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The AstraZeneca COVID-19 vaccine has an apparent Achilles’ heel—it does not prevent mild to moderate illness in those infected with the South African variant of the virus, a new study shows.

In response, South Africa said it was halting rollout of the AstraZeneca vaccine in the country, where the variant—known by the designations 501Y.V2 or B.1.351—first emerged. It has now become the dominant strain in the country, responsible for more than 90% of new infections. It has also made the jump to Europe and North America.

The South African variant includes significant mutations to the coronavirus’s spike protein, which is the portion of the virus most of the vaccines currently being used have targeted to prompt an immune response. So far, there is no evidence the South African variant is more lethal, but some preliminary studies suggest it may spread more easily than the original virus.

The new research, which has not yet been peer-reviewed, did not examine whether the AstraZeneca vaccine, which the British-Swedish pharmaceutical company developed along with scientists from the University of Oxford, can still prevent severe COVID-19 illness.

That’s because the clinical trial, which was conducted by researchers from Oxford and South Africa’s University of Witwatersrand, used 2,000 volunteers whose average age was just 31. Young people are much less likely to develop severe COVID-19 symptoms, and so the trial could not accurately assess whether the vaccine offered protection against hospitalizations or deaths.

Preliminary data from Johnson & Johnson’s one-shot coronavirus vaccine showed that it did prevent severe COVID-19 in those infected with the South African strain. The vaccine from Novavax also seemed to still be able to prevent severe illness, clinical trials showed, even if it was less effective in stopping more mild forms of the disease.

Because J&J’s vaccine uses a similar technology to AstraZeneca’s, some researchers, including Shabir Madhi, who led the clinical trial of the AstraZeneca vaccine in South Africa, said they were hopeful that further studies would show the AstraZeneca immunization was still capable of preventing severe COVID-19 symptoms too.

But, in the meantime, South Africa said it would accelerate deployment of the J&J vaccine, of which it has ordered 9 million doses so far. It plans to begin vaccinating health care workers with that vaccine, as well as the one produced by Pfizer, in the coming weeks.

Salim Abdool Karim, chairman of South Africa’s ministerial advisory committee on COVID-19, said that the country would still deploy the AstraZeneca vaccine but that it would “need to do it wisely with a stepped approach.”

Sarah Gilbert, one of the Oxford researchers who created the vaccine AstraZeneca is manufacturing, said her team was working on a booster shot that would hopefully confer immunity to the South African variant.

“We are working with AstraZeneca to optimize the pipeline required for a strain change should one become necessary,” she said in a statement released by Oxford.

Moderna and several other vaccine manufacturers have also said they are already at work on new versions of their vaccines to address the new variants.

The AstraZeneca conundrum

But with the AstraZeneca vaccine already requiring two doses to provide effective immunity, the new data about its poorer efficacy against the South African strain means that many countries that were planning on using the AstraZeneca vaccine as one of their primary weapons against the pandemic will now face the added logistical hurdle of trying to provide millions of people with a potential third booster shot before next winter.

The same challenge may also dog many of the other two-shot vaccines that have been authorized so far. Most of these vaccines use the original coronavirus’s spike protein to trigger an immune response. But in the South African variant of the virus, the structure of that protein is significantly altered.

The South African variant of the virus has now been detected in many countries around the globe, including the U.S. and the U.K. It is one of several new strains of the virus that are complicating efforts to end the pandemic.

This may be a particular problem in the U.K., where more than 10 million people have already received a first dose of the AstraZeneca vaccine, and where the government is fighting a desperate race against time to contain the spread of the South African strain of the virus, which has recently been detected in the country.

The new data may also mean the J&J vaccine will become more important in ending the pandemic globally. Although the J&J shot had a lower overall effectiveness than some of the other vaccines—just 72% in clinical trials held in the U.S. and less than that in trials held in other regions of the world where different strains are more prevalent—the fact that it requires only a single dose makes it much easier to administer logistically, particularly if everyone will now need a booster that targets the South African strain.

In addition, the J&J vaccine can be kept at normal refrigerator temperatures, giving it an advantage over the vaccines from Pfizer and Moderna, both of which must be stored in super-freezing conditions until shortly before they are administered.