Top federal health officials warned in a briefing Tuesday morning that the omicron variant is rapidly spreading in the United States and could peak in a massive wave of infections as soon as January, according to new modeling from the Centers for Disease Control and Prevention.

The prevalence of omicron jumped sevenfold in a single week, CDC modeling shows, and at such a pace, the highly mutated variant of the coronavirus could ratchet up pressure on a health system already strained in many places as the delta variant continues its own late-autumn surge.

The warning of an imminent surge came even as federal officials and pharmaceutical companies signaled that they don’t currently favor creating a new omicron-specific vaccine. Based on the data so far, they believe that existing vaccines plus a booster shot are an effective weapon against omicron.

The CDC briefing Tuesday detailed two scenarios for how the omicron variant may spread through this country. The worst case scenario has spooked top health officials, who fear a fresh wave, layered on top of delta and influenza cases in what one described as “a triple whammy,” could overwhelm health systems and devastate communities, particularly those with low vaccination rates.

“I’m a lot more alarmed. I’m worried,” said Marcus Plescia, chief medical officer for the Association of State and Territorial Health officials, who participated in the call. The CDC, normally cautious in its messaging, told the public health officials that “we got to get people ready for this,” he said.

He noted that the omicron surge, if it materializes as forecast, would be taking place as delta continues its onslaught and during the time of year when influenza cases often peak.

Officials stress that early data shows individuals who are fully vaccinated and received a booster shot remain largely protected against severe illness and death from omicron. But they worry about how few Americans have been boosted to date. Over 54 million people in the United States have gotten the additional shots, out of 200 million who are “fully vaccinated,” according to the CDC.

The newest modeling scenarios have been shared among senior administration officials as they discuss politically fraught decisions about how, when and whether to take new steps to suppress the virus and keep hospitals from being overwhelmed.

The second scenario outlines a smaller omicron surge in the spring. It’s unclear which scenario is more likely. The modeling was done by experts tapped by CDC Director Rochelle Walensky in August to deliver real-time outbreak forecasting and analytics.

“They’re considering the information at the highest levels right now, and thinking through how to get the public to understand what the scenarios mean,” said one federal health official familiar with the briefing. “It looks daunting.”