The Oxford-AstraZeneca Covid jab gives limited protection against mild disease caused by the South Africa variant, the firm said early trials had suggested.
It also said it had not yet fully determined whether the vaccine protects against severe disease caused by the more transmissible coronavirus variant.
The preliminary findings from a small study of more than 2000 people have not yet been peer-reviewed.
More than 100 cases of the South Africa variant have been found in the UK.
The preliminary findings, first reported by the Financial Times, suggest the vaccine offers limited protection against mild and moderate disease caused by the variant.
The study is due to be published on Monday.
A spokesman for AstraZeneca said they had not yet been able to properly establish whether the jab would prevent severe disease and hospitalisation caused by the South Africa variant because those involved in the study had predominantly been young, healthy adults.
But the company expressed confidence that the vaccine would offer protection against serious cases, because it created neutralising antibodies similar to those of other coronavirus vaccines.
A spokesman for AstraZeneca said the company and the University of Oxford had started adapting the vaccine against the South Africa variant, adding that a new vaccine to work against mutated versions of the virus could be ready to deploy in the autumn if needed.
Dr Michael Head, a senior research fellow in global health at the University of Southampton, said that if the Oxford-AstraZeneca vaccine was less protective against mild disease but prevented severe disease this would “still be a pretty good outcome”.
“I don’t think we need to be too alarmed by [the reported findings] as yet but we do need to see the full study to work out what the implications really are,” he said.
NHS England medical director of primary care Dr Nikki Kanani encouraged people to take a vaccine when offered, adding that evidence shows they are “very protective” – particularly against hospitalisation and death from Covid.
However, she said scientists will have to keep looking at how coronavirus vaccines are working as they will likely have to be given on a yearly basis to “reflect any changes” in variants of the virus, like the flu jab.
It comes as the company on Saturday said its vaccine provided good protection against the variant first discovered in Kent, which is now dominant in the UK.
Current vaccines were designed around earlier versions of coronavirus, but scientists believe they should still work against the new ones, although it is not yet clear how well against different mutations.
Early results suggest the Pfizer-BioNTech vaccine protects against the new variants.
Data on two new coronavirus vaccines that could be approved soon – one from Novavax and another from Janssen – appear to offer some protection.
And early results from Moderna suggest its vaccine is still effective against the South Africa variant.
Experts say vaccines could be redesigned and tweaked to be a better match for new variants in a matter of weeks or months if necessary.