British drug manufacturer AstraZeneca is likely to run an additional global trial to assess the efficacy of its Covid-19 vaccine, its chief executive has said, after questions over the results from its late-stage study.
Pascal Soriot said that further research is needed but the additional testing is unlikely to affect regulatory approval in Europe.
AstraZeneca and its partner, the University of Oxford, announced on Monday that it was seeking regulatory approval for the vaccine after it showed an average 70% effectiveness.
That rate jumped to 90% when an initial half-dose then a full dose was given, similar to that in rival vaccines in development by Pfizer/BioNTech and Moderna.
But US scientists have said the higher rate of effectiveness came during tests in people aged 55 and under, and was discovered by accident during the clinical trials.
Head of the Oxford Vaccine Group, Andrew Pollard, said this week that further evidence will be available next month, but the result was still "highly significant".
"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 News.
He said he expected that to be another "international study but this one could be faster because we know the efficacy is high, so we need a smaller number of patients".
The additional trial is not likely to delay regulatory approval in the European Union, Mr Soriot said.
There are high hopes for the AstraZeneca/Oxford vaccine, which Mr Pollard has hailed as a "vaccine for the world" given that it could be cheaper to make, and easier to store and distribute.
It can be stored, transported and handled at normal refrigerated conditions of between 2C and 8C (36-46F) for at least six months.
Pfizer/BioNTech's offering requires temperatures of -70C, driving up costs and potentially making it out of reach for lower and middle-income countries.
AstraZeneca/Oxford has also promised to provide its vaccine to the developing world on a non-profit basis.
Top UK scientific adviser says AstraZeneca vaccine works
The British government's Chief Scientific Adviser Patrick Vallance has said the main point about the AstraZeneca vaccine is that it works.
"The headline result is the vaccine works and that's very exciting," he said during a news conference with British Prime Minister Boris Johnson.
Chief Medical Adviser Chris Whitty, answering the same question, said there was always scientific debate about virtually everything.
"The key thing from our point of view is to leave this in the hands of the regulator.
"They will make an assessment with lots of data that is not currently in the public domain on efficacy and on safety," he said.