The first known US case was a fully vaccinated person in California who returned from South Africa on Nov 22 and tested positive seven days later.
Heavily mutated Omicron is rapidly becoming the dominant variant of the coronavirus in South Africa less than four weeks after it was first detected there, and the United States on Wednesday became the latest country to identify an Omicron case within its borders.
The first known US case was a fully vaccinated person in California who returned to the United States from South Africa on November 22 and tested positive seven days later.
The person had mild symptoms and was in self-quarantine, Dr Anthony Fauci, the top US infectious disease official, told reporters at the White House.
Late on Tuesday, airlines in the United States were told to hand over the names of passengers arriving from parts of southern Africa hit by Omicron, according to a US Centers for Disease Control and Prevention letter seen by Reuters.
Key questions remain about the new variant, which has been found in two dozen countries, including Spain, Canada, Britain, Austria and Portugal. The UAE reported its first case on Wednesday, the second Gulf country after Saudi Arabia.
Early indications suggesting Omicron may be markedly more contagious than previous variants have rattled financial markets, fearful that new restrictions could choke off a tentative recovery from the economic ravages of the pandemic.
South Africa's National Institute for Communicable Diseases (NICD) said early epidemiological data suggested Omicron was able to evade some immunity, but existing vaccines should still protect against severe disease and death.
It said 74% of all the virus genomes it had sequenced last month had been of the new variant, which was first found in a sample taken on November 8 in Gauteng, South Africa's most populous province.
The number of new cases reported in South Africa doubled from Tuesday to Wednesday.
World Health Organization (WHO) epidemiologist Maria van Kerkhove told a briefing that data on how contagious Omicron was should be available "within days."
BioNTech's CEO said the vaccine it makes in a partnership with Pfizer was likely to offer strong protection against severe disease from Omicron.
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 "US detects first case of Omicron variant in person who returned from South Africa")
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