One study, involving around 2,000 people, found the vaccine offered "minimal protection" against mild and moderate cases of Covid-19. The government says it will use other vaccines in the meantime.
South Africa on Sunday said it would suspend its rollout of the Oxford-AstraZeneca vaccine after initial trials showed "disappointing" results against the B.1.351 variant of COVID-19 .
The country has received 1 million doses of the jab. It had planned to start using it to vaccinate frontline healthcare workers from mid-February.
The study, involving around 2,000 people, found the vaccine offered "minimal protection" against mild and moderate cases of COVID-19. It has not yet been peer-reviewed.
"We have decided to put a temporary hold on the rollout of the (AstraZeneca) vaccine. More work needs to be done," said South African Health Minister Zweli Mkhize.
Scientists will be studying whether or not the AstraZeneca vaccine is effective in preventing severe disease and death against the new variant, Mkhize said.
South Africa still eyes vaccination drive
The government will instead offer vaccines produced by Johnson & Johnson and Pfizer in the coming weeks.
This newer strain is more infectious and currently accounts for more than 90% of the COVID-19 cases in the country.
South Africa plans to vaccinate at least 67% of its population by the end of the year, or around 40 million people.
It has recorded nearly 1.5 million infections and more than 46,000 deaths from the virus.
Developers vow modified jab by autumn
Developers of theOxford-AstraZeneca vaccinesay they will have a modified jab ready by the end of this summer.
Sarah Gilbert, lead researcher for the Oxford team, told the BBC on Sunday that "we have a version with the South African spike sequence in the works."
"It looks very likely that we can have a new version ready to use in the autumn," she added.
The early results for the AstraZeneca vaccine against the South African variant could have far-reaching implications in the fight against coronavirus on the continent.
Many African nations had been planning to use the AstraZeneca shot owing to its affordability and the fact that it can be kept in ordinary refrigerators
COVAX, an international procurement initiative for poorer countries, has bought the AstraZeneca vaccine in bulk from the Serum Institute of India.