Public health experts fear that the highly contagious variant cannot be stopped without harsh measures that the public will no longer tolerate.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.”Vaccinations in San Ramon, Calif., this month.Jim Wilson/The New York TimesA line for Covid testing in Lower Manhattan.Gabby Jones for The New York TimesHow 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.The Coronavirus Pandemic: Key Things to KnowCard 1 of 4The Omicron variant.

Federal health officials on Friday updated public guidance about how the coronavirus spreads, emphasizing that transmission occurs by inhaling very fine respiratory droplets and aerosolized particles, as well as through contact with sprayed droplets or touching contaminated hands to one’s mouth, nose or eyes.The Centers for Disease Control and Prevention now states explicitly — in large, bold lettering — that airborne virus can be inhaled even when one is more than six feet away from an infected individual. The new language, posted online, is a change from the agency’s previous position that most infections were acquired through “close contact, not airborne transmission.”As the pandemic unfolded last year, infectious disease experts warned for months that both the C.D.C. and the World Health Organization were overlooking research that strongly suggested the coronavirus traveled aloft in small, airborne particles. Several scientists on Friday welcomed the agency’s scrapping of the term “close contact,” which they criticized as vague and said did not necessarily capture the nuances of aerosol transmission.“C.D.C. has now caught up to the latest scientific evidence, and they’ve gotten rid of some old problematic terms and thinking about how transmission occurs,” said Linsey Marr, an aerosol expert at Virginia Tech.The new focus underscores the need for the federal Occupational Safety and Health Administration to issue standards for employers to address potential hazards in the workplace, some experts said.“They hadn’t talked much about aerosols and were more focused on droplets,” said David Michaels, an epidemiologist at George Washington School of Public Health and head of O.S.H.A. in the Obama administration.He and other researchers expressed concern that the C.D.C. has not yet strengthened its recommendations on preventing exposure to aerosolized virus. The new information has significant implications for indoor environments, and workplaces in particular, Dr. Michaels said. Virus-laden particles “maintain their airborne properties for hours, and they accumulate in a room that doesn’t have good ventilation.”“There’s more exposure closer up,” Dr. Michaels said. “But when you’re further away, there’s still a risk, and also these particles stay in the air.”Donald Milton, an aerosol scientist at the University of Maryland, agreed that federal officials should provide better guidelines for keeping workplaces safe. “We need better focus on good respirators for people who have to be close to other people for long periods of time,” Dr. Milton said. “A surgical mask, even if it’s tucked in on the edges, is still not really going to give you enough protection if you’re in a meatpacking plant elbow to elbow all day long with other people.”Health care workers, bus drivers and other workers may also require respirators, Dr. Michaels said. Customers in retail stores should continue to maintain distance from one another and to wear masks, he added; good ventilation is paramount in these settings.Dr. Marr pointed out that one updated page on the C.D.C. website, entitled “How Covid-19 Spreads,” says that inhaling the virus when people are far apart is “uncommon.” The statement is “misleading and potentially harmful,” Dr. Marr said. “If you’re in a poorly ventilated environment, virus is going to build up in the air, and everyone who’s in that room is going to be exposed.”

Americans who are fully vaccinated against the coronavirus no longer need to wear masks outdoors if they’re walking, running, hiking or biking alone, with members of their household, or if they attend small outdoor gatherings, federal health officials announced on Tuesday.The Centers for Disease Control and Prevention stopped short of telling those people that they could shed their masks altogether in outdoor settings — citing the worrying risk that remains for transmitting the coronavirus, unknown vaccination levels among people in crowds and the still high-caseloads in some regions of the country.Federal health officials and President Biden were announcing the updated advice on Tuesday, linking the news with the administration’s public campaign to get most American adults vaccinated by summer and trying to offer reassurances that some semblance of normal life can return.But the C.D.C. is maintaining advice on other safety measures, saying vaccinated adults should continue wearing masks and staying six feet apart in large public spaces, like outdoor performance or sports events, indoor shopping malls and movie theaters, where the vaccination and health status of others would be unknown. And they still should avoid medium and large gatherings, crowds and poorly ventilated spaces, officials said.“I welcome less restrictive guidelines about masking outdoors,” said Linsey Marr, an aerosol scientist at Virginia Tech. “We know that transmission outdoors is much less likely to occur than indoors, because the virus cannot accumulate in the air outdoors. It’ll become rapidly diluted.”But the guidelines themselves, which outline different masking recommendations for a variety of scenarios, seem overly complex, she said. “I can’t remember this. I would have to carry around a sheet of paper — a cheat sheet with all these different stipulations.” She added: “I worry that this is not as helpful as it could be.”Americans have been whipsawed on the issue of mask-wearing advice since the beginning of the pandemic, when top health officials said people did not need them — in part because of severe shortages of protective gear for health care workers on the front lines.And mask restrictions since then have been a patchwork from state to state, despite growing evidence of a mask’s protection for individuals and those around them. Many states have already lifted restrictions they had put in place for indoor and outdoor activities. Others like New York, however, have maintained mask-wearing requirements even for outdoor spaces, citing the threat of potentially more contagious variants.But the pace of vaccinations has helped influence some easing of those limits. So far, about 42 percent of Americans have received at least one dose of a Covid-19 vaccine, and 29 percent have received both doses of the two vaccines requiring double shots.Centers for Disease Control and PreventionThe vaccines are highly effective at preventing people from becoming seriously ill from the coronavirus.“Scientifically the vaccines are good enough that it’s highly unlikely that someone who’s vaccinated is going to be exposed to enough virus outdoors to have a breakthrough infection,” Dr. Marr said.Early evidence also suggests that vaccinated people may be significantly less likely to transmit the virus, but the exact risks are not yet known.Some experts also wondered if the new directives were confusing, by establishing different standards for those who are vaccinated and those who are not, even though it is impossible to know who is who.“It’s not like you can go up to someone in public and say, ‘You don’t have a mask on – are you vaccinated?’” said Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco. “Those who aren’t vaccinated will promptly take their mask off outdoors because no one can check.”But, she said, that is probably fine, since the risk of transmission in outdoor settings is very low, absent close or prolonged contact with someone.Dr. Mercedes Carnethon, an epidemiologist at Northwestern University, said the relaxed guidelines signal that “if you’re outside in a group of individuals who you know well, then it is safe to be without a mask if you were vaccinated. I don’t think that it goes so far as to change what our behavior needs to be in outdoor settings where we don’t know people, and we can’t distance.”Masking and distancing are still generally recommended when gathering with unvaccinated people from more than one other household or with an unvaccinated person who is at high risk of severe illness from Covid, or who lives with a vulnerable person.And there are scenarios in which wearing a mask outdoors can still be an important social signal, Dr. Carnethon said. For instance, no vaccine has yet been authorized for children under 16. “And when we’re going to require children to wear masks, at school and on the playground when they’re at school,” she said, “I think that it is responsible for the adults in the situation to model that behavior and normalize mask wearing even when outside.”A growing body of research indicates that the risk of spreading the virus is far lower outdoors than indoors. Viral particles disperse quickly outdoors, experts say, meaning brief encounters with a passing walker or jogger pose very little risk of transmission.But most if not all of the research about viral transmission outside was done before the vaccine was available.A recent systematic review of studies that examined the transmission of the coronavirus and other respiratory viruses among unvaccinated individuals concluded that fewer than 10 percent of infections occurred outdoors and that the odds for indoor transmission were 18.7 times higher than outdoors. (The odds of super-spreading events were 33 times higher indoors.)