The Centers for Disease Control and Prevention on Friday offered a new strategy to help communities across the country live with the coronavirus and get back to some version of normal life.The new guidelines suggest that 70 percent of Americans can now stop wearing masks, and no longer need to social distance or avoid crowded indoor spaces.The recommendations no longer rely only on the number of cases in a community to determine the need for restrictions such as mask wearing. Instead, they direct counties to consider three measures to assess risk of the virus: new Covid-related hospital admissions over the previous week and the percentage of hospital beds occupied by Covid patients, as well as new coronavirus cases per 100,000 people over the previous week.

The agency issued a new set of recommendations intended to help communities live with the virus and get back to normal life.The Centers for Disease Control and Prevention on Friday offered a new strategy to help communities across the country live with the coronavirus and get back to some version of normal life.The new guidelines suggest that 70 percent of Americans can now stop wearing masks, and no longer need to social distance or avoid crowded indoor spaces.The recommendations no longer rely only on the number of cases in a community to determine the need for restrictions such as mask wearing. Instead, they direct counties to consider three measures to assess risk of the virus: new Covid-related hospital admissions over the previous week and the percentage of hospital beds occupied by Covid patients, as well as new coronavirus cases per 100,000 people over the previous week.Based on these three factors, counties can calculate whether the risk to their residents is low, medium or high, according to the agency, and only areas of high risk should require everyone to wear a mask. But unvaccinated people should wear masks even in low-risk areas, the agency said.The agency had endorsed universal masking in schools since July, regardless of virus levels in the community, but the new guidelines recommend masking in schools only in counties at high risk.The new guidelines are being released as the coronavirus is in retreat across the country. Case numbers have dropped to levels not seen before the surge of the Omicron variant, and hospitalizations have been plummeting. About 58,000 people are hospitalized with Covid nationwide, but those numbers have fallen by about 44 percent in the past two weeks.Several experts said the new guidelines were appropriate for the country’s current situation. Although the number of cases nationwide is still high, “we’re well past the surge,” said Linsey Marr, an aerosol scientist at Virginia Tech. “We don’t need to be operating in emergency mode anymore.”But many places have already shed pandemic restrictions. Most states have eased rules for mask-wearing, and some, like New Jersey, have announced plans to lift mandates even in schools. Others are poised to end indoor mask mandates in the coming weeks. An official recommendation from the C.D.C. may hold some sway in districts that have been more cautious.Under the C.D.C.’s previous criteria, 95 percent of the counties in the United States were considered high risk. Using the new criteria, fewer than 30 percent of Americans are living in areas with a high level of risk, the agency said.The new set of guidelines gives people a framework for adapting precautions as virus levels change, Dr. Rochelle Walensky, the C.D.C.’s director, told reporters on Friday.“We want to give people a break from things like masking when our levels are low, and then have the ability to reach for them again should things get worse in the future,” she said. “We need to be prepared and we need to be ready for whatever comes next.”Those who are particularly vulnerable because of their age, health status or occupation may choose to take extra precautions, regardless of the risk level in their community, she added.The availability of high-quality masks such as N95 respirators allows individuals at high risk to continue to protect themselves even if others around them are not taking precautions, Dr. Marr said.She added that it was good that the agency would continue to monitor cases because hospital rates can lag by two to three weeks. “By the time hospitals are overwhelmed, it’s too late,” she said.But Dr. Walensky said C.D.C. scientists tested models with data from previous surges to confirm that the new method of calculating risk would have detected the surges early.The Omicron surge made it clear that because so many Americans have some immunity to the virus through vaccinations or prior infection, counties may see high numbers of cases and yet comparatively few that involve serious illness. The new guidelines nod to that reality, and allow for a more sustainable approach to living with the virus, public health experts said.“It just looked wrong that the whole country was a single shade of red,” said Jennifer Nuzzo, an epidemiologist at Johns Hopkins University’s Bloomberg School of Public Health.Though a growing number of political leaders, public health experts and ordinary citizens now support the easing of restrictions — at least temporarily, others are still wary. They note that millions of people in the United States — including children under 5 — and billions around the world remain unvaccinated, making the emergence of a dangerous new variant not just possible but likely.The Coronavirus Pandemic: Key Things to KnowCard 1 of 4A new C.D.C. framework.

