Dec 28 (Reuters) - The Omicron variant was estimated to be 58.6% of the coronavirus variants circulating in the United States as of Dec. 25, according to data from the U.S. Centers for Disease Control and Prevention (CDC) on Tuesday. The agency also revised down the Omicron proportion of cases for the week ending Dec. 18 to 22% from 73%, citing additional data and the rapid spread of the variant that in part caused the discrepancy. "We had more data come in from that timeframe and there was a reduced proportion of Omicron," a CDC spokesperson said. "It’s important to note that we’re still seeing a steady increase in the proportion of Omicron." 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. Since then, the strain has rapidly spread across the world and driven a surge in U.S. infections, causing widespread flight cancellations and dashing hopes for a more normal holiday season. The Delta variant, which had been the dominant strain in the past few months, accounts for 41.1% of all U.S. COVID-19 cases as of Dec. 25, the public health agency"s data showed. Former U.S. Food and Drug Administration Commissioner Scott Gottlieb said on Twitter that if the CDC"s new estimate of Omicron prevalence was precise, then it suggests that a good portion of the current hospitalizations may still be driven by Delta infections. The agency said the data includes modeled projection that may differ from weighted estimates generated at later dates.