Dr. Anthony Fauci told reporters the person was a traveler who returned from South Africa on 22 November and tested positive on 29 November.
Dr. Fauci said the person had been vaccinated but had not received a booster shot and was experiencing “mild symptoms”.
Clusters of cases have also been identified in about two dozen other nations.
“We knew it was only a matter of time before the first case of omicron would be discovered in the United States,” said Dr. Fauci.
He said the person was recovering and added: “I think what is happening now is another example of why it is important for people to be vaccinated. But also to boost. Boost is very important. “
Officials said they had contacted anyone who had close contact with the person and that they had all been tested negative.
Genomic sequencing was performed at the University of California, San Francisco, and the sequence was confirmed by the Centers for Disease Control and Prevention.
The Centers for Disease Control and Prevention is taking steps to tighten U.S. test rules for travelers from abroad, including requiring a test for all travelers within one day of boarding a flight to the United States regardless of vaccination status.
It also considered mandating post-arrival testing.
Officials said these measures would only “buy time” for the country to learn more about the new variant and take appropriate precautions, but that given its transferability it was inevitable.
Much remains to be known 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.
Dr. Fauci, the top US expert on infectious diseases, said there would be more knowledge about the Omicron strain in two to four weeks while scientists grow and test laboratory samples of the virus.
The California Department of Public Health credited the state’s “large-scale testing and early detection systems” for identifying the case.
“We recognize that everyone is exhausted and the news of a new variant can be overwhelming. It is important that we jointly focus on the things we know to prevent the spread of COVID-19 and its variants,” said state health officials. in a statement.
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Sir. Biden has tried to mute the alarm over the omicron variant, saying it was a cause for concern, but “not a cause for panic.”
Sir. The bite and public health officials have become more intrusive in their prayers for more Americans to be vaccinated – and for those who have been vaccinated to get booster shots to maximize their protection against the virus.