AstraZeneca reported on March 22 that its COVID-19 vaccine provided strong protection among all adults in a long-anticipated U.S. study, raising hopes that the findings could help rebuild public confidence in the beleaguered shot in other countries and moving a step closer to clearance for American use.

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AstraZeneca said the vaccine was 79% effective overall at preventing symptomatic cases of COVID-19—including in older people—and that none of the study volunteers who were vaccinated were hospitalized or developed severe disease. The company also said its experts did not identify any safety concerns related to the vaccine, including finding no increased risk of rare blood clots identified in Europe.

The findings bolster AstraZeneca's prior research in Britain and other countries, and add to real-world evidence that the shots are offering good protection as they're used more widely. But confidence in the vaccine has been repeatedly hit because of concerns about how data was reported from some previous trials, confusion over its efficacy in older adults and a recent scare over clotting.

AstraZeneca said it will seek clearance in the United States "in the coming weeks," putting it on track to arrive just as the country is projected to have a big boost in supplies of three other vaccines—from Pfizer, Moderna and Johnson & Johnson—that already are in use.

AstraZeneca's interim results are based on 141 COVID-19 cases in the 30,000-person trial, but officials declined to tell reporters during a news conference Monday how many were in study volunteers who received the vaccine and how many in those who got dummy shots. Two-thirds of the volunteers received vaccine.

"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."

A Food and Drug Administration advisory committee will publicly debate the evidence behind the shots before the agency decides whether to allow emergency use. Ruud Dobber, an AstraZeneca executive vice president, said that if the FDA OK's the vaccine, the company will deliver 30 million doses immediately—and another 20 million within the first month.

The AstraZeneca shot, which has been authorized in more than 70 countries, is a pillar of a U.N.-backed project known as COVAX that aims to get COVID-19 vaccines to poorer countries, and it has also become a key tool in European countries' efforts to boost their sluggish vaccine rollouts. That important role in the global strategy to stamp out the pandemic make doubts about the shot especially worrying.

Stephen Evans, of the London School of Hygiene & Tropical Medicine, said the new data could help to allay concerns about the vaccine.

"The benefits of these results will mainly be for the rest of the world where confidence in the AZ (AstraZeneca) vaccine has been eroded, largely by political and media comment," he said.

Dr. Paul Hunter, a professor of medicine at the University of East Anglia, said the results were reassuring but that more details were needed to back up AstraZeneca's claim that the vaccine was completely effective at preventing severe disease and hospitalization.

"It would be good to know how many severe cases occurred in the control group and so what the confidence intervals are for this 100% figure," said Hunter, who was not connected to the study. "But this should add confidence that the vaccine is doing what it is most needed for."

Scientists had hoped the U.S. study would clear up some of the confusion about just how well the shots really work, particularly in older people. Previous research suggested the vaccine was effective in younger populations, but there was no solid data proving its efficacy in those over 65, often those most vulnerable to COVID-19.

Britain first authorized the vaccine based on partial results from testing in the United Kingdom, Brazil and South Africa that suggested the shots were about 70% effective. 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 was offering seniors protection.

AstraZeneca's vaccine development was rocky in the U.S., too. Last fall, the FDA suspended the company's study 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 rare blood clots—even as international health agencies insisted the benefits of the vaccine outweighed the risks. 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. It recommended adding a warning about these cases to the vaccine's leaflet.

It's not unheard of for such rare problems to crop up as vaccines are rolled out since trials typically look at tens of thousands of people, and some issues are only seen once the shot is used in millions of people.

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.

The AstraZeneca shot 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 coronavirus's spike protein's genetic material into the body that in turn produces some harmless protein. That primes the immune system to fight if the real virus comes along.

Two other companies, Johnson & Johnson and China's CanSino Biologics, make COVID-19 vaccines using the same technology but using different cold viruses.

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—Maria Cheng and Lauran Neergaard