The study from South Africa's University of the Witwatersrand and Oxford University showed the vaccine had significantly reduced efficacy against the South African variant, according to a Financial Times report published on Saturday.

Among coronavirus variants currently most concerning for scientists and public health experts are the so-called British, South African and Brazilian variants, which appear to spread more swiftly than others.

"In this small phase I/II trial, early data has shown limited efficacy against mild disease primarily due to the B.1.351 South African variant," an AstraZeneca spokesman said in response to the FT report.

"However we have not been able to properly ascertain its effect against severe disease and hospitalisation given that subjects were predominantly young healthy adults."

The company said it believes its vaccine can protect against severe disease, given that the neutralising antibody activity is equivalent to that of other COVID-19 vaccines.

While thousands of individual changes have arisen as the virus mutates into new variants, only a tiny minority are likely to be important or change the virus in an appreciable way, according to the British Medical Journal.

"Oxford University and AstraZeneca have started adapting the vaccine against this variant and will advance rapidly through clinical development so it is ready for Autumn delivery should it be needed," the AstraZeneca spokesman said.

The trial involving more than 2000 people has not been peer-reviewed, the Financial Times said.

On Friday Oxford said their vaccine had similar efficacy against the British coronavirus variant as it did to the previously circulating variants.

Australian Associated Press