The omicron variant is causing an increasing share of coronavirus infections in the U.S., though its climb to dominance has been shallower 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.

Omicron has swept through the U.S. since being detected in California on Dec. 1. In the region encompassing New York and New Jersey, for example, omicron 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.

While data suggests omicron may produce milder illness than delta and other strains, the variant could stress 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.