The CDC significantly reduced its estimate for how prevalent the omicron variant of COVID-19 is in the United States on Tuesday, saying that the new variant was only responsible for 22.5% of new cases in the week that ended Dec. 18, not the alarming 73.2% that it had originally estimated last week.
For the week ending Dec. 25, the agency says omicron accounted for 58.6% of all new cases.
Jasmine Reed, a spokesperson for the CDC, noted that there was "a wide predictive interval posted in last week’s chart," and the downward revision was partly due to the "speed at which Omicron was increasing."
"CDC’s models have a range, and… we’re still seeing steady increase in the proportion of Omicron," Reed told Fox News Digital. "In some regions in the country, Omicron accounts for ~ 90% or more of cases."
Dr. Jerome Adams, the former surgeon general in the Trump administration, explained that the revision was likely partly due to a responder bias caused by a testing quirk with omicron called "S gene dropout," in which one of the three target genes is not detected. When that gene is not detected, it can be an immediate marker for omicron.
"A lot of people were seeing this S dropout on the tests even before they got the follow-up genetic testing, and so those samples were disproportionately more likely to be sent in for sequencing," Adams told Fox News Digital.
As the CDC collects more data, it can more accurately pinpoint the proportions of each variant throughout the country, according to Dr. Li Tang, an associate faculty member in the department of biostatistics at St. Jude Children's Research Hospital.
"Earlier, they probably relied on a small number of available sequences. It should be also noted, although the confidence interval now is narrower, the range is still big, covering from 41.5% to 74%, suggesting large uncertainty," Tang told Fox News Digital.
The new estimates mean that the delta variant was still dominant for most of December, though omicron has a slight edge now.
Regardless, COVID-19 cases have been surging in recent weeks, as the United States set a single-day record for new cases on Monday with 441,278 confirmed infections.
The seven-day average on Monday was 240,408, which is roughly quadruple the low in the fall of 64,206 on Oct. 24.
Omicron was most prevalent in the South, the Pacific Northwest, the Midwest and the New York area in the week that ended on Dec. 25, according to CDC data.
"It’s also important for people to understand that in the grand scheme of things, they really were probably just a week or two ahead of what we’re going to see anyway, because omicron is spreading so quickly that it is going to be 73% by the time you look at this week’s or next week’s numbers," Adams said.