Clinical trial data on two Covid-19 vaccines shows that a coronavirus variant first identified in South Africa is lessening their ability to protect against the illness, underscoring the need to vaccinate vast numbers of people as quickly as possible, scientists said.

The vaccines — from Novavax Inc and Johnson & Johnson — were significantly less effective at preventing Covid-19 in trial participants in South Africa, where the potent new variant is widespread, compared with countries in which this mutation is still rare, according to preliminary data released by the companies.

“Clearly, the mutants have a diminishing effect on the efficacy of the vaccines,” Dr Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said in a briefing. “We can see that we are going to be challenged.”

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