WASHINGTON (AP) — A person in California who had been vaccinated against COVID-19 became the first in the U.S. to have an identified case of the omicron variant, the White House announced Wednesday as scientists continue to study the risks posed by the new virus strain.

Dr. Anthony Fauci told reporters that the person was a traveler who returned from South Africa on Nov. 22 and tested positive on Nov. 29. Fauci said the person was vaccinated but had not received a booster shot and was experiencing “mild symptoms.”

San Francisco health officials also gave an update on COVID-19 in San Francisco.

“We are still learning about the Omicron variant, but we are not back to square one with this disease. From what we know now, San Francisco is relatively well positioned to handle COVID-19 and its variants because of our high vaccination rates, our high booster uptake, and other local health measures such as masking and testing,” said Dr. Grant Colfax, Director of Health. “We will stay alert and vigilant and do what we need to do to protect ourselves. This means getting vaccinated, getting your booster, wearing a mask indoors, and taking the other steps we know help slow the spread.” 

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.

“We knew that it was just a matter of time before the first case of omicron would be detected in the United States,” Fauci said.

Officials said they had contacted everyone who had close contact with the person and they had all tested negative.

Genomic sequencing was conducted at the University of California, San Francisco and the sequence was confirmed by the Centers for Disease Control and Prevention.

“San Francisco has one of the highest vaccination rates and lowest death rates in the country because of the actions our residents have taken from the beginning of this pandemic to keep each other safe. We knew that it was only a matter of time until the Omicron variant was detected in our city, and the work that we have done to this point has prepared us to handle this variant. We continue to encourage everyone to get vaccinated, get boosted, and take steps to keep each other safe,” said Mayor London Breed.  

In a joint statement, the state and San Francisco health departments said:

“As expected, and thanks to California’s large-scale testing and early detection systems, the State of California and the San Francisco Department of Public Health have confirmed a case of the Omicron variant in California. Our partners at the University of California, San Francisco identified this case through their sequencing capabilities. California is continuing to monitor the variant’s presence and progress through the state’s robust Whole Genome Sequencing surveillance.

We must remain vigilant against this variant, but it is not a cause for panic. To help detect and prevent the spread of this new variant, the State of California is increasing COVID-19 testing at our airports for arrivals from countries identified by the Centers for Disease Control and Prevention (CDC). We recognize that everyone is exhausted, and the news of a new variant can be overwhelming. It is important that we collectively focus on the things we know prevent the spread of COVID-19, and its variants. Individuals should (1) get vaccinated and boosted; (2) wear your mask in indoor settings; (3) get tested if you have symptoms; and (4) stay home if you are sick.

As we continue to learn more about the Omicron variant and how it can impact all of us, we will share that information through covid19.ca.gov and cdph.ca.gov.”

The Centers for Disease Control and Prevention is taking steps 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.

Much remains unknown about the new variant, including whether it is more contagious than previous strains, whether it makes people more seriously ill, and whether it can thwart the vaccine. Fauci, the top U.S. infectious disease expert, said more would be known about the omicron strain in two to four weeks as scientists grow and test lab samples of the virus.

The announcement of the first U.S. case comes before President Joe Biden plans to outline his strategy on Thursday to combat the virus over the winter. Biden has tried to quell alarm over the omicron variant, saying it was a cause for concern but “not a cause for panic.”

Biden and public health officials have grown more urgent in their pleas for more Americans to get vaccinated — and for those who have been vaccinated to get booster shots to maximize their protection against the virus.