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.Â
France reported a case in the Ile de France region of greater Paris on Thursday.
US President Joe Biden is working on a strategy to fight COVID-19 this winter and sources briefed on the matter said one step would be extending requirements for travellers to wear masks through mid-March. A formal announcement is expected on Thursday.
The White House also plans to announce stricter testing for international visitors.
French government adviser Jean-Francois Delfraissy told BFM television the "true enemy" for now was still the Delta variant, spreading in a fifth wave.
"We should see a progressive rise of the Omicron variant, which will take over from Delta," possibly by the end of January, he said.
The French Omicron infection followed a case found in the French Indian Ocean island of Reunion last month.
Airlines in the United States were told to hand over the names of passengers arriving from parts of southern Africa, a US Centers for Disease Control and Prevention letter showed.
Much remains unknown about Omicron, which was first found on November 8 in South Africa and has spread to at least two dozen countries, just as parts of Europe were suffering a surge in infections of the better-known Delta variant as winter sets in.
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.
World Health Organisation (WHO) epidemiologist Maria van Kerkhove told a briefing that data should be available "within days" on Omicron's contagiousness.
BioNTech's chief executive said the vaccine it makes in a partnership with Pfizer was likely to offer strong protection against severe disease from Omicron.
And GlaxoSmithKline said that a lab analysis of the antibody-based COVID-19 therapy it is developing with US partner Vir had shown the drug is effective against the Omicron variant.
This contrasts with Regeneron's study of its COVID-19 antibody drug, which it said could be less effective against Omicron, adding to fears about the efficacy of existing treatments. Moderna's top boss raised similar concerns about the company's vaccine.
Indications suggesting Omicron may be markedly more contagious than previous variants have rattled financial markets, fearful that new restrictions could choke a tentative recovery from the economic impact of the pandemic.
About 56 countries were reportedly implementing travel measures to guard against Omicron as of November 28, the WHO said.
In the latest restrictions, South Korea, which confirmed its first five cases of the Omicron variants on Wednesday, halted quarantine exemptions for fully vaccinated inbound travellers for two weeks as daily coronavirus case numbers rose to a new high.
The European Union brought forward the start of its vaccine rollout for 5-to-11-year-olds to December 13, as the president of the EU's executive body said there was a "race against time" to stave off the new variant.
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