Omicron made up 59% new US cases last week; CDC revises past count

The omicron variant accounted for an estimated 58.6 percent of COVID-19 cases in the U.S. for the week ending Dec. 25, down from previous estimates, according to new data released Dec. 28 by the CDC.

The CDC had previously said the variant accounted for 73.2 percent of new infections for the week ending Dec. 18. On Dec. 28, the agency revised that count to 22.5 percent, which falls outside of the previously predicted range. CDC officials said the disparity occured because of the rapid rate in which the omicron variant spreads, according to Politico. The first known U.S. case of the omicron variant was detected Dec. 1 in California. 

"There was a wide predictive interval posted in last week's chart, in part because of the speed at which omicron was increasing," Jasmine Reed, spokesperson for the CDC, told Politico. "We had more data come in from that timeframe and there was a reduced proportion of omicron."

The CDC revised its variant proportion model Dec. 25, narrowing its confidence range of omicron's prevalence. Experts told The New York Times that they weren't surprised by the revisions, and CDC estimates are just that — estimates. Cases can only be confirmed via genetic sequencing, which is only conducted on a portion of U.S. samples. Estimates will likely become more accurate as more data about omicron is collected.

The current CDC model suggests that the delta variant still accounts for 41.1 percent of U.S. cases. The CDC has started providing weekly omicron prevalence estimates every Tuesday.


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