The United States identified its first case of the new Omicron coronavirus variant in California, the U.S. Centers for Disease Control and Prevention said on Wednesday.

The person was a traveler who returned to the United States from South Africa on Nov. 22 and tested positive seven days later and suffered mild symptoms, officials said.

That patient was fully vaccinated but did not have a booster shot, according to Dr. Anthony Fauci, a top U.S. infectious disease official, who briefed reporters at the White House.

“The individual is self-quarantining and all close contacts have been contacted and all close contacts thus far have tested negative,” said Fauci, who also serves as an adviser to President Joe Biden. “The individual was fully vaccinated and experienced mild symptoms which are improving at this point.”

For days, U.S. health officials have said the new variant -first detected in South Africa and announced on Nov. 25 – was likely already in the United States as dozens of other countries also detected its arrival.

Omicron, dubbed a “variant of concern” by the World Health Organization, is being studied to see if it is more contagious or causes severe illness than other variants.

The variant has also been detected in several countries including Spain, Canada, Britain, Austria and Portugal.