(Bloomberg) -- The World Health Organization said that a strain of coronavirus recently discovered by South African researchers is a variant of concern, posing a threat that could confound countries’ efforts to slow the spread of Covid-19.

The WHO assigned the Greek letter omicron to the variant, which had been known as B.1.1.529, following a meeting by a panel of experts Friday to discuss the strain. 

In South Africa, “this variant has been detected at faster rates than previous surges in infection, suggesting that this variant may have a growth advantage,” the WHO said Friday.

Scientists say B.1.1.529 carries a high number of mutations in its spike protein, which plays a key role in the virus’ entry into cells in the body. It’s also what is targeted by vaccines, so if the protein changes enough, it raises concern that the mutations could protect it against vaccines.

The more the virus circulates, the more opportunities it has to mutate, WHO spokesman Christian Lindmeier said before the panel’s meeting. Preliminary evidence suggests omicron may present a greater risk of reinfection than other variants of concern, the WHO said Friday.

The WHO uses “variants of concern” to signify strains that pose additional risks to global public health. So far, it has identified four of them, in addition to omicron. The WHO also calls some strains “variants of interest” if they warrant close monitoring because of their emerging risk.

South African Health Minister Joe Phaahla said Thursday the variant, first sequenced on Nov. 11, was of serious concern, and many countries followed the announcement with travel restrictions to the region. Susan Hopkins, chief medical adviser to the U.K. Health Security Agency, said the strain is the most worrying yet.

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