British drugmaker AstraZeneca said its Covid-19 vaccine developed with the University of Oxford appeared to offer only limited protection against mild disease caused by the South African variant of the virus, based on early data from a trial.

The jab showed significantly reduced efficacy against the 501Y.V2 viral variant, which is dominant in South Africa, according to the randomised, double-blind study seen by the Financial Times on Saturday.

"A two-dose regimen of (the vaccine) did not show protection against mild-moderate Covid-19 due to (the South African variant)," the study indicated, adding that efficacy against severe Covid-19, hospitalisations and deaths was not yet determined.

The study, with a relatively small sample size of more than 2,000 individuals, hasn't yet been peer-reviewed and is due to be published on Monday.

AstraZeneca said it had not been able to properly ascertain the effect of the vaccine on severe disease and hospitalisation caused by the South African variant in the study given most of the participants were young, healthy adults.

"We do believe our vaccine could protect against severe disease, as neutralising antibody activity is equivalent to that of other Covid-19 vaccines that have demonstrated activity against more severe disease, particularly when the dosing interval is optimised to 8-12 weeks," it said.

While all Covid-19 vaccines so far have largely held up against the B.1.1.7 variant that emerged in the UK, the strain that originated in South Africa has been more worrying, the report said.

Both Johnson & Johnson and Novavax have said their vaccines were less effective against the strain in clinical trials conducted in South Africa, the report said.

In trials, both vaccines offered complete protection against severe disease and death in relation to Covid-19, it added.

Moderna has said it will test a booster shot and a reformulated vaccine to target the South African variant, after studies showed its vaccine was significantly less effective.

BioNTech-Pfizer said their vaccine was slightly less effective in a lab study using a pseudovirus with some mutations from the 501Y.V2 variant, but have not published results of tests against the variant itself.

The 501Y.V2 variant, dominant in South Africa, has recently been discovered in countries all over the world, including the US and the UK.