CDC Lowers Estimate of Omicron Prevalence in the United States

In an update that illustrates just how challenging it is to track the spread of a fast-moving virus, the U.S. Centers for Disease Control and Prevention on Tuesday lowered its estimate of how prevalent the omicron variant is in the United States. The latest agency data peg omicron's prevalence at 59 percent, a sharp drop from its estimate last week of 73 percent of all COVID-19 cases.

"The 73 percent got a lot more attention than the confidence intervals [which measure the range of certainty on an estimate], 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," David O'Connor, Ph.D., a virologist at the University of Wisconsin-Madison, told The New York Times. Genetic sequencing is the only way to confirm which variant is involved in a particular case and that is not done on every sample, he explained.

While predictions are likely to become more accurate as researchers learn more about omicron, even the new 59 percent estimate is likely to be revised in future weeks, experts said.

"I just want people to be very aware that that is an estimate, that's not actually from sequence-confirmed cases," Nathan Grubaugh, Ph.D., an epidemiologist at the Yale School of Public Health in New Haven, Connecticut, told The Times. "With omicron in particular, it's been very difficult to have any sort of projections, because things are changing just so so rapidly."

In Connecticut, where Grubaugh is tracking probable omicron samples, the variant is responsible for more than 80 percent of cases, while in Wisconsin, where O'Connor is tracking cases, about half were omicron in just three days.

Estimating more precisely will be important to the patients who have COVID-19 because their treatments may vary, depending on whether they are infected with the delta variant or omicron. Two of the three monoclonal antibody treatments available in the United States do not work against omicron. This has affected hospital decision-making about whether to give these treatments to patients. Administrators from three New York hospitals all said they would stop giving patients the two treatments that are ineffective against omicron, The Times reported.