South Africa has temporarily suspended its rollout of the AstraZeneca-Oxford University coronavirus vaccine after a small clinical trial revealed the shot provided only minimal protection from mild and moderate illness caused by the virus variant that is widely circulating in the country.

The variant, called B.1.351, appears to be better at evading the body's immune system and has helped fuel a massive second wave of COVID-19 cases there.

The findings are not definitive, but they do add to concerns that currently available vaccines may be less effective against new variants and may need to be adjusted. People who have already been vaccinated could potentially need booster shots.

"It is time, unfortunately, for us to recalibrate our expectation of COVID-19 vaccines," Shabir Madhi, a professor of vaccinology at the University of the Witwatersrand in Johannesburg and chief investigator on the new study, said during a televised press briefing Sunday. He called the findings "a reality check" that the coronavirus will continue to mutate and that perhaps it's time to rethink the goals for mass vaccination campaigns.

"These findings also force us to recalibrate thinking about how to approach the pandemic virus and shift the focus from the aspirational goal of herd immunity against transmission to the protection of all at risk individuals in the population against severe disease," Madhi said.

Sarah Gilbert, a professor of vaccinology at the University of Oxford, said researchers are already working with AstraZeneca to adjust the vaccine so it protects better against new variants and to develop booster shots, if necessary.

The findings, which have not yet been peer-reviewed, were based on a study of around 2,000 participants whose median age was 31. Madhi said that the AstraZeneca vaccine had been showing a 75% efficacy against mild to moderate COVID cases until the B.1.351 strain became dominant in South Africa; after that, the efficacy dropped to just 22% percent, based on 42 symptomatic cases. However, the number of cases involved was too small to draw firm conclusions.

At the briefing, Madhi noted that what really matters is whether a vaccine can prevent serious cases — the ones that overwhelm hospitals and lead to deaths. The trial did not assess that, because the participants were too young to be at high risk of severe disease. In a statement, AstraZeneca said it believes its vaccine will still be effective at preventing severe disease. Salim Abdool Karim, an epidemiologist at Columbia University and a member of a committee advising South Africa's government on COVID-19, echoed that sentiment.

"We would expect, based on what we know, there's a reasonable likelihood that even though it does not prevent mild disease, it should prevent or may prevent severe disease or hospitalization," Karim told South Africa's SABC News. In general, vaccines tend to be better at preventing severe disease than mild to moderate cases.

Other COVID vaccines have also shown reduced effectiveness against the B.1.351 strain. Moderna has said it is working on a booster shot for its vaccine to protect against the variant from South Africa.

Despite this looming concern, there's good news from South Africa as well. Another trial there found that the vaccine made by Johnson & Johnson was 57% effective at preventing moderate to severe disease caused by the variant. The country is now awaiting doses of the Johnson & Johnson vaccine, which could arrive within a week or so.

The findings come just days after South Africa, the African country hardest-hit by the pandemic, welcomed the first 1 million doses of the AstraZeneca vaccine. The government had planned to start vaccinating this month, but on Sunday, South Africa Minister of Health Dr. Zweli Mkhize said those plans would be put on temporary hold while scientists and other experts determine how to most effectively use the AstraZeneca vaccine. It's possible they may decide to use it with certain low-risk populations, such as young people.

The study's findings also have serious implications for COVAX, an international effort to provide vaccines to lower-resourced countries. Last week, COVAX announced plans to start sending the first of 90 million doses of the AstraZeneca vaccine to African nations this month, many of which are battling COVID cases fueled by the B.1.351 strain. The AstraZeneca vaccine also accounts for the vast majority of doses that COVAX has allocated to countries in other regions of the world.

Nurith Aizenman, Jaclyn Diaz and Eyder Peralta contributed to this story.

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