Coronavirus: Doubts over AstraZeneca's vaccine data risk delaying approval

London - Several scientists have raised doubts about the robustness of results showing the shot was 90% effective.

Days after grabbing headlines with its Covid-19 "vaccine for the world", 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 per cent effective in a sub-group of trial participants who, by error initially, received a half dose followed by a full dose.

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"All we have to go on is a limited data release," said Peter Openshaw, a professor of experimental medicine at Imperial College London. "We have to wait for the full data and to see how the regulators view the results," he said, adding that US and European regulators "might possibly take a different view" from each other.

British drugmaker AstraZeneca said on Monday that its experimental vaccine, developed with Oxford University, prevented on average 70 per cent of Covid-19 cases in late-stage trials in Britain and Brazil.

While the success rate was 90 per cent in the sub-group of volunteers, the efficacy was 62 per cent if the full dose was given twice, as it was for most participants.

That is well above the 50 per cent efficacy required by US regulators. Europe's drug regulator has said it will not set a minimum level of efficacy for potential vaccines.

At the heart of concerns, however, is that the trial's most promising result of 90 per cent comes from a sub-group analysis - a technique many scientists say can produce spurious readings.

"Sub-group analyses in randomised controlled trials are always fraught with difficulties," said Paul Hunter, a professor of medicine at Britain's University of East Anglia.

He said, in particular, such analyses increase the risk of "type 1 errors" - in other words, where an intervention is considered to be effective when it is not.

This is in part because the number of participants is greatly reduced in a sub-group - making it harder to be confident that a finding is not just down to chance differences or similarities among participants.

"In order to have faith in the results," Hunter said, any sub-group analysis "should be sufficiently powered" with large numbers of volunteers to take readings from.

Only 2,741 volunteers were in the sub-group that gave the 90 per cent efficacy read-out, a fraction of the tens of thousands in trials that resulted in the above 90 per cent efficacy data released earlier this month for Pfizer-BioNTech's and Moderna's vaccines.

'DEVIL IN THE DETAIL'

AstraZeneca said the administering of the half dose was reviewed and approved by independent data safety monitors and the UK regulator, adding that the regulator publicly confirmed there was "no concern".

"We are in discussions with regulators around the world to evaluate these findings and we look forward to the publication of the peer-reviewed results, which has now been submitted to the journal," a spokesperson added.

Oxford University did not respond immediately to a request for comment.

The US regulator, the Food and Drug Administration (FDA), has not commented on AstraZeneca's vaccine trial results. The European Medicines Agency said on Thursday it would "assess data on the efficacy and safety of the vaccine in the coming weeks once they have been received from the company".

"It is still possible that the difference (in efficacy) is a random difference," he added. "It's unlikely, but it's still possible."