How widespread is omicron? Why the CDC lowered its estimate of variant’s prevalence
The Centers for Disease Control and Prevention drastically lowered its estimate on the new omicron coronavirus variant’s prevalence in the United States — prompting confusion and questions from some about how widespread the variant actually is in the country.
Previously, the CDC estimated that omicron — which seems to be able to spread and evade COVID-19 vaccines more easily — accounted for about 73% of COVID-19 cases in the United States as of the week ending Dec. 18. But the agency has since revised that estimate to about 23% of cases during that time frame, meaning delta was still the dominant variant as it’s estimated to have accounted for 77% of cases.
During the week ending Saturday, Dec. 25, the most recent data available, the CDC estimates that omicron accounts for about 59% of cases in the United States. That makes it the dominant variant ahead of the highly contagious delta variant, which accounts for an estimated 41% of cases.
Experts have said it’s not surprising that the CDC lowered its estimate on omicron’s prevalence as data and information are changing quickly during the variant’s rapid spread throughout the country.
Why the CDC lowered its estimate
The CDC uses a model called Nowcast to estimate coronavirus variants’ prevalence in the United States.
It can take two to three weeks after a specimen is collected to determine its genetic sequence, according to the CDC — meaning officials don’t immediately know what variant of the coronavirus a person is infected with when they test positive for COVID-19. Additionally, genetic sequencing isn’t performed on all coronavirus samples.
Nowcast uses genomic surveillance data from previous weeks to provide a more recent estimate on a variant’s spread, the CDC says. Those estimates, however, can change as the CDC gets more data.
The estimates also use confidence intervals, or a range of possible values, The New York Times reported.
Jasmine Reed, a CDC spokesperson, told Politico that the change in the agency’s estimate was due to the rate at which the omicron variant is spreading.
“There was a wide predictive interval posted in last week’s chart, in part because of the speed at which omicron was increasing,” Reed said, according to Politico. “We had more data come in from that time frame and there was a reduced proportion of omicron.”
But a CDC spokesperson told Reuters “it’s important to note” that the agency is “still seeing a steady increase in the proportion of omicron” cases.
Experts told The New York Times it’s likely the CDC’s current estimate on omicron’s prevalence will change, too.
“I just want people to be very aware that that is an estimate, that’s not actually from sequence-confirmed cases,” Nathan Grubaugh, an epidemiologist at the Yale School of Public Health, told the NYT. “With omicron in particular, it’s been very difficult to have any sort of projections, because things are changing just so so rapidly.”
Experts weigh in on the revision
David O’Connor, a virologist at the University of Wisconsin-Madison, told The New York Times that the CDC’s original estimate on omicron’s prevalence during the week of Dec. 18 “seemed high” and noted that the number was based on a “relatively small number of sequences.”
“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,’” O’Connor told the outlet. “Without more data, it can be really hard to know which one it’s going to be.”
Ashish Jha, dean of the School of Public Health at Brown University, told NBC News that he also wasn’t “totally surprised” to see the CDC’s estimate drop.
“In a quickly moving pandemic, you’re always going to have these kinds of estimates that get revised as more data comes in,” Jha told NBC.
He went on to say that the revision suggests there’s “still a lot of delta out there.”
“There’s at least one potential upside to this estimate change, which is a lot of the hospitalizations we’re seeing right now may be from delta,” Jha said.
Early data has suggested that omicron could be associated with less severe infections than past variants — though more research has yet to be done, McClatchy News previously reported.
Scott Gottlieb, former commissioner of the U.S. Food and Drug Administration, made similar comments on Twitter.
“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,” he wrote.
This story was originally published December 29, 2021 at 1:32 PM with the headline "How widespread is omicron? Why the CDC lowered its estimate of variant’s prevalence."