The Centers for Disease Control and Prevention is expected to loosen its guidelines for when and where Americans should wear masks to prevent the spread of the coronavirus, allowing most people to go without them in public indoor spaces, according to two federal officials familiar with the matter.The policy is expected to be announced on Friday afternoon, according to an administration official. The Associated Press first reported the change on Thursday.Under the current guidelines, the agency recommends that anyone living in areas with substantial or high transmission of the coronavirus, as defined by case counts, should wear masks in public indoor spaces like gyms, movie theaters and full-capacity houses of worship. That means that people living in 95 percent of the counties in the United States should continue wearing masks indoors. The country is recording an average of about 76,000 new cases per day, a roughly 66 percent drop from two weeks ago.The forthcoming recommendations are expected to hinge on newly defined metrics to determine whether people in a particular geographical area are at high risk from the virus. They will place less emphasis on case counts and give more weight to hospitalizations as a key measure of risk, according to a federal official who is familiar with the plans but was not authorized to speak about them.The guidelines are likely to factor in the capacity of hospitals in a local area as an important indicator of the level of risk. With hospitalizations declining across the nation, that may allow the great majority of Americans to drop their masks. About 60,000 people are hospitalized with Covid nationwide, but those numbers have dropped by about 44 percent in the past two weeks.Most states have already eased rules for mask-wearing, and some, like New Jersey, have announced plans to lift mandates even in schools. Others are poised to end indoor mask mandates in the coming weeks. But an official recommendation from the C.D.C. may hold some sway in districts that have been more cautious.Many businesses have shifted to requiring only proof of vaccinations. But masks are currently required on public transportation, including airplanes. The current mandate is scheduled to remain in force through March 18, and may be extended. Last year, the C.D.C. was sharply criticized for lifting mask restrictions too soon, only to have the Delta variant of the virus surge throughout the nation. This time, however, many public health experts support the easing of mask guidelines.Recent polls have indicated that the public’s patience with Covid restrictions is waning. Nearly half of Americans surveyed thought the nation should “learn to live with” the pandemic “and get back to normal,” according to a recent Yahoo News/YouGov survey. About 70 percent of Americans believe “it’s time we accept Covid is here to stay and we just need to get on with our lives,” according to a recent poll by Monmouth University.

For more than a year, the Centers for Disease Control and Prevention has collected data on hospitalizations for Covid-19 in the United States and broken it down by age, race and vaccination status. But it has not made most of the information public.When the C.D.C. published the first significant data on the effectiveness of boosters in adults younger than 65 two weeks ago, it left out the numbers for a huge portion of that population: 18- to 49-year-olds, the group the data showed was least likely to benefit from extra shots, because the first two doses already left them well-protected.The agency recently debuted a dashboard of wastewater data on its website that will be updated daily and might provide early signals of an oncoming surge of Covid cases. Some states and localities had been sharing wastewater information with the agency since the start of the pandemic, but it had never before released those findings.Two full years into the pandemic, the agency leading the country’s response to the public health emergency has published only a tiny fraction of the data it has collected, several people familiar with the data said.Much of the withheld information could help state and local health officials better target their efforts to bring the virus under control. Detailed, timely data on hospitalizations by age and race would help health officials identify and help the populations at highest risk. Information on hospitalizations and death by age and vaccination status would have helped inform whether healthy adults needed booster shots. And wastewater surveillance across the nation would spot outbreaks and emerging variants early.Without the booster data for 18- to 49-year-olds, the outside experts whom federal health agencies look to for advice had to rely on numbers from Israel to make their recommendations on the shots.Kristen Nordlund, a spokeswoman for the C.D.C., said the agency has been slow to release the different streams of data “because basically, at the end of the day, it’s not yet ready for prime time.” She said the agency’s “priority when gathering any data is to ensure that it’s accurate and actionable.”Another reason is fear that the information might be misinterpreted, Ms. Nordlund said.Dr. Daniel Jernigan, the agency’s deputy director for public health science and surveillance said the pandemic exposed the fact that data systems at the C.D.C., and at the state levels, are outmoded and not up to handling large volumes of data. C.D.C. scientists are trying to modernize the systems, he said.

