Omicron Now Accounts for 73 Percent of COVID Cases in U.S., CDC Says

The Omicron coronavirus variant is now the dominant strain in the U.S., making up 73 percent of new infections last week, the Centers for Disease Control and Prevention said Monday.

The highly contagious Delta variant had been dominant for months in the U.S. As of the end of November, Delta accounted for more than 99.5 percent of virus infections, according to CDC data.

Omicron's share of infections saw a nearly six-fold increase in just one week, the CDC said. In some areas of the U.S., Omicron accounts for even higher percentages of COVID-19 cases. This includes the New York area, the Pacific Northwest, industrial Midwest and the Southeast, where estimates indicate that the strain is responsible for 90 percent of new infections.

It took less than a month after scientists in Africa flagged Omicron for the variant to spread to nearly 90 countries and become dominant in the U.S.

Dr. Francis Collins, the director for the National Institutes of Health, warned during an appearance on NPR's "Weekend Edition" on Sunday that the U.S. could soon be recording as many as 1 million new cases per day. She urged people to keep following health measures and restrictions, even as the world approaches two years in the pandemic.

"I know people are tired of this," Collins said. "I'm tired of it too, believe me. But the virus is not tired of us. It's having a great old time changing its shape every couple of months, coming up with new variants and figuring out ways to be even more contagious."

Much about the Omicron variant remains unknown, including whether it causes more or less severe illness. Early studies suggest the vaccinated will need a booster shot for the best chance at preventing Omicron infection but even without the extra dose, vaccination still should offer strong protection against severe illness and death.

"All of us have a date with Omicron," said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. "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."

Adalja said he was not surprised by the CDC data showing Omicron overtaking delta in the U.S., given what was seen in South Africa, the U.K. and Denmark. He predicted spread over the holidays, including breakthrough infections among the vaccinated and serious complications among the unvaccinated that could stress hospitals already burdened by delta.

CDC's estimates are based on thousands of coronavirus specimens collected each week through university and commercial laboratories and state and local health departments. Scientists analyze their genetic sequences to determine which versions of the COVID-19 viruses are most abundant.

In the week that ended Dec. 11, Omicron's share of new infections in the U.S. increased to 2.9 percent from 0.4 percent the week before, the CDC previously reported.

But the CDC on Tuesday said they are revising some of the earlier numbers, after analyzing more specimens. The new numbers indicate that about 13 percent of the infections the week of Dec. 11 were Omicron, and not 3 percent, CDC officials said.

The Associated Press contributed to this report.