Dec. 29, 2021 -- The CDC has significantly revised the estimate of the percentage of new COVID-19 infections in the U.S. caused by the Omicron variant.

As of Saturday, Omicron accounted for about 59% of U.S. infections, according to the latest CDC data updated Tuesday. Last week, the CDC said Omicron made up 73% of all cases as of Dec. 18, which has now been revised to 22.5% of cases.

“We had more data come in from that time frame, and there was a reduced proportion of Omicron,” a CDC spokesperson told Reuters. “It’s important to note that we’re still seeing a steady increase in the proportion of Omicron.”

With the revision, the Delta variant now accounts for 41% of U.S. cases and was still the dominant variant in the U.S. in mid-December.

“Setting aside the question of how the initial estimate was so inaccurate, if CDC’s new estimate of #Omicron prevalence is precise then it suggests that a good portion of the current hospitalizations we’re seeing from COVID may still be driven by Delta infections,” Scott Gottlieb, MD, a former FDA commissioner, wrote in a Twitter post on Tuesday.

The Omicron variant is quickly spreading across the U.S. and driving a surge in U.S. infections, which are now averaging more than 267,000 new cases per day, according to the data tracker from The New York Times.

Public health experts told the newspaper that they weren’t surprised by the CDC’s revisions, given that early data is always an estimate within a range known as a “confidence interval.” The actual number of variant cases can only be confirmed with genetic sequencing, which is done with a portion of samples across the country.

“It’s like playing Name That Tune, and trying to say, based on just the first note, if the song is ‘Ice Ice Baby’ by Vanilla Ice or ‘Under Pressure,’” David O’Connor, PhD, a virologist at the University of Wisconsin-Madison, told The New York Times.

“Without more data, it can be really hard to know which one it’s going to be,” he said.

The new estimate of 59% is also a confidence interval calculation and could be revised, the newspaper reported. The percentage of Omicron varies across the country as well. In Wisconsin, for instance, the variant made up half of the cases on the University of Wisconsin-Madison campus in 3 days, O’Connor told the newspaper. Meanwhile, in Connecticut, Omicron makes up more than 80% of cases.

“With Omicron in particular, it’s been very difficult to have any sort of projections because things are changing just so, so rapidly,” Nathan Grubaugh, PhD, an epidemiologist at the Yale School of Public Health who is tracking Omicron samples in Connecticut, told the newspaper.

The estimates will likely become more accurate as scientists collect more data on Omicron and test more samples.

“I just want people to be very aware that this is an estimate, that’s not actually from sequence-confirmed cases,” Grubaugh said.

Show Sources

CDC: “COVID Data Tracker: Variant Proportions.”

Reuters: “U.S. CDC estimates Omicron variant to be 58.6% of cases, revises projection.”

Twitter: @ScottGottliebMD, Dec. 28, 2021.

The New York Times: “Coronavirus in the U.S.: Latest Map and Case Count, Dec. 29, 2021,” “The CDC significantly lowers its estimate of Omicron’s prevalence nationwide.”

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