The Cambridge-based pharmaceutical company that teamed up with Oxford University to develop a coronavirus vaccine has expressed the need for another global trial.

AstraZeneca chief executive Pascal Soriot said it had discovered by accident that using a half dose, followed by a full dose at least one month apart was most effective in preventing COVID-19.

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

AstraZeneca is set to run an additional global trial to assess the efficacy of the vaccine when half a dose is given first.

On Monday, it revealed the experimental vaccine was 90 per cent effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.

The other dosing regimen showed 62 per cent efficacy when given as two full doses at least one month apart.

Instead of adding the trial arm to an ongoing US process, a new study would be run to evaluate the lower dosage that performed better than a full amount in AstraZeneca’s studies, Mr Soriot said.

Mr 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 US and EU regulatory approval.

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

Clearance from the US 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, he said.

Authorisation 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.

-with AAP