That mutant, which belongs to a lineage known as CAL.20C, seemed to have popped up in July but lay low till November. Then it began to quickly spread.

CAL.20C accounted for more than half of the virus genome samples collected in Los Angeles laboratories on Jan. 13, according to a new study that has not yet been published.

“We had our own problem that didn’t cross over from Europe,” said Jasmine Plummer, a research scientist at Cedars-Sinai Medical Center in Los Angeles, who worked on the new study. “It really originated here, and it had the chance to start to emerge and surge over the holiday.”…

But Eric Vail, the director of molecular pathology at Cedars-Sinai, said it was possible that CAL.20C is playing a large part in the surge of cases that has overwhelmed Southern California’s hospitals. “I’m decently confident that this is a more infectious strain of the virus,” Dr. Vail said.