccine did not raise the overall risk of blood clots, but could not rule out that it was connected to two very rare types of clots.

WATCH | Vaccine co-creator highlights positive U.S. trial results for AstraZeneca's shot:

AstraZeneca vaccine co-creator says U.S. trial data show vaccine is safe and effective

2 years ago
Duration 1:51
Jenner Institute director and AstraZeneca vaccine co-creator Adrian Hill tells Power & Politics data from a U.S. trial show that the AstraZeneca vaccine is safe and effective against COVID-19.

France, Germany, Italy and other countries subsequently resumed their use of the shot on Friday, with senior politicians rolling up their sleeves to show the vaccine was safe.

French Prime Minister Jean Castex, 55, received his first dose of the AstraZeneca jab last week live on TV, as did 56-year-old British Prime Minister Boris Johnson. In Quebec, Health Minister Christian Dubé, 64, also got a dose of the vaccine last week. 

Health Canada said in a release last week that based on an assessment of the available data, it believed that the benefits of the vaccine outweighed the risks.

"As the vaccine rollout continues in Canada, Health Canada will continue to monitor the use of all COVID-19 vaccines closely," the agency said.

1 of 4 'viral vector' vaccines

The U.S recently agreed to send 1.5 million doses to Canada and another 2.5 million doses to Mexico. When those vaccines will arrive in Canada wasn't immediately clear, but it could be this week, Procurement Minister Anita Anand said.

AstraZeneca said Monday it would continue to analyze the U.S. data in preparation for submitting it to the FDA in the coming weeks. It said the data would also soon be published in a peer-reviewed journal.

The AstraZeneca vaccine is what scientists call a "viral vector" vaccine. The shots are made with a harmless virus, a cold virus that normally infects chimpanzees. It acts like a Trojan horse to carry the spike protein's genetic material into the body, which in turn produces some harmless protein. That primes the immune system to fight if the real virus comes along.

Three other vaccine makers, Johnson & Johnson; China's CanSino Biologics, which makes the Convidecia vaccine; and Russia's Gamaleya Research Institute, which developed the Sputnik V vaccine, make COVID-19 vaccines using the same technology but with different cold viruses.

The AstraZeneca shot has become a key tool in European countries' efforts to boost their sluggish vaccine rollouts. It is also a pillar of a UN-backed project known as COVAX that aims to get COVID-19 vaccines to poorer countries.


  • An earlier version of this story reported that AstraZeneca-Oxford's vaccine was 79 per cent effective. In fact, a clinical trial demonstrates a shot's efficacy compared with a placebo, not effectiveness in the real world.
    Mar 22, 2021 11:43 AM ET


  • An earlier version of this story mistakenly said that only three companies currently produce viral vector COVID-19 vaccines. In fact, four of the current vaccines are viral vector vaccines: AstraZeneca-Oxford, Johnson & Johnson, Convidecia from CanSinoBIO Biologics and Russia's Sputnik V.
    Mar 22, 2021 9:34 PM ET
  • This story has been updated to replace the incorrect description that the AstraZeneca vaccine is "100 per cent effective" in preventing the severe outcomes of COVID-19; in fact, the company says the vaccine has 100 per cent efficacy to prevent those outcomes. Effectiveness refers to outcomes in "the real world," while efficacy refers to outcomes in clinical trials.
    Apr 30, 2021 12:09 PM ET

With files from CBC News and The Canadian Press

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