LONDON (NewsNation Now) —  AstraZeneca and Oxford University’s COVID-19 vaccine was 79% effective in a large U.S. trial at preventing symptomatic illness, and was 100% effective against severe or critical disease and hospitalization, the drugmaker said on Monday.

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. 

Investigators said the vaccine was effective across 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.”

The AstraZeneca shot 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.

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.”

The early findings from the U.S. study are just one set of information AstraZeneca must submit to the Food and Drug Administration.

In a statement, the drugmaker said it will continue to analyze the data and prepare for the primary analysis to be submitted to the FDA for Emergency Use Authorization in the coming weeks.

An FDA advisory committee will then publicly debate the evidence behind the shots before the agency decides whether to allow emergency use of the vaccine.

The fresh data comes after many countries began resuming use of the vaccine after the European Medicines Agency (EMA) and the World Health Organization (WHO) said the benefits outweighed the risks following investigations into reports of blood clots.

AstraZeneca said an independent safety committee conducted a specific review of the blood clots in the U.S. trial, as well as cerebral venous sinus thrombosis (CVST), which is an extremely rare blood clot in the brain, with the help of an independent neurologist.

The London-listed firm said the panel found “no increased risk of thrombosis or events characterized by thrombosis among the 21,583 participants receiving at least one dose of the vaccine. The specific search for CVST found no events in this trial.”

(Reporting by Ludwig Burger in Frankfurt, Pushkala Aripaka and Muvija M in Bengaluru; Editing by Josephine Mason, Mark Potter, Sherry Jacob-Phillips and Edmund Blair)

The Associated Press and Reuters contributed to this report. Reporting by Ludwig Burger, Pushkala Aripaka and Muvija M.