A flurry of new studies suggest that several parts of the immune system can mount a sustained, potent response to any coronavirus variant.As people across the world grapple with the prospect of living with the coronavirus for the foreseeable future, one question looms large: How soon before they need yet another shot?Not for many months, and perhaps not for years, according to a flurry of new studies.Three doses of a Covid vaccine — or even just two — are enough to protect most people from serious illness and death for a long time, the studies suggest.“We’re starting to see now diminishing returns on the number of additional doses,” said John Wherry, director of the Institute for immunology at the University of Pennsylvania. Although people over 65 or at high risk of illness may benefit from a fourth vaccine dose, it may be unnecessary for most people, he added.Federal health officials including Dr. Anthony S. Fauci, the Biden administration’s top Covid adviser, have also said that they are unlikely to recommend a fourth dose before the fall.The Omicron variant can dodge antibodies — immune molecules that prevent the virus from infecting cells — produced after two doses of a Covid vaccine. But a third shot of the mRNA vaccines made by Pfizer-BioNTech or by Moderna prompts the body to make a much wider variety of antibodies, which would be difficult for any variant of the virus to evade, according to the most recent study, posted online on Tuesday.The diverse repertoire of antibodies produced should be able to protect people from new variants, even those that differ significantly from the original version of the virus, the study suggests.“If people are exposed to another variant like Omicron, they now got some extra ammunition to fight it,” said Dr. Julie McElrath, an infectious disease physician and immunologist at Fred Hutchinson Cancer Research Center in Seattle.What’s more, other parts of the immune system can remember and destroy the virus over many months if not years, according to at least four studies published in top-tier journals over the past month.Specialized immune cells called T cells produced after immunization by four brands of Covid vaccine — Pfizer-BioNTech, Moderna, Johnson & Johnson and Novavax — are about 80 percent as powerful against Omicron as other variants, the research found. Given how different Omicron’s mutations are from previous variants, it’s very likely that T cells would mount a similarly robust attack on any future variant as well, researchers said.This matches what scientists have found for the SARS coronavirus, which killed nearly 800 people in a 2003 epidemic in Asia. In people exposed to that virus, T cells have lasted more than 17 years. Evidence so far indicates that the immune cells for the new coronavirus — sometimes called memory cells — may also decline very slowly, experts said.“Memory responses can last for ages,” said Wendy Burgers, an immunologist at the University of Cape Town who led one of the studies, published in the journal Nature. “Potentially, the T-cell response is extremely long lived.”Throughout the pandemic, a disproportionate amount of research attention has gone to antibodies, the body’s first line of defense against a virus. That’s partly because these molecules are relatively easy to study: They can be measured from a drop of blood.Analyzing immune cells, by contrast, requires milliliters of blood, skill, specialized equipment — and a lot of time. “It’s orders of magnitude slower and more laborious,” Dr. Burgers said.Few labs have the wherewithal to study these cells, and their findings lag weeks behind those on antibodies. Perhaps as a result, scientists have frequently overlooked the importance of other parts of the immune system, experts said.“Most people don’t even know what they are — a lot of doctors and scientists are not completely clear what a T cell is,” said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston who led one of the T-cell studies.Researchers found that all four vaccines studied — Pfizer-BioNTech, Moderna, Johnson & Johnson and Novavax — produce T cells that retain much of their potency against the Omicron variant.Kenny Holston for The New York Times“Fundamentally, I would argue that T cells are probably more important than what many people have given them credit for,” Dr. Barouch said.Antibodies spike after every shot of vaccine — or after each exposure to the virus — and inevitably decline within a few weeks to months.Waning antibody levels after two vaccine doses prompted federal officials to recommend boosters for everyone older than 12. The extra shots fortified antibody levels and helped to contain Omicron’s spread, but they too appear to lose some of their ability to prevent infections within four months, according to recent data from the Centers for Disease Control and Prevention.Antibodies recognize two or three key parts of the spike protein, a protrusion on the outside of the coronavirus that allows it to latch on to human cells. But T cells detect many more parts of the spike, and so are less likely to fail when the virus gains mutations in some of them.Vaccines also encode a memory of the virus in B cells, which can churn out fresh batches of antibodies within four or five days after a new exposure to the virus.The Coronavirus Pandemic: Key Things to KnowCard 1 of 3C.D.C. data.

