AstraZeneca said its COVID-19 vaccine had a 76 per cent efficacy rate at preventing symptomatic illness in a new analysis of its major U.S. trial — slightly lower than the level announced earlier this week in a report that was criticized for using outdated information.

U.S. health officials had publicly rebuked the drugmaker for not using the most up-to-date information when it published an interim analysis on Monday that said the vaccine had a 79 per cent efficacy rate.

The latest data was based on 190 infections among more than 32,400 participants in the United States, Chile and Peru. The earlier interim data was based on 141 infections through Feb. 17.

"The primary analysis is consistent with our previously released interim analysis, and confirms that our COVID-19 vaccine is highly effective in adults," Mene Pangalos, executive vice president of BioPharmaceuticals R&D at AstraZeneca, said in a statement.

AstraZeneca said it plans to seek U.S. emergency use authorization in the coming weeks and the latest data has been presented to the independent trial oversight committee, the Data Safety Monitoring Board.

AstraZeneca reiterated on Thursday that the shot, developed with Oxford University, was 100 per cent effective against severe or critical forms of the disease. It also said the vaccine showed 85 per cent efficacy in adults 65 years and older.

"A lot of us were waiting for this large, well-constructed and reported Phase III study," said Paul Griffin, a professor at the University of Queensland.

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.

"This appears to be a very effective vaccine with no safety concerns. Hopefully, this should now give people the confidence that this vaccine is the right one to continue to use moving forward," Griffin said, noting that he and his parents have taken the vaccine.

The updated 76 per cent efficacy rate compares with rates of about 95 per cent for vaccines from Pfizer-BioNTech and Moderna.

The AstraZeneca vaccine is, however, seen as crucial in tackling the spread of COVID-19 across the globe, not just due to limited vaccine supply but also because it is easier and cheaper to transport than rival shots. It has been granted conditional marketing or emergency use authorization in more than 70 countries.

The highly unusual rebuke from U.S. health authorities had marked a fresh setback for the vaccine that was once hailed as a milestone in the fight against the COVID-19 pandemic, but has been dogged by questions over its effectiveness and possible side effects.

The shot has faced questions since late last year when the drugmaker and Oxford University published data from an earlier trial with two different efficacy readings as a result of a dosing error.

Then this month, more than a dozen countries temporarily suspended giving out the vaccine after reports linked it to a rare blood clotting disorder in a very small number of people.

The European Union's drug regulator said last week the vaccine was clearly safe, but Europeans remain skeptical about its safety.

Countries including Germany, France, Italy and Spain said they would use the vaccine after the update from the European Medicines Agency, but the Danish government has decided to keep its suspension of the AstraZeneca vaccines for three more weeks.

Denmark paused the use of the AstraZeneca shot as a precautionary measure on March 11, after reports that a 60-year-old woman died with blood clots in several parts of her body a week after she received the vaccine. News that a second person in Denmark had died after getting the AstraZeneca vaccine was also reported.

Danish health authorities said they have no evidence the vaccine was responsible for either death. Norway and Sweden also have paused the use of the AstraZeneca vaccine.

Health Canada said this week it's updating the label on the AstraZeneca-Oxford and Covishield COVID-19 vaccines to add information about "very rare reports of blood clots associated with low levels of blood platelets," but says the shot remains safe and that the benefits of getting the vaccine outweigh the risks.