• The preliminary findings from a small study of more than 2,000 people have not yet been peer-reviewed.
• Annual vaccines or a booster in the autumn could be required to combat variants, the vaccine minister said.
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.
Annual vaccines or a booster in the autumn could be required to combat variants, the vaccine minister said.
More than 100 cases of the South Africa variant have been found in the UK.
The preliminary findings from a small study of more than 2,000 people have not yet been peer-reviewed.
First reported by the Financial Times, the study suggests 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.
Vaccine minister Nadhim Zahawi told BBC's Andrew Marr Show: "We see very much probably an annual or booster in the autumn and then an annual (jab), in the way we do with flu vaccinations where you look at what variant of virus is spreading around the world, rapidly produce a variant of vaccine and then begin to vaccinate and protect the nation."
Prof Sarah Gilbert, the architect of the Oxford-AstraZeneca vaccine, said researchers were already working on a new vaccine designed to combat the South African variant.
"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. It looks very much like it will be available for the autumn," Prof Gilbert told Andrew Marr.
"We're already working on the first part of the manufacturing process in Oxford, that will be passed on to other members of the manufacturing supply chain as we go through the spring."