In late December, scientists in California began searching coronavirus samples for a fast-spreading new variant that had just been identified in Britain.

They found it, though in relatively few samples. But in the process, the scientists made another unwelcome discovery: California had produced a variant of its own.

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.