Dec 28 (Reuters) - The Omicron variant was estimated to be 58.6% of the coronavirus variants circulating in the United States for the week ending Dec. 25, according to data from the U.S. Centers for Disease Control and Prevention (CDC).

The fast-spreading variant was first detected in southern Africa and Hong Kong in November, with the first known case in the United States identified on Dec. 1 in a fully vaccinated person who had traveled to South Africa.

The agency also revised down the Omicron proportion for the week ending Dec. 18 to 22% from 73%, saying that there was a wide predictive interval posted in last week's chart, in part because of the speed at which Omicron was increasing.

The Delta variant accounts for 41.1% of all U.S. COVID-19 cases as of Dec. 25, according to the public health agency's data on Tuesday.

Former U.S. Food and Drug Administration Commissioner Scott Gottlieb said on Twitter that if the CDC's new estimate of Omicron prevalence is precise then it suggests that a good portion of the current hospitalizations may still be driven by Delta infections.

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The agency said the data includes modeled projection that may differ from weighted estimates generated at later dates.

(Reporing by Ankur Banerjee; Editing by Aditya Soni and Shailesh Kuber)