WASHINGTON (AP) — A person in California became the first in the U.S. to have an identified case of the omicron variant of COVID-19, a U.S. official told The Associated Press on Wednesday. It comes as scientists continue to study the risks posed by the new strain of the virus.

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The Biden administration moved late last month to restrict travel from Southern Africa where the variant was first identified and had been widespread. Clusters of cases have also been identified in about two dozen other nations. The official could not discuss the matter publicly and spoke to the AP on condition of anonymity.

The Centers for Disease Control and Prevention was moving to tighten U.S. testing rules for travelers from overseas, including requiring a test for all travelers within a day of boarding a flight to the U.S. regardless of vaccination status. It was also considering mandating post-arrival testing.

Officials said those measures would only “buy time” for the country to learn more about the new variant and to take appropriate precautions, but that given its transmissibility its arrival in the U.S. was inevitable.