AstraZeneca – the company collaborating with Oxford University on a coronavirus vaccine – is set to run an extra trial of the jab.
Chief executive Pascal Soriot said it followed the accidental discovery that it performed best when half a dose was given, followed by a full one.
“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 says the new study will be international
It is likely to be an “international study, but this one could be faster because we know the efficacy is high so we need a smaller number of patients”, he added.
The development is unlikely to hold up regulatory approval in the UK, Mr Soriot said.
Earlier this week, it was announced the vaccine is up to 90% effective in preventing COVID-19 – when half a dose is given first.
When two full doses were given at least a month apart, it had an efficacy of 62%.
A total of 2,741 people were on the course that proved 90% effective, while 8,895 were given two full doses.
Professor Andrew Pollard, director of the Oxford Vaccine Group, has said the 90% effectiveness finding has already met the “necessary statistical evidence as required by regulators”.
But concerns about the jab have been raised by the scientific head of the US’s Operation Warp Speed – the programme to deliver vaccines across America.
Moncef Slaoui told reporters the half-dose regime was given only to people aged 55 and under.
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Other vaccines, from Pfizer and Moderna, were recently shown to be 95% and 94.5% effective respectively.
Shares in AstraZeneca – a FTSE 100 company – ended the day 0.7% down in London.