Omicron is now the dominant coronavirus strain in the United States, accounting for 73 percent of new cases between Dec. 12 and 18, the Centers for Disease Control and Prevention said on Monday.

The Delta variant had been the driving force in U.S. infections for more than five months, with CDC data showing that at the end of last month, more than 99.5 percent of coronavirus cases were Delta. Omicron was first detected in southern Africa in late November, and has since been found in 90 countries. It’s estimated that last week, more than 650,000 Omicron infections were reported in the United States. 

In New York, the Pacific Northwest, and the Southeast, it’s believed that at least 90 percent of new infections are because of Omicron, The Associated Press reports. The numbers are “stark,” CDC Director Dr. Rochelle Walensky said, but “they’re not surprising,” as they follow the growth seen in other countries. She also anticipates that over time, “Delta will be crowded out by Omicron.”

Researchers are still trying to determine if Omicron causes a more severe illness; both Moderna and Pfizer have said in lab tests, a third dose of their COVID-19 vaccines increased immune response against Omicron. Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security, told AP he expects Omicron will further spread during the holidays.

“All of us have a date with Omicron,” Adalja said. “If you’re going to interact with society, if you’re going to have any type of life, Omicron will be something you encounter, and the best way you can encounter this is to be fully vaccinated.”