US health authorities on Wednesday reported the first confirmed case of the omicron variant in the country.
The case was found in the state of California, the Centers for Disease Control and Prevention said.
Anthony Fauci, the chief medical adviser to US President Joe Biden, said the infected patient had recently returned from South Africa.
He said the person returned on November 22, tested positive on November 29, and is self-quarantining.
The person had been vaccinated but had not received a booster shot. They were experiencing "mild symptoms," Fauci said.
All of the person's close contacts tested negative for the virus.
"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.
Since the emergence of the omicron variant, the United States has moved to tighten travel regulations for international arrivals, requiring a negative test for all travelers within a day of boarding a flight, regardless of vaccination status. Authorities are also considering mandating post-arrival testing.
However, these measures would only buy time while the new variant is studied.
It is unknown whether the new variant is more contagious, whether it is more dangerous, and whether it evades vaccines. However, its large number of mutations has the World Health Organization, and international governments, worried about its spread.
According to a tally kept by Johns Hopkins University, the US has suffered the most deaths from the coronavirus pandemic, with more than 780,000 people dying from the virus.
Biden is due on Thursday to announce his plans to combat the virus over the winter.