The agency has withheld critical data on boosters, hospitalizations and, until recently, wastewater analyses.For more than a year, the Centers for Disease Control and Prevention has collected data on hospitalizations for Covid-19 in the United States and broken it down by age, race and vaccination status. But it has not made most of the information public.When the C.D.C. published the first significant data on the effectiveness of boosters in adults younger than 65 two weeks ago, it left out the numbers for a huge portion of that population: 18- to 49-year-olds, the group the data showed was least likely to benefit from extra shots, because the first two doses already left them well-protected.The agency recently debuted a dashboard of wastewater data on its website that will be updated daily and might provide early signals of an oncoming surge of Covid cases. Some states and localities had been sharing wastewater information with the agency since the start of the pandemic, but it had never before released those findings.Two full years into the pandemic, the agency leading the country’s response to the public health emergency has published only a tiny fraction of the data it has collected, several people familiar with the data said.Much of the withheld information could help state and local health officials better target their efforts to bring the virus under control. Detailed, timely data on hospitalizations by age and race would help health officials identify and help the populations at highest risk. Information on hospitalizations and death by age and vaccination status would have helped inform whether healthy adults needed booster shots. And wastewater surveillance across the nation would spot outbreaks and emerging variants early.Without the booster data for 18- to 49-year-olds, the outside experts whom federal health agencies look to for advice had to rely on numbers from Israel to make their recommendations on the shots.Kristen Nordlund, a spokeswoman for the C.D.C., said the agency has been slow to release the different streams of data “because basically, at the end of the day, it’s not yet ready for prime time.” She said the agency’s “priority when gathering any data is to ensure that it’s accurate and actionable.”Another reason is fear that the information might be misinterpreted, Ms. Nordlund said.Dr. Daniel Jernigan, the agency’s deputy director for public health science and surveillance said the pandemic exposed the fact that data systems at the C.D.C., and at the state levels, are outmoded and not up to handling large volumes of data. C.D.C. scientists are trying to modernize the systems, he said.“We want better, faster data that can lead to decision making and actions at all levels of public health, that can help us eliminate the lag in data that has held us back,” he added.The C.D.C. also has multiple bureaucratic divisions that must sign off on important publications, and its officials must alert the Department of Health and Human Services — which oversees the agency — and the White House of their plans. The agency often shares data with states and partners before making data public. Those steps can add delays.“The C.D.C. is a political organization as much as it is a public health organization,” said Samuel Scarpino, managing director of pathogen surveillance at the Rockefeller Foundation’s Pandemic Prevention Institute. “The steps that it takes to get something like this released are often well outside of the control of many of the scientists that work at the C.D.C.”The performance of vaccines and boosters, particularly in younger adults, is among the most glaring omissions in data the C.D.C. has made public.Last year, the agency repeatedly came under fire for not tracking so-called breakthrough infections in vaccinated Americans, and focusing only on individuals who became ill enough to be hospitalized or die. The agency presented that information as risk comparisons with unvaccinated adults, rather than provide timely snapshots of hospitalized patients stratified by age, sex, race and vaccination status.President Biden joined a