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The Centers for Disease Control and Prevention (CDC) and health authorities in California confirmed the first case of the newly discovered COVID-19 variant Omicron in the U.S. on Wednesday, saying an individual who had recently traveled to South Africa tested positive for the strain.

“The individual, who was fully vaccinated and had mild symptoms that are improving, is self-quarantining and has been since testing positive,” the CDC said in a statement. “All close contacts have been contacted and have tested negative.”

The health agency said the emergence of the variant “emphasizes the importance of vaccination, boosters, and general prevention strategies needed to protect against COVID-19. Everyone 5 and older should get vaccinated boosters are recommended for everyone 18 years and older.”

South Africa reported the variant to the World Health Organization last week, and the agency labeled Omicron a “variant of concern.” The Biden administration responded by announcing international travel restrictions from eight southern African countries.

The individual who tested positive returned to the U.S. from South Africa on November 22, the CDC said. The University of California, San Francisco conducted genomic sequencing of the individual’s test, and the CDC confirmed it was the Omicron variant.

The variant’s emergence in the U.S. comes amid the busy holiday travel season. According to auto club AAA, over 53 million Americans were estimated to have traveled for the Thanksgiving holiday, an increase from last year.

How vaccines perform against the variant remains unclear. On “Face the Nation” on Sunday, Dr. Scott Gottlieb, a former commissioner of the Food and Drug Administration and a member of Pfizer’s board, conveyed confidence in the vaccines.

“People who have looked closely at this sequence … those individuals feel reasonably confident that three doses of vaccine is going to be protective,” Gottlieb said. “Now, that could give a really strong impetus to trying to get more people boosted.”