AstraZeneca and the University of Oxford have said their vaccine has been found to provide only limited protection against mild and moderate disease caused by the South African variant of Covid-19 in early data from a small trial.
However, Oxford vaccine researchers say a version ofthe jab that works against news variants should be available by the autumn.
The study, which has not been peer-reviewed, was conducted by South Africa's University of the Witwatersrand and Oxford University.
It analysed the E484K mutation in more than 2,000 people, with most of the participants considered young and healthy.
In a statement, an AstraZeneca spokesperson said early data from the "small phase I/II trial" showed "limited efficacy against mild disease primarily due to the B.1.351 South African variant".
In a separate press release on Sunday, Oxford confirmed the study findings, adding that the vaccine appeared to provide minimal protection for "mild-moderate" Covid-19 infections from the South African variant.
The study did not provide clarity, however, on whether the vaccine was effective against severe disease caused by the South African variant, given that the participants were predominantly young healthy adults.
The AstraZeneca spokesperson said the company believes the vaccine could protect against severe disease, with the neutralising antibody activity appearing similar to that demonstrated by other Covid-19 vaccines that have been found to protect against severe disease.
They said researchers have already started adapting the vaccine against the South African variant and would "advance rapidly through clinical development so that it is ready for Autumn delivery, should it be needed".
Oxford vaccine lead researcher Professor Sarah Gilbert provided the same assurances in an interview with the BBC's Andrew Marr Show on Sunday.
Saying her team has "a version with the South African spike sequence in the works," Prof Gilbert said: "It's not quite ready to vaccinate people with yet, but as all of the developers are using platform technologies, these are ways of making a vaccine that are very quick to adapt."
"This year we expect to show that the new version of the vaccine will generate antibodies that recognise the new variant. Then it will be very much like working on flu vaccines," she said.
"It looks very much like it will be available for the autumn," she added.
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