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 Pascal Soriot 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, 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 percent 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."