Health experts question claims vaccine can protect up to 90 percent of people against disease caused by SARS-CoV-2 as reported efficacy was a result of a mistake in dosing a small number of subjects.
AstraZeneca Plc Chief Executive Pascal Soriot has said that the company is likely to run an additional global trial to assess the efficacy of its Covid-19 vaccine.
Instead of adding the trial arm to an ongoing US process, a new trial would be run to evaluate a lower dosage that performed better than a full amount in AstraZeneca's studies, Bloomberg News report said on Thursday.
AstraZeneca is facing tricky questions about its success rate that some experts say could hinder its chances of getting speedy US and EU regulatory approval.
Several scientists have raised doubts about the robustness of results showing the shot was 90 percent effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.
Earlier on Monday, the British drugmaker said that the vaccine could be around 90 percent effective, when administered as a half dose followed by a full dose a month later, citing data from late-stage trials in Britain and Brazil.
"The reason we had the half-dose is serendipity," Mene Pangalos, the head of AstraZeneca's non-oncology research and development, told Reuters.
Half dose vs full
A larger group who had received two full doses, as planned, resulted in an efficacy read-out of 62 percent, leading to an overall efficacy of 70 percent across both dosing patterns.
Around the time when Astra was initiating its partnership with Oxford at the end of April, university researchers were administering doses to trial participants in Britain.
They soon noticed expected side effects such as fatigue, headaches or arm aches were milder than expected, he said.
"So we went back and checked ... and we found out that they had underpredicted the dose of the vaccine by half," said Pangalos.
He added the company decided to continue with the half dose and administer the full dose booster shot at the scheduled time.