WASHINGTON (REUTERS) - AstraZeneca said on Thursday (March 25) that its Covid-19 vaccine was 76 per cent effective at preventing symptomatic illness and completely stopped severe or critical forms of the disease, citing a new analysis of up-to-date results for its major US trial.

US health officials earlier in the week publicly rebuked the drugmaker for using outdated information when calculating that the vaccine was 79 per cent effective.

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 US emergency use authorisation 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.

"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," he said, adding 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 authorisation in more than 70 countries.

The highly unusual rebuke from US 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.

Canada's health department said on Wednesday that it has updated the product label for AstraZeneca's Covid-19 vaccine to provide information on "very rare reports of blood clots associated with low levels of blood platelets."

Health Canada continues to back the vaccine, saying that it had not received any reports of these blood clots to date.

Canada, which is using AstraZeneca doses made at the Serum Institute of India, has received 500,000 doses and expects to get 1.5 million more by May.

More than a dozen countries this month 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 sceptical about its safety.