Public health officials said the infected person, who had mild symptoms, returned to the United States from South Africa on November 22, testing positive seven days later.
Dr. Anthony Fauci, the top U.S. infectious disease official, informed reporters at the White House that the patient was fully vaccinated but did not have a booster shot.
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Fauci confirmed that the person is in self-quarantine and all of the patient's close contacts have tested negative so far.
The new variant, which has shaken up markets given signs it could spread quickly and evade some of the defenses provided by vaccines, has posed numerous questions, and has been found in two dozen countries, including Spain, Canada, Britain, Austria and Portugal.
Fauci stated it might take two weeks or more to gain insight into how easily the variant spreads from person to person, how severe the disease it causes may be, and whether it bypasses protections provided by vaccines on the market.
Fauci, who serves as an adviser to President Joe Biden, said, "We don't have enough information right now," adding that the variant's molecular profile "suggests that it might be more transmissible, and that it might elude some of the protection of vaccines, but we don't know that now... We have to be prepared that there's going to be a diminution in protection."
U.S. health officials have been saying for days now that the new variant first detected in southern Africa and announced on November 25 was likely already in the United States as dozens of other countries also detected its presence.
Biden, who had been briefed by his team on the first known case, according to spokeswoman Jen Psaki, said on Wednesday before the Omicron case was announced: "This new variant is a cause for concern but not a cause for panic."
The United States has barred basically all foreigners who have been in one of eight southern African countries. On Tuesday, the U.S. Centers for Disease Control and Prevention (CDC) told airlines to disclose names and other information of passengers who have been to those countries.