Even as President Biden on Tuesday outlined new plans for battling the highly contagious Omicron variant, public health experts warned that the measures would not be sufficient to prevent a grim rise in infections and hospitalizations over the next few weeks.

The administration’s strategy includes doubling down on vaccination campaigns and propping up hospitals as they confront a large influx of patients. Federal officials will direct resources, including Army doctors, to support health care systems and distribute rapid tests to Americans.

But Mr. Biden explicitly ruled out lockdowns and other harsh measures of the kind put in place as the pandemic first unfolded in early 2020. In interviews on Tuesday, some scientists argued that the variant’s rapid spread requires more vigorous mitigation measures.

Some expressed frustration and alarm about what they described as a timid public health response, and bemoaned the apparent lack of will among politicians and society at large for more aggressive steps.

The crisis is brewing just as Americans prepare to travel to holiday gatherings, college students return home for vacation, and young and old converge for New Year’s parties or set off on trips that may further spread the virus.

Federal health officials asked health care providers on Monday to advise their patients to conduct rapid home tests for Covid before holiday gatherings, and ask their guests to do the same. But while the tests are sold over the counter, prices start at $14 for a two-pack, and many stores are sold out.

And in sharp contrast with the advice given out last year, Mr. Biden encouraged people to gather and celebrate the holidays, so long as they were vaccinated and took standard precautions.

At the same time, he warned that the variant was spreading at unprecedented speed, and said there would be Omicron infections among the vaccinated, apparently resigned to the fact that even those who have received boosters may get infected with the highly contagious variant.

“I still can’t quite wrap my head around how quickly this is moving,” said Joseph Fauver, a genomic epidemiologist at the University of Nebraska Medical Center. “I think it’s going to be really bad. I don’t know how else to put it.”

It is not yet clear whether the variant causes milder illness than earlier variants. But there is a concern among some scientists that the notion has gained wide circulation and that the pandemic-weary public has let down its guard.

“This is an incredibly contagious pathogen, and we don’t know yet its impact on severity and death,” said Galit Alter, an immunologist and virologist affiliated with the Ragon Institute of Massachusetts General Hospital, M.I.T. and Harvard.

“We have to reestablish the importance and rigor of the first wave,” she added. “We are back in ‘flatten the curve’ mode.”

Saskia Popescu, an infection prevention epidemiologist based in Arizona, said that Mr. Biden’s steps must be accompanied by greater vigilance at the community level.

Indoor gatherings should be limited in areas of high transmission, and masks should be worn even at large events held outside, she said. Restaurants must have adequate outdoor seating and ventilation, and should check patrons’ vaccination status for indoor dining.

“Now is the time to reinforce safety measures, and I think people are hesitant because everyone is burned out, but the truth is that we need them now more than ever,” she said.

Omicron spreads so quickly that the United States cannot afford to wait to observe how things play out in other countries, as happened in previous waves, said Dr. Jacob Lemieux, of the Massachusetts Consortium on Pathogen Readiness.

Nor can Americans “bet the farm” on the variant producing less severe disease, he said. The vaccines and booster shots encouraged by Mr. Biden should help reduce the incidence of severe disease, but the vaccinations are most effective two weeks after administration; in the meantime, those who have not gone for their shots are highly susceptible.

The quick spread of the variant is likely to strain already overburdened hospitals and leave vulnerable Americans, including older adults and people who are immunocompromised, at risk.

“We need to double down on keeping them protected,” said Dr. Megan Ranney, an emergency physician and the academic dean of Brown University’s School of Public Health. “Decreasing community spread in general helps them.”

How to do that? Proposals range from making vaccinations and negative Covid tests mandatory to board domestic flights to renewing the preventive behaviors recommended since the start of the pandemic, such as washing hands frequently, wearing masks in enclosed public spaces, avoiding crowds and keeping windows open for ventilation.

“We have been through this many, many times,” said Alessandro Vespignani, director of the Network Science Institute at Northeastern University in Boston. “At this point we know that there is a portfolio of interventions that can be layered on top of each other.”

Experts have recommended distributing free, high-quality masks alongside rapid tests, and creating a robust public educational campaign to ensure that people know how and when to use those tests.

Hundreds of public health experts, aerosol scientists, heath providers and advocates signed a letter Monday urging the federal government to encourage the wearing of masks indoors regardless of vaccination status, saying the precaution can be swiftly implemented and is highly effective.

The Biden administration plans to provide 500 million free rapid tests to Americans — a good start, experts said. But the tests are only expected to be available in January, after many experts fear the Omicron surge will be well underway, and the number is likely to be insufficient, as tests are intended to be used frequently.

People will also have to use a website to request the free tests. At the moment, retail outlets in cities like New York have run out of rapid tests, so many Americans cannot easily get a diagnosis before a gathering or a flight.

“People right now are having to work too hard to do the things that are necessary to prevent infection and transmission,” said Bertha Hidalgo, an epidemiologist at the University of Alabama at Birmingham School of Public Health.

Ideally, she said, the tests would be made more widely available in places that people already routinely visit, such as schools and workplaces.

Despite Mr. Biden’s advice on Tuesday, Americans planning family celebrations with grandparents or other potentially vulnerable individuals, or planning New Year’s Eve festivities with friends, should reconsider, some experts said.

“If you’re having a holiday gathering right now, chances are that one in 10 people in that room is infected and doesn’t know it yet,” said Michael Osterholm, the director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“If you really want to protect yourself during the surge, you have to limit the contact you have with people in public settings and in your own home.”

The Biden administration’s decision not to close entertainment venues, shops or restaurants, and to keep schools open with new testing procedures, points to the stark choice facing many governments as Omicron spreads, said Dr. Salim Abdool Karim, an associate member of the Ragon Institute and director of the Centre for the AIDS Programme of Research in South Africa.

“We can let people decide for themselves and businesses decide for themselves about the risk, or take structured action and say, ‘These things should not be done,’” he said. Given the fast spread of the virus, the choices must be made quickly: “Time is not on your side,” Dr. Abdool Karim warned.

In the United States, many hospitals are already straining under the Delta surge amid an exodus of staff. Hospitals and nurses have started pleading directly with the public to take the pandemic seriously.

While welcoming Mr. Biden’s proposals, Rick Pollack, the president of the American Hospital Association, said on Tuesday that health care workers “have been pushed to the brink.” He reiterated a call for all Americans to get vaccinated and boosted as soon as possible.

In Rhode Island, there is little evidence that current measures are enough to contain the latest surge. In some hospitals, emergency room waits have stretched to more than 12 hours, and doctors are treating patients in parking lots, Dr. Ranney said.

“There are no nurses,” she said. “There are no beds. There is no way to get an IV in folks out in the waiting room. There’s nothing you can do.”

The administration’s plan to mobilize the National Guard to help shore up overwhelmed hospitals and increase the number of hospital beds is desperately needed, she added.

Standards of care may need to be reassessed, experts said. Staffing shortages may require infected health care workers to continue working if possible, despite the risk to patients.

Exasperated health workers have been pleading with the public to take every step possible to protect themselves — and to keep the health care system from crumbling.

“Get the vaccine and the booster, for God’s sake,” said Mary Turner, an intensive care nurse who is president of the Minnesota Nurses Association. “The nurses are at a breaking point.”

“A year and a half ago, I likened what we were going into as a war, and we were soldiers going to battle,” Ms. Turner said. “And I’m telling you, we’re losing the war.”