A new trial's expected to be run on the Oxford AstraZeneca Covid-19 vaccine, after questions were raised about its performance.
The makers are applying for approval for the jab, after tests proved it was 70 per cent effective.
That jumped to 90 per cent when an initial half-dose was given, followed by a full dose.
But US scientists claim the higher rate of effectiveness only occurred in people aged 55 and under, and was discovered by accident.
Leading immunologist, Trinity Professor Luke O'Neill doesn't think it will affect it being approved in Europe:
"They will re-run that trial probably now, and they'll do it upfront. They'll start with a low dose and then try a high and that can still give a 90% efficacy. The FDA are the ones saying we don't like the look of this. The European agencies haven't said anything yet but AstraZeneca have said we're still expecting it to go down the approval route but time will tell."