The omicron Covid-19 variant has quickly overtaken delta as the dominant strain of the virus across the U.S., accounting for 90% of the cases in some parts of the country, CDC director Dr. Rochelle Walensky said Wednesday.

The variant makes up more than 73% of the cases in the United States as of Saturday, according to the latest data released by the Center for Disease Control and Prevention. Last week, U.S. health officials said omicron accounted for 2.9% of all cases sequenced through Dec. 11, but later revised that number up to 12.6%.

Walensky said the highly mutated and contagious strain accounts for up to 90% of the infections in the eastern Atlantic states, parts of the Midwest, South and northern Pacific states.

“This rapid increase in the proportion of omicron circulating around the country is similar to what we’ve seen across the world,” she told reporters during a White House Covid-19 press briefing.

This story is developing. Check back for updates.