Further research is needed on AstraZeneca's Covid-19 vaccine, the drug maker said on Thursday, after questions were raised over the protection it provides.
AstraZeneca and its partner, the University of Oxford, said on Monday that it was seeking approval for the vaccine after it showed an average 70 per cent effectiveness.
That rose to 90 per cent when an initial half-dose then a full dose was given, similar to that in rival vaccines being developed 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 found by accident during the clinical trials.
The head of the Oxford Vaccine Group, Andrew Pollard, said this week that further evidence would 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," AstraZeneca chief executive Pascal Soriot told Bloomberg.
Mr Soriot said he expected it would 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 new trial was not likely to delay regulatory approval in Britain and the EU, he said.
There are high hopes for the AstraZeneca-Oxford vaccine, which Mr Pollard has praised as a "vaccine for the world" given that it could be cheaper to make, and easier to store and distribute.
It can be stored, moved and handled at normal refrigerated conditions of between 2°C and 8°C for at least six months.
Pfizer-BioNTech's offering requires temperatures of minus 70°C, driving up costs and potentially making it out of reach for lower and middle-income countries.
AstraZeneca and Oxford have promised to provide their vaccine to the developing world on a non-profit basis.
The World Health Organisation said it looked forward to the full publication of data from the trial.
"On a review of the detailed data, we will be better positioned to understand the vaccine performance," it said.
The chief medical officer for England, Prof Chris Whitty, also cautioned against drawing premature conclusions and urged patience until data was published in peer-reviewed journals.
"It's always a mistake to make too many judgments early and in particular before the independent regulator has had a chance to look at the results," Prof Whitty said.
Helen Fletcher, professor of immunology at the London School of Hygiene and Tropical Medicine, said the safety data available on the vaccine had been "very robust".
"It's possible that a lower initial vaccine dose could result in higher vaccine efficacy," Prof Fletcher said.
"More is not necessarily better when it comes to vaccines and immunotherapies.
"It's also possible that a strong immune response to the first vaccine could effectively block an immune response to the second shot of the same virus."
Her colleague, professor of pharma-coepidemiology, Stephen Evans, said speculation about the age of volunteers in the trials was "not useful to anyone".
"We have good grounds for trusting that the regulation in this high-profile area will be done as carefully or more carefully for these vaccines than for any others in the past," Prof Evans said.
Gillies O'Bryan-Tear, from the UK Faculty of Pharmaceutical Medicine, said "the eventual efficacy rate may change", but "the validity of the low dose-high dose group results are unlikely to be questioned".
"Even if they [regulators] decided to ignore the results of the low dose-high dose group, the study of the high dose-high dose patients will still be strongly significant, but I think that is unlikely," he said.