The Centers for Disease Control and Prevention estimated that the Omicron variant now accounts for about 59% of all Covid cases in the United States, a significant decrease from the agency’s previous estimate. The update shows how difficult it is to keep up with the fast-spreading variant in real time and how poorly the agency has communicated its uncertainty, experts said.

Last week, the CDC said Omicron accounted for about 73% of variants circulating in the United States during the week ending December 18. But in its review, the agency said the variant accounted for around 23% of cases that week.

In other words, Delta, which has dominated U.S. infections since the summer, was still ruling the United States that week. This could mean that a significant number of current hospitalizations for Covid were due to infections from Delta, Dr Scott Gottlieb, former Commissioner of the Food and Drug Administration, suggested on Twitter. Hospitalizations usually delay several weeks after the initial infections.

Experts said they weren’t surprised by the revisions, given that the CDC’s estimates are rough guesses, with a wide range of possible values ​​known as “confidence intervals.” Omicron cases can only be confirmed by genetic sequencing, which is only done on a portion of the samples across the country.

And Omicron is still spreading extremely quickly.

Still, they said the CDC had done a poor job of communicating the uncertainty of its estimates.

“The 73 percent have received a lot more attention than confidence intervals, and I think this is just one example where scientists are trying to project an air of confidence into 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 estimate of 73% “seemed high”. The agency came up with an estimate based on a “relatively small number of sequences,” he added.

“It’s like playing Name That Tune and trying to tell, based only on the first note, if the song is Ice Ice Baby by Vanilla Ice, or Under Pressure,” said Dr. O’Connor. “Without more data, it can be very difficult to know which one it’s going to be. “

The new estimate of 59% is also a rough calculation, experts said, and will most likely be revised in the coming weeks.

“I just want people to know that this is an estimate, which is not actually from sequence confirmed cases,” said Nathan Grubaugh, epidemiologist at the Yale School of Public Health. “With Omicron in particular, it’s been very difficult to have projections because things change so quickly.”

Dr Grubaugh, who tracks probable samples from Omicron in Connecticut, said the variant accounted for over 80% of cases there, although he also notes that the country is heterogeneous and the variant likely has a different prevalence in different places.

“I don’t know how the CDC constructed their algorithm, but humans created these programs, and humans are fallible,” said Massimo Caputi, molecular virologist at Florida Atlantic University School of Medicine. “At the end of the day, you can predict as much as you want, but you have to look at the numbers you have on hand.”

Dr O’Connor, who follows Omicron in Wisconsin, said the variant accounted for half of the cases on the University of Wisconsin-Madison campus in just three days. “If I was making a betting prediction it wasn’t so much that the 73 percent number was wrong, but the timing of it was wrong,” he said.

These predictions will likely become more accurate over time as more data on Omicron is collected.

More precise figures will be needed to intelligently distribute Covid treatments. One of Omicron’s big challenges is the variant’s ability to thwart two of the three monoclonal antibody treatments, which can prevent serious illness in Covid-19 patients. Thus, some hospitals have started to reduce these treatments; administrators from NewYork-Presbyterian, NYU Langone and Mount Sinai have all said they will stop giving patients the two treatments that are ineffective against Omicron. But the drugs could still help those infected with Delta.

“If you still have these Delta cases, stopping the monoclonal drugs means that not everyone who would have benefited from them will get them at all,” said Dr O’Connor.

Dr O’Connor said scientists and healthcare providers need to do a better job of communicating the uncertainty of the predictions they share with the public. “Having the humility to recognize that there is a lot that no one knows that is unknowable right now is going to be really important.”