The variant’s arrival also makes it all the more imperative that Americans receive vaccinations in great numbers, and more quickly, scientists said. A pathogen that spreads easily is more difficult to contain, and a greater percentage of the population must be inoculated to turn back the pandemic…

[G]iven the mutant’s apparent contagiousness, scientists fear that its toehold in the United States augurs another difficult chapter in the pandemic. Gov. Gavin Newsom of California announced on Wednesday that a case of the variant had been discovered in the state.

Officials in San Diego County later identified the patient as a 30-year-old-man who had not traveled outside the United States, suggesting the virus was transmitted by someone else in the community — a sign that the new version is already spreading. A household contact of the man has developed symptoms, the officials said, and is being tested.