Soriot said he didn't expect the additional trial to hold up regulatory approvals in the UK and European Union.

Clearance from the US Food and Drug Administration may take longer because the regulator is unlikely to approve the vaccine on the basis of studies conducted elsewhere, especially given the questions over the results, he said. Authorisation in some countries is still expected before the end of the year, he said.

Loading

AstraZeneca and its CEO are facing scrutiny as the drugmaker responds to growing confusion over the vaccine.

The company's late-stage data initially increased confidence that the world would soon have multiple shots to combat the pathogen, following positive reports from front-runners Pfizer and Moderna. But scant disclosures and the manufacturing discrepancy have sparked doubts among scientists and investors.

Australia has advance purchasing agreements for all three vaccines, as well as an agreement with the University of Queensland for their vaccine which has just begun phase-three trials.

Different rates

AstraZeneca and its partner, the University of Oxford, reported on Monday that a lower initial dose of the vaccine, followed by a full dose, produced a 90 per cent efficacy rate, compared with 62 per cent for two full doses.

A day after the data was unveiled, the head of the US vaccine program known as Operation Warp Speed said that the regimen showing the higher level of effectiveness was tested in a younger population. He also said that the half-dose was given to some people because of an error in the quantity of vaccine put into some vials. None of those details were disclosed in AstraZeneca or Oxford's original statements.

The company has previously said it was considering adding a new arm to its US trial to test the lower dosage.

Loading

AstraZeneca and researchers have declined to provide more data ahead of a peer-reviewed analysis that is expected to be published in the coming weeks. Results have been submitted to an undisclosed journal, AstraZeneca said in a statement.

AstraZeneca's is one of three vaccines that could be approved before the end of the year. Pfizer and Moderna, which have both created vaccines using messenger RNA, published data earlier this month that showed their vaccines were about 95 per cent effective, and Pfizer has applied to US regulators for emergency approval.

There is added pressure on the AstraZeneca shot to succeed because it is easier to store and the company is selling it at cost during the pandemic, which means many low- and middle-income countries are relying on it.

Bloomberg