AstraZeneca says advanced trial data from a U.S. study on its COVID vaccine shows it is 79% effective.

Although AstraZeneca's vaccine has been authorized in more than 50 countries, it has not yet been given the green light in the U.S. The U.S. study comprised 30,000 volunteers, 20,000 of whom were given the vaccine while the rest got dummy shots. The results were announced Monday.

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

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.

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 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 is offering seniors protection.

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.

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.