SAN FRANCISCO (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.

In a morning news conference from the White House, Dr. Anthony Fauci confirmed the first case of the omicron variant is in San Francisco.

Dr. Fauci said the person was a traveler who returned from South Africa on Nov. 22 and tested positive on Nov. 29. The person was vaccinated but had not received a booster shot.

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 California and San Francisco Departments of Public Health have confirmed that a recent case of COVID-19 among an individual in California was caused by the Omicron variant (B.1.1.529). The individual was a traveler who returned from South Africa on November 22, 2021. The individual, who was fully vaccinated and had mild symptoms that are improving, is self-quarantining and has been since testing positive. All close contacts have been contacted and have tested negative.
Genomic sequencing was conducted at the University of California, San Francisco and the sequence was confirmed at CDC as being consistent with the Omicron variant. This will be the first confirmed case of COVID-19 caused by the Omicron variant detected in the United States. 

Centers for Disease Control

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.

The Associated Press contributed to this story.