(Reuters) — AstraZeneca is likely to run an additional global trial to assess the efficacy of its COVID-19 vaccine using a lower dosage, its chief executive was quoted as saying on Thursday amid questions over the results from its late-stage study.

Instead of adding the trial to an ongoing U.S. process, AstraZeneca might launch a fresh study to evaluate a lower dosage of its vaccine that performed better than a full dosage, Pascal Soriot was quoted as saying in a Bloomberg News report.

“Now that we’ve found what looks like a better efficacy we have to validate this, so we need to do an additional study,” he said.

Soriot said it would 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.”

The news comes as AstraZeneca faces questions about its success rate that some experts say could hinder its chances of getting speedy U.S. and EU regulatory approval.

Several scientists have raised doubts about the robustness of results released on Monday showing the experimental vaccine was 90% effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.

Soriot said he did not expect the additional trial to delay UK and European regulatory approvals.

Asked about the Bloomberg report, an AstraZeneca spokesman said there was “strong merit in continuing to further investigate the half-dose/ full dose regimen.”

“We are further evaluating the data and will work with regulators on the best approach for further evaluation,” he said.

“This would add to data from existing trials which are currently being prepared for regulatory submission.”

AstraZeneca told Reuters earlier on Thursday that administering of the half dose had been reviewed and approved by independent data safety monitors and the UK regulator, adding that the regulator publicly confirmed there was “no concern.”

Clearance from the U.S. Food and Drug Administration (FDA) may take longer though because the agency is unlikely to approve the vaccine based on studies carried out elsewhere, especially given the questions over the results, Soriot said.

Authorization in some countries is still expected before the end of the year, he added.

AstraZeneca research chief Mene Pangalos told Reuters on Monday that researchers had stumbled upon the half-dose regime by accident, saying a sub-group of the trial was given a smaller initial dose by mistake.

Earlier he had said that the firm would start discussions with the FDA to change the design of its experimental COVID-19 vaccine trial to add the more-effective dosage regime.

Running an additional trial might not be too much of a complication for the British drugmaker in the race to develop a successful vaccine to help tame the pandemic, which has killed more than a million people and roiled the global economy.

Helen Fletcher, professor of immunology at the London School of Hygiene & Tropical Medicine, said that another trial would not necessarily delay getting a green light as efficacy in the higher dose regime still met the World Health Organization’s target, and it was not unusual to run new studies on approved vaccines.

“It’s entirely possible AZ and Oxford could license the high dose and then quickly seek an amendment to use the low dose when they have sufficient data,” she said.

The vaccine is one of three that could get approved before the end the year. Earlier this month, Pfizer and Moderna reported that their vaccines were about 95% effective in preventing illness, setting the bar sky-high.

Even so, the AstraZeneca shot developed with Oxford University is cheaper to make, easier to distribute and faster to scale up than its rivals.

A peer-reviewed analysis of the trial data will be published in a medical journal in the coming weeks.

The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) is continuing its rolling review of the vaccine, the regulator’s Chief Executive June Raine said in an email on Thursday.

“Any vaccine must undergo robust clinical trials in line with international standards, with oversight provided by the Medicines and Healthcare products Regulatory Agency (MHRA), and no vaccine would be authorised for supply in the UK unless the expected standards of safety, quality and efficacy are met,” she said.

The European Medicines Agency did not immediately respond to requests for comment on the potential for another study.

(Reporting by Sabahatjahan Contractor in Bengaluru and Alistair Smout in London Writing by Josephine Mason; Editing by Alexander Smith, Pravin Char, Elaine Hardcastle and Tom Brown)