The Omicron variant is causing an increasing share of coronavirus infections in the U.S., though its climb to dominance has been gentler than earlier estimates indicated, according to an updated federal model.
Omicron accounted for an estimated 58.6% of sequenced U.S. virus cases in the week ending Dec. 25, the Centers for Disease Control and Prevention’s Nowcast model showed Tuesday, up from an estimated 22.5% a week earlier. The once-dominant Delta variant accounted for 41.1% of cases in the most recent period, according to the CDC.
Nowcast estimates levels of variant prevalence based on genomic-sequencing data.
The week-earlier figure marks a substantial revision from a previous estimate, which said the Omicron variant was responsible for 73% of sequenced infections. That reading suggested that Omicron had rocketed to dominance practically overnight, leaping from just 3% of all cases in the preceding week.
A CDC spokesperson said that additional data that came in caused the agency to reduce the earlier proportion of Omicron. The agency is still seeing a steady increase in the incidence of the variant, the spokesperson said.
Read more: What Actually Worries U.S. Doctors About Omicron
Omicron has swept through the U.S. since its detection in California on Dec. 1. In the region encompassing New York and New Jersey, for example, the variant accounts for an estimated 88% of cases, according to the CDC. But there are still some areas of the country where Delta remains dominant, including some Midwestern states and New England, the model suggests.
Omicron’s rise has coincided with the holiday season in the U.S., stoking fears of another surge that could overwhelm the already exhausted health-care system. An average of 206,577 cases are recorded every day, according to the CDC, nearing levels not seen since last year’s winter wave.
The new variant may have a shorter incubation period and may be more likely to cause reinfections than other strains, the CDC said Tuesday in a separate report. The agency’s findings were based on an omicron outbreak among a six-person household, which included one person who was fully vaccinated, four previously infected people, and one person who tested positive for the first time.
While the median time from infection to the appearance of symptoms has been estimated at five days or more for other variants, the incubation period in this group was about three days, the researchers said. People with confirmed prior infections said their symptoms were similar to or milder than earlier, the CDC said, echoing earlier research suggesting that Omicron causes less severe illness than other strains.
“The horrific scenes that we saw a year ago—intensive care units being full, lots of people dying prematurely—that is now history in my view, and I think we should be reassured that that’s likely to continue,” said John Bell, regius professor of medicine at the University of Oxford in England. He spoke to BBC Radio 4’s Today program after the U.K. government said it wouldn’t tighten COVID-19 restrictions before the end of the year.
Despite the reports of milder illness, the variant still threatens to put stress on the economy. U.S. carriers canceled thousands of flights around the busy Christmas holiday because of staffing shortages.
The CDC on Monday shortened the quarantine period for people who have been exposed to the virus and isolation time for those who have tested positive. People in both groups should continue to wear a mask when they resume activity, the CDC said. The new measures could minimize disruption to workplaces and schools amid the surge.