What is the Omicron variant?

The World Health Organization has called Omicron a “variant of concern” and on Monday warned that the global risks posed by it were “very high,” despite what officials described as a multitude of uncertainties. Cases have been identified in 20 countries so far, including Britain, Italy, Belgium and the Netherlands. On Wednesday, officials announced that Omicron had been detected in the United States — in a traveler who returned to California from South Africa. The individual, who was fully vaccinated, had mild symptoms and was improving, health officials said. The nation’s top infectious disease expert, Dr. Anthony S. Fauci, said that contact tracers had identified all those who had close contact with the infected individual and that so far, all of them had tested negative for the virus.