The makers of the Oxford University and AstraZeneca coronavirus vaccine say they plan to conduct an additional global trial to assess the drug after questions were raised over some of its original results.

Scientists had expressed doubts about the robustness of results showing the shot was 90% effective in a sub-group of trial participants who were accidentally given a half dose followed by a full dose, instead of two full doses. The difference was caused by a manufacturing error.

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

The average efficacy of the vaccine was reported as just over 70%, taking into account both the group that received an initial half dose and the group that received two full doses.

Soriot said he did not expect the additional trial to hold up regulatory approvals in the UK and European Union – but it is thought the concerns over the trial could hold up approval in the US.

The UK has pinned high hopes on this particular vaccine, ordering 100m doses and planning an ambitious, rapid rollout. Other jabs, made by Pfizer/BioNTech and Moderna, have also reported high success rates, but Britain has ordered fewer doses. 

At a press conference on Thursday, the matter was put to chief medical officer Professor Chris Whitty and the UK’s chief scientific adviser Sir Patrick Vallance. 

Professor Whitty said: “There’s always debate about virtually everything. The key thing to do is leave this in the hands of the regulator.”

The Medicines and Healthcare products Regulatory Agency will have access to fuller data than has been made public, he added, and will make an assessment based on what is put before it.

The Department for Health and Social Care has been approached for fuller comment.

On Monday, Oxford and AstraZeneca announced preliminary results from trials showed an average efficacy of 70% – a figure reached by pooling the results from two different dosing regimens.

One month apart, one set of volunteers received two identical doses while the other received a half-dose, and then a full dose. In the first group, the efficacy was 62%. In the second, it was 90% – but it has since been admitted that no older people received this regimen, making it hard to be sure whether they would respond so well in the real world.