A new coronavirus variant called 'Omicron' is now a 'variant of concern,' the WHO said

• The WHO named a new coronavirus variant, Omicron, a "variant of concern."
• Public-health experts worry its large number of mutations may pose a high risk of reinfection.

The World Health Organization designated the new coronavirus variant B.1.1.529 a "variant of concern" on Friday and named it "Omicron."

South Africa first reported the variant to the WHO on November 24, according to a WHO press release. Researchers there detected the first known case on November 9.

South Africa's coronavirus cases began to rise sharply around the same time that the variant was detected, the WHO said. The country's average daily cases have risen 13-fold since November 9, from around 275 to 3,700 per day.

Omicron cases have also been reported in Botswana, Hong Kong, Israel, and Belgium.

"This variant has a large number of mutations, some of which are concerning," the WHO said on Friday. "Preliminary evidence suggests an increased risk of reinfection with this variant, as compared to other" variants of concern.

The WHO reserves the term "variants of concern" for select coronavirus strains, like Delta, that are more transmissible than other strains, cause more severe disease, or are more resistant to vaccines. Omicron's high number of mutations could increase the odds that vaccines and other treatments won't be as effective against the variant, Insider's Dr. Catherine Schuster-Bruce reported.

Several vaccine manufacturers said Friday that they were testing how well existing vaccines protect against Omicron infections, and expect results in the coming weeks.

Dr. Anthony Fauci told CNN's "New Day" on Friday there is "no indication" this variant is currently in the US. But the US is notoriously slow to sequence coronavirus cases relative to countries like South Africa.

"I would not be surprised if it's already landed in the US," Katelyn Jetelina, an epidemiologist at UTHealth School of Public Health, told Insider.