The company wants the new test to confirm the 90% efficacy rate that the shot showed in a portion of an existing trial, Chief Executive Officer Pascal Soriot said. It’s favoring that option rather than adding an arm to a separate study that’s already underway in the U.S.

Questions are mounting over one of the fastest-moving shots after the company acknowledged that a lower dosage level that appeared more effective resulted from a manufacturing discrepancy. The company and its partner, the University of Oxford, didn’t initially disclose the error and other key details, leading to concern over their transparency.

“Now that we’ve found what looks like a better efficacy we have to validate this, so we need to do an additional study,” Soriot said in his first interview since the data were released. It will probably be another “international study, but this one could be faster because we know the efficacy is high so we need a smaller number of patients.”

Soriot said he didn’t expect the additional trial to hold up regulatory approvals in the U.K. and European Union.

The U.K. government said Friday it had asked the medicines regulator to assess whether the Astra-Oxford vaccine was suitable for temporary authorization. The unusual step comes after the government amended legislation in light of the pandemic to allow the U.K. to approve a vaccine ahead of the European regulator, which Britain is still subject to until January.

Clearance from the U.S. 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, Soriot said. Authorization in some countries is still expected before the end of the year, he said.

“The question for us was, will we need the U.S. data to get approval in the U.S. or can we get approval in the U.S. with international data, and it was never clear,” said Soriot, who is in quarantine after arriving in Australia. “Now with those results it’s more likely that we will need the U.S. data.”

Astra 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 Inc. and Moderna Inc. But scant disclosures and the manufacturing discrepancy have sparked doubts among scientists and investors.

Different Rates

Astra and Oxford reported Monday that a lower initial dose of the vaccine, followed by a full dose, produced a 90% efficacy rate in a smaller set of participants, compared with 62% for two full doses.

A day after the data were unveiled the head of Operation Warp Speed, the U.S. vaccine program, said that the regimen showing the higher level of effectiveness was tested in a younger population. He also said 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 Astra or Oxford’s original statements.

Soriot disputed the idea that the half-dose regimen was an error, saying that after researchers realized the dosing discrepancy they formally changed the trial protocol with the blessing of regulators.

“I won’t tell you we expected the efficacy to be higher,” said Soriot. But “people call it a mistake — it’s not a mistake.”

Astra shares closed 0.7% lower in London. They have declined about 7% this week amid questions about the trial results.

The company has previously said it might add a new arm to its U.S. trial to test the lower dosage.

Astra 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, Astra said in a statement.

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

There’s added pressure on the Astra shot to succeed because it’s 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.