The C.D.C. significantly lowers its estimate of Omicron's prevalence nationwide [View all]

[BumRushDaShow]

Source: New York Times

The Centers for Disease Control and Prevention estimated that the Omicron variant now accounts for roughly 59 percent of all Covid cases in the United States, a significant decrease from the agency’s previous estimate. The update shows how hard it is to track the fast-spreading variant in real time and how poorly the agency has communicated its uncertainty, experts said. Last week, the C.D.C. said that Omicron accounted for approximately 73 percent of variants circulating in the United States in the week ending Dec. 18. But in its revision, the agency said the variant accounted for about 23 percent of cases that week.

In other words, Delta, which has dominated U.S. infections since summer, still reigned in the United States that week. That could mean that a significant number of current Covid hospitalizations were driven by infections from Delta, Dr. Scott Gottlieb, a former commissioner of the Food and Drug Administration, suggested on Twitter. Hospitalizations typically lag several weeks behind initial infections. Experts said they were not surprised by the revisions, given that the C.D.C.’s estimates are rough guesses, with a wide range of possible values known as “confidence intervals.”

Cases of Omicron can only be confirmed by genetic sequencing, which is only performed on a portion of samples across the country. And Omicron is still spreading extremely fast. Still, they said the C.D.C. did a poor job communicating the uncertainty of its estimates. “The 73 percent got a lot more attention than the confidence intervals, and I think this is one example among many where scientists are trying to project an air of confidence about what’s going to happen,” said David O’Connor, a virologist at the University of Wisconsin-Madison. Dr. O’Connor said he initially thought the initial 73 percent point estimate “seemed high.” The agency came up with the estimate based on a “relatively small number of sequences,” he added.

“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,” Dr. O’Connor said. “Without more data it can be really hard to know which one it’s going to be.” The new estimate of 59 percent is also a rough calculation, experts said, and will most likely be revised in future weeks. “I just want people to be very aware that that is an estimate, that’s not actually from sequence-confirmed cases,” said Nathan Grubaugh, an epidemiologist at the Yale School of Public Health. “With Omicron in particular, it’s been very difficult to have any sort of projections, because things are changing just so so rapidly.”

Read more: https://www.nytimes.com/live/2021/12/28/world/omicron-covid-vaccine-tests/the-cdc-significantly-lowers-its-estimate-of-omicrons-prevalence-nationwide

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Interesting because I had just checked for updates on their bar charts.

The one done 12/18/21 was this -



The latest is this -



I think they use models that estimate a certain infection rate based on what data they have and then extrapolate from there. However with a still-small number of actual sequencing being done, the estimates will have a higher bias towards those actually doing sequencing, thus introducing a lot of uncertainty. But as other locales start ramping their sequencing up, those places may not be seeing the surge yet.