virtual meeting with the White House Covid-19 Response Team in December. Cheriss May for The New York TimesBut the C.D.C. has been routinely collecting information since the Covid vaccines were first rolled out last year, according to a federal official familiar with the effort. The agency has been reluctant to make those figures public, the official said, because they might be misinterpreted as the vaccines being ineffective.Ms. Nordlund confirmed that as one of the reasons. Another reason, she said, is that the data represents only 10 percent of the population of the United States. But the C.D.C. has relied on the same level of sampling to track influenza for years.Some outside public health experts were stunned to hear that information exists.“We have been begging for that sort of granularity of data for two years,” said Jessica Malaty Rivera, an epidemiologist and part of the team that ran Covid Tracking Project, an independent effort that compiled data on the pandemic till March 2021.A detailed analysis, she said, “builds public trust, and it paints a much clearer picture of what’s actually going on.”Concern about the misinterpretation of hospitalization data broken down by vaccination status is not unique to the C.D.C. On Thursday, public health officials in Scotland said they would stop releasing data on Covid hospitalizations and deaths by vaccination status because of similar fears that the figures would be misrepresented by anti-vaccine groups.But the experts dismissed the potential misuse or misinterpretation of data as an acceptable reason for not releasing it.“We are at a much greater risk of misinterpreting the data with data vacuums, than sharing the data with proper science, communication and caveats,” Ms. Rivera said.When the Delta variant caused an outbreak in Massachusetts last summer, the fact that three-quarters of those infected were vaccinated led people to mistakenly conclude that the vaccines were powerless against the virus — validating the C.D.C.’s concerns.But that could have been avoided if the agency had educated the public from the start that as more people are vaccinated, the percentage of vaccinated people who are infected or hospitalized would also rise.“Tell the truth, present the data,” said Dr. Paul Offit, a vaccine expert and adviser to the Food and Drug Administration. “I have to believe that there is a way to explain these things so people can understand it.”Knowing which groups of people were being hospitalized in the United States, which other conditions those patients may have had and how vaccines changed the picture over time would have been invaluable, Dr. Offit said.Relying on Israeli data to make booster recommendations for Americans was less than ideal, Dr. Offit noted. Israel defines severe disease differently than the United States, among other factors.“There’s no reason that they should be better at collecting and putting forth data than we were,” Dr. Offit said of Israeli scientists. “The C.D.C. is the principal epidemiological agency in this country, and so you would like to think the data came from them.”It has also been difficult to find C.D.C. data on the proportion of children hospitalized for Covid who have other medical conditions, said Dr. Yvonne Maldonado, chair of the American Academy of Pediatrics’s Committee on Infectious Diseases.The academy’s staff asked their partners at the C.D.C. for that information on a call in December, according to a spokeswoman for the A.A.P., and were told it was unavailable.The Coronavirus Pandemic: Key Things to KnowCard 1 of 3Queen Elizabeth tests positive.

The Latest on Vaccines for Kids Under 5Apoorva MandavilliReporting on the coronavirusBut isn’t there new data on the vaccine’s effectiveness in children under 5?As the trial continued through December, some children became infected with the Omicron variant, giving the companies insight into the vaccine’s performance in the real world, and prompting the Food and Drug Administration to consider authorizing two doses of the vaccine. But as the number of infections grew, the data did not support that decision.

