The nation's top health officials warned Wednesday that the fast-rising omicron variant "undoubtedly" compromises the protection of two doses of Pfizer and Moderna vaccines, putting the U.S. at risk of a tidal wave of fresh COVID cases in the next month if more people don’t get vaccinated and sign up for booster shots.
The good news, though, is that booster shots mostly reconstitute protection, reducing the need for the U.S. to roll out an entirely new vaccine formula specific to omicron.
"Booster vaccine regimens work against omicron,” said Dr. Anthony Fauci, the president's chief medical adviser.
The speculation about what happens next has rattled many Americans as they prepare to travel for the holidays. Cases of the new variant have been doubling every two days, with a sevenfold increase in the prevalence of the omicron variant in the last week-- proving itself to be even more transmissible than the delta variant.
"We expect to see the proportion of omicron cases here in the United States continue to grow in the coming weeks," said Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention.
Health officials said the best bet is wearing a mask indoors and improving ventilation, in addition to vaccinations and boosters.
"Those are the tools we have. If we didn't have these tools, I would be telling you to really, really be worried. But we have tools. So, get vaccinated, get boosted," Fauci said.
The White House on Wednesday sought to tamp down any speculation of lockdowns. Jeff Zients, Biden’s chief coordinator on the COVID response, said they weren't necessary.
"We know how to keep our kids in school and our businesses open and we're not going to shut down our economy in any way," he told reporters on Wednesday. "We're going to keep our schools and our businesses open."
Still, whether the U.S. faces shutdowns again is largely in the hands of state and local officials who typically have splintered ideas on how to handle surges.
In New York, a statewide mask mandate for all indoor public places was to take effect Jan. 15 unless businesses already have a vaccine requirement in place. In Texas, the governor has tried to ban mask mandates and is fighting a federal mandate that large businesses either require vaccines or weekly testing.
New York University and Princeton University joined Cornell University this week in canceling events and moving winter exams online. Cornell declared "alert level red" after finding 900 cases, including a "significant number" of students infected with the omicron variant.
The omicron variant was believed to have originated in southern Africa, whose lower-income countries have struggled to obtain and distribute vaccines needed to tamp down outbreaks. In recent weeks, scientists have been collecting real-world data while conducting lab studies on how the virus responds to antibodies induced by the vaccine.
The latest research found that booster shots significantly improved protection against disease. Yet only 55 million Americans have received boosters, making many Americans vulnerable.
"The omicron variant undoubtedly compromises the effects of a two-dose mRNA vaccine induced antibodies and reduces the overall protection," Fauci said. But early studies "indicate that boosters reconstitute the antibody titers (numbers) and enhance the vaccine protection against omicron," he added.
In one study cited by the CDC, nursing home residents with a booster have 10 times lower rates of getting COVID compared to people who are unvaccinated or vaccinated but without a booster.
The CDC has been looking at various scenarios involving a triple whammy this winter – COVID-related hospitalizations stemming from omicron or delta, along with cases of seasonal flu. The worst case scenario is a peak in January with cases slowly trending downward by March, though the data informing the forecast is still sparse.
The dreary possibility was discussed in a phone call on Tuesday with public health organizations, which said the message from the CDC was to take steps now to blunt the impact.
Chrissie Juliano, executive director of Big Cities Health Coalition who participated in the call, said her takeaway from the discussion was that there are a lot of unknowns with omicron.
But it's also clear what has worked against every variant so far -- masks in public indoor settings, vaccines, and now, boosters for everyone eligible.
"We do have tools in place and we do know what to do. But we need to make sure that those things happen," Juliano said.
ABC News producer Arielle Mitropoulos contributed to this report.