The omicron variant has overtaken delta as the dominant coronavirus variant in the United States: As of Friday, more than 73% of new cases in the country were caused by omicron, according to data the Centers for Disease Control and Prevention posted Monday.
The data, for the week ending Dec. 18, found that the prevalence of the delta variant fell to just over 26%. Delta had been the dominant variant in the U.S. since July.
In certain areas of the country, the omicron variant accounts for more than 90% of new cases, including New York and New Jersey, as well as parts of the Midwest, the South and the Pacific Northwest.
“This sharp rise in omicron cases was expected and is similar to what has been seen worldwide,” the CDC said in a statement.
Still, the speed at which omicron outcompeted delta left experts astonished.
“This rate of takeoff is impressive even for me,” said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. The variant is “blisteringly fast.”
“Amazing how fast it spreads,” said Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia.
It’s still unknown whether the omicron variant causes less severe illness than previous strains. Early data suggests it may be milder, but Hanage said that doesn’t necessarily mean people can breathe a sigh of relief. The variant’s ability to evade the body’s immune defenses, plus its high transmissibility, is concerning.
“Imagine if everybody in the world got the common cold exactly at the same time. What would happen?” he said.
Offit said there should be “lots of cases,” as the existing COVID-19 vaccines appear to be not as good at protecting against mild disease as they are against previous strains. But, he added, there should also be “disproportionately fewer hospitalizations and deaths.”
Omicron overtook the previously dominant delta variant with startling speed. The new highly mutated variant was first detected in South Africa in November; the first case in the U.S. was found Dec. 1.
The CDC also revised its estimates of omicron’s prevalence for the week ending Dec. 11 to 12% of all new cases, up from 3%.
This story first appeared on NBCNews.com.