South Africa has moved to halt its rollout of the Oxford/AstraZeneca vaccine after preliminary and limited evidence showed it failed to protect against mild and moderate forms of disease caused by a coronavirus variant first detected in the country.

Distribution of the AstraZeneca jab, scheduled to begin this month, would be put on hold to study its effects including on severe cases in more detail “until the scientists give us clear indications as to what we need to do”, Zweli Mkhize, South Africa’s health minister, said on Sunday.

A small trial led by Oxford university and South Africa’s Wits university found that the AstraZeneca vaccine had low efficacy against milder illness with the 501Y.V2 variant, according to findings first reported by media on Saturday.

The paper, which has not yet been peer reviewed, did not study the jab’s effects on stopping severe illness.

The South African government had been planning to begin jabs for frontline health workers this month using 1.5m doses of the AstraZeneca vaccine that are being supplied by India’s Serum Institute. It will now accelerate rollout of other vaccines it has ordered.

“We can still proceed with this rollout, but we need to do it wisely with a stepped approach,” said Salim Abdool Karim, chairman of South Africa’s ministerial advisory committee on Covid-19.