FOX 11's sister station KTVU confirmed with the mayor's office that the new variant was detected in San Francisco.
"This is the first case of COVID-19 caused by the omicron variant detected in the United States," Dr. Anthony Fauci, the top U.S. infectious disease expert, said at the White House Wednesday.
According to Fauci, the individual who tested positive for the variant 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 and was experiencing "mild symptoms," Fauci said.
According to the CDC, the person is self-quarantining and has been since testing positive. All close contacts of the individual 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.
"We knew that it was just a matter of time," Fauci told reporters at the White House.
The omicron variant was first classified by the World Health Organization on Nov. 26. It was first detected in Africa and Europe. There are many unanswered questions surrounding the new variant, including whether it is more contagious than previous strains, whether it makes people more seriously ill, and whether the vaccine is effective against it.
"Vaccines continue to be our best way through the pandemic by safely protecting us against severe illness from COVID-19 and its variants. We are doubling down on our vaccination and booster efforts to ensure that all Californians have access to safe, effective, and free vaccines that can prevent serious illness and death," state Public Health Director Tomos Aragon said.
The Los Angeles County Department of Public Health addressed the new variant on Saturday.
"Although more studies are needed to determine whether the Omicron variant is more contagious, more deadly or resistant to vaccine and treatments than other COVID-19 strains, as with any variant of concern that is found in other countries, there are important steps we all need to take to protect ourselves from COVID-19 and from emerging variants of concern," the department said.
Officials urged anyone at least 5 years old who has not been vaccinated to do so, and those who had their last shot six months ago to get a booster shot. "The vaccines are effective against the Delta variant and earlier strains of the virus, which allows us to remain hopeful that the approved vaccines will also provide some protection against Omicron," the statement said.
Officials also reminded people to wear a mask when indoors or at large outdoor mega-events regardless of vaccination status.
Tune in to FOX 11 Los Angeles for the latest Southern California news.
CNS contributed to this report.