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The first U.S. case of the omicron variant has been identified in California in a traveler who returned from South Africa on Nov. 22, the Centers for Disease Control and Prevention said Wednesday.

The patient had mild symptoms that were improving and was in isolation, officials said.

Much remains unknown about the highly mutated variant, which scientists fear could be more transmissible and more resistant to vaccines. The new variant has been identified in more than 20 countries since it was first identified in southern Africa last week.

Here’s what to know