But in its study, due to be published Monday, the pharma group said it could still have an effect on severe disease — although there is not yet enough data to make a definitive judgement.
None of the 2,000 participants in the trial developed serious symptoms, the FT said, but AstraZeneca said the sample size was too small to make a full determination.
“We may not be reducing the total number of cases but there is still protection against deaths, hospitalisations and severe disease,” said Sarah Gilbert, who led the development of the vaccine with the Oxford Vaccine Group.
It could also be “some time” before they determine its effectiveness for older people in fighting the strain, which is a growing presence in Britain, she told BBC television.
“We might have to put it together from a number of studies,” she said.
Researchers are currently working to update the vaccine, and “have a version with the South African spike sequence in the works” that they would “very much like” to be ready for the autumn, said Gilbert.
UK vaccines minister Nadhim Zahawi said the government’s strategy to combat the spread of the strain was to continue with its mass vaccination programme “as rapidly as possible” as well as “hyper-local surge testing” in areas where it is detected.
Britain is in the midst of a massive vaccination drive, which it sees as its way out of one of the worst outbreaks in the world that has seen more than 112,000 fatalities among those testing positive for the virus.
It has so far vaccinated over 11 million people using either the Pfizer/BioNTech or Oxford/Astrazeneca shots.
The AstraZeneca vaccine has been the source of an ugly row with the European Union, which is angry that the Anglo-Swedish firm was unable to meet the delivery target agreed with Brussels.
France, Germany and Switzerland are also among countries to recommend the jab not be used in the elderly due to a lack of data.