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 briefing detailed two scenarios for how the omicron variant, first identified in South Africa three weeks ago, 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.

“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.”

“The implications of a big wave in January that could swamp hospitals . . . we need to take that potential seriously,” said the official, who spoke on the condition of anonymity because they were not authorized to discuss policy deliberations.

The CDC’s internal modeling is consistent with that of several academic groups in the United States and with data from the United Kingdom, Denmark and Norway. New restrictions have already been imposed in the United Kingdom and other countries in Europe that were seeded with omicron earlier.

The Biden administration’s strategy relies heavily on vaccination, including boosters and testing. When President Joe Biden announced his “action plan” on Dec. 2 for fighting the virus this winter, he noted “it doesn’t include shutdowns or lockdowns but widespread vaccinations and boosters and testing and a lot more.”

A reformulated vaccine that’s omicron-specific is not currently planned as part of the toolbox. Senior administration health officials and vaccine experts at vaccine companies said there is no evidence such a major switch in the vaccine design is necessary.

They cited the data that suggests the original vaccine, coupled with a booster shot, provides protection against severe illness caused by omicron. So far, they noted, the vaccines have successfully countered every variant. That view could change in the next two weeks as more data comes in involving laboratory tests and the spread of omicron.

Switching the vaccine has sweeping implications. If it is changed too early, that limits the ability to deal with another variant down the road - one that might potentially be more dangerous than omicron.

“We have to be careful not to repeat mistakes of the past,” said one administration official who spoke on the condition of anonymity because he was not authorized to talk about the issue. “If there is a change needed, we want to make it, but we don’t want to end up making a change if we don’t really need it. It costs time, money and effort.”

Experts say that it’s impossible to keep changing the vaccines or giving different boosters because there is not enough manufacturing capacity and other resources.

Anthony S. Fauci, Biden’s chief medical adviser on the coronavirus response, said in an interview Monday “there isn’t any compelling reason right now to drop everything and make an omicron-specific vaccine, as opposed to continue to administer vaccine for people who are unvaccinated and boosting people who are vaccinated.”

Speaking Tuesday in an interview on NBC’s “Today” Show, CDC Director Rochelle Walensky said of omicron, “It is more transmissible, and we’re seeing that in other countries as well, that it’s rapidly becoming the more predominant strain, but I want to emphasize that we have the tools now.”

Between Dec. 4 and Dec. 11, omicron likely jumped from a mere 0.4 percent of new infections in the U.S. to 2.9 percent, according to the new CDC data. In New York and New Jersey, omicron already accounts for 13 percent of new cases, Walensky said.

In the Houston Methodist hospital system, omicron accounted for 13 percent of new cases in a four-day period leading up to Dec. 8, according to James Musser, chair of pathology and genomic medicine. He expects that percentage to approach 20 percent when new numbers are published Wednesday. The omicron variant was first detected in Houston on Nov. 29.

Musser said his hospital system is ready for whatever comes next: “We’ve had 21 months of this now, and we’re sort of - I hate to say it, because it’s tragic - but we’re sort of skilled in the art of how to handle this.”

Though the delta variant remains dominant in the United States, and is the driver of the recent surge in hospitalizations, particularly in the Upper Midwest and Mountain West, omicron continues to show signs that it is dramatically more transmissible. Importantly, it has dozens of mutations that make it a more slippery foe when encountered by neutralizing antibodies, the immune system’s first line of defense.

That was reinforced Tuesday with the release of a large study from researchers in South Africa, the country that first warned the world of the emergence of the new variant in late November. The new study confirms that vaccines are significantly less effective at preventing infections with omicron, but still usually prevent severe disease.

The study also found that the people infected with omicron so far have had a 29 percent lower chance of being hospitalized than people infected with delta.

Infectious-disease experts caution that what happens in South Africa, which has a relatively young population, may not be repeated in northern hemisphere countries with older populations.

The CDC modelers also based their forecasts in part on data coming out of Denmark, Norway and the United Kingdom, Plescia said. Looking at the Norway data, the modelers said Norway cases could reach 300,000 quickly, “and there’s not that many people in Norway,” he added.

Public health officials think there is adequate supply of personal protective equipment in the United States to protect against another wave. But the country is not ready with sufficient testing capability, Plescia said. States will need to work with their hospital systems to get them ready to expand capacity, he said.

“The hope is that it is going to be less severe, but the concern is that the numbers could be so great, even if proportionally less people have to be hospitalized, the numbers are much higher and a lot of people are going to be really sick and overwhelm things,” Plescia said.

The messaging to the public will be even more difficult. Two things that would help enormously - less travel during Christmas, and more consistent wearing of masks - are not likely to happen, because people are so tired of the pandemic and have tuned out many public health messages, he aded.