The bad news, coming nearly a week after a million doses of the AstraZeneca-Oxford vaccine arrived in South Africa, was a big setback for the country.

By Benjamin Mueller, Rebecca Robbins and Lynsey Chutel

South Africa halted use of the AstraZeneca-Oxford coronavirus vaccine on Sunday after evidence emerged that the vaccine did not protect clinical trial volunteers from mild or moderate illness caused by the more contagious virus variant that was first seen there.

The findings were a devastating blow to the country’s efforts to combat the pandemic.

Scientists in South Africa said on Sunday that a similar problem held for people who had been infected by earlier versions of the coronavirus: The immunity they acquired naturally did not appear to protect them from mild or moderate cases when they were reinfected by the variant, known as B.1.351.

The developments, coming nearly a week after a million doses of the AstraZeneca-Oxford vaccine arrived in South Africa, were an enormous setback for the country, where more than 46,000 people are known to have died from the virus.

They were also another sign of the dangers posed by new mutations in the coronavirus. The B.1.351 variant has spread to at least 32 countries, including the United States.

The number of cases evaluated as part of the studies outlined by South African scientists on Sunday were low, making it difficult to pinpoint just how effective or not the vaccine might be against the variant.

And because the clinical trial participants who were evaluated were relatively young and unlikely to become severely ill, it was impossible for the scientists to determine if the variant interfered with the AstraZeneca-Oxford vaccine’s ability to protect against severe Covid-19, hospitalizations or deaths.

The scientists said, however, that they believed the vaccine might protect against more severe cases, based on the immune responses detected in blood samples from people who were given it. If further studies show that to be the case, South African health officials will consider resuming use of the AstraZeneca-Oxford vaccine, they said.

The new research findings have not been published in a scientific journal. But the discovery that the AstraZeneca-Oxford product showed minimal efficacy in preventing mild and moderate cases of the new variant added to the mounting evidence that B.1.351 makes current vaccines less effective.

Pfizer and Moderna have both said that preliminary laboratory studies indicate that their vaccines, while still protective, are less effective against B.1.351. Novavax and Johnson & Johnson have also sequenced test samples from their clinical trial participants in South Africa, where B.1.351 caused the vast majority of cases, and both reported lower efficacy there than in the United States.

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