The first U.S. Omicron case was reported in California, the C.D.C. said today, signaling the arrival of a potentially dangerous new phase of the pandemic in the country.
The patient, who returned to the state from South Africa on Nov. 22, is in isolation, and aggressive contact tracing is underway. The individual was fully vaccinated, was not hospitalized and had mild symptoms that were improving. People in close contact with the individual have tested negative.