The highly contagious omicron variant is evident in 50% to 70% of new coronavirus cases in parts of the most populous state heading into the holidays, California health officials said Thursday.

Omicron has quickly become the most common form of the coronavirus in the U.S., accounting for 73% of all cases, federal officials said in the latest nationwide estimate Monday. The U.S. recorded its first confirmed omicron infection just three weeks ago in a San Francisco resident who had recently traveled to South Africa.

It is now being “detected at increasing proportions in wastewater in multiple regions throughout California,” the California Department of Public Health said.