It’s not clear whether three doses of the Pfizer-BioNTech vaccine will adequately protect young children. But the F.D.A. may authorize the first two doses anyway.The Food and Drug Administration’s upcoming review of the Pfizer-BioNTech coronavirus vaccine for young children is without precedent in recent history.Next week, scientific advisers to the agency will decide whether to endorse two doses of the vaccine for children 6 months through 4 years of age, before clinical trials have shown the full course — three doses — to be effective. Such an authorization would be a first for the agency, many experts say.In fact, interim results from the trial suggested that two doses of the vaccine did not produce a strong immune response in children aged 2 through 4. Results from trials of the third dose are expected in a few weeks.The companies applied for authorization at the urging of the F.D.A., also highly unusual. The fast-moving pandemic has forced federal health officials to make important decisions with limited data before, and they argue that it’s important to begin vaccinating young children now, before a new, potentially more dangerous variant arrives.But the agency’s review of incomplete data as a basis for authorization has alarmed some experts.“We’ve never done that before, that’s what gives me some pause,” said Dr. Gregory Poland, founder and director of the Mayo Vaccine Research Group in Minnesota, and editor in chief of the journal Vaccine. “I don’t like that there isn’t more data.”The third dose is likely to build up immunity in young children, Dr. Poland and several other experts said, but it is not guaranteed to do so. With the ebbing of the Omicron surge, many scientists feel the agency could afford to wait for results on the third shots, which are expected in just a few weeks.Authorizing a vaccine before that may undermine the public’s trust in the regulatory process, and deter parents who are already anxious about immunizing their children, they warn. What if the third dose simply doesn’t work, and millions of parents have already given their children the first two doses?Although children generally do not become seriously ill when infected with the coronavirus, more of them have been hospitalized during the Omicron surge than at any other point in the pandemic. But multiple studies have shown that children who are hospitalized with Covid tend to have conditions that predispose them to severe illness, including diabetes, chronic lung disease or heart problems.Instead of authorizing the vaccine for all 18 million children aged 6 months to 4, the agency might consider recommending it only for children at high risk until more evidence becomes available, some experts said.Still, the spate of infections during the Omicron surge has left some parents eager for a vaccine.“On one side, parents are desperate to get their kids protected. On another side, there is extreme distrust,” said Natalie Dean, a biostatistician at Emory University in Atlanta. “The whole process will need to be approached with care and a lot of transparency.”Today’s 3 Key Reads About Covid1. Mask Debate: Health experts agree masks should come off in schools. But they differ on when.2. How Americans Feel: The U.S. public is frustrated with the pandemic. A wave of new polls shows how much.3. Canada’s Trucker Protests: With no end in sight, the demonstrations in Ottawa are reverberating beyond Canada’s borders.Scientific advisers to the F.D.A. will meet on Feb. 15 to weigh the current data, which will be released on Friday. The Centers for Disease Control and Prevention could recommend the two-dose regimen for the youngest children shortly after that.The Biden administration has promised to respect the recommendations of the advisers. “Please know that the F.D.A. will not cut any corners in their review process,” Dr. Vivek Murthy, the surgeon general, told reporters last week. “They know that they are the gold standard that all of us rely on.”As with the booster recommendations for all adults, the push to immunize children is part of the administration’s plan for the future, according to two federal officials familiar with the discussions: Omicron may be on its way out, but children should be protected before the next variant arrives.“We are also concerned by the notable increase in reports of children experiencing Covid-19 long haul symptoms, including in some cases children developing autoimmune diseases and Type 1 diabetes after having had Covid-19,” Stephanie Caccomo, a spokeswoman for the F.D.A., said.Pediatric doses of the Pfizer-BioNTech vaccine approved for 5- to 11-year-olds.Taylor Glascock for The New York TimesEven if vaccination of young children begins in April, it will be summer before they have had three doses, noted Dr. Diego Hijano, a pediatric infectious disease specialist at St. Jude Children’s Research Hospital, and an investigator for the Pfizer-BioNTech trial. “For sure, by summer we may have a variant of concern that’s spreading around.”But other researchers said preparing for the future was not a compelling enough reason to get ahead of the third-dose clinical trial. The risk-benefit calculus for young children now is very different from that of adults at the start of the pandemic, Dr. Poland said.“When we’re making these considerations for kids, we’re not making it in the smoke and fog and chaos of war,” he said.“I would, as a vaccinologist, just have to sit and think about it a little bit,” Dr. Poland added of the F.D.A.’s decision. “I can just guess that that puzzling is going to take a lot longer for the majority of America’s parents.”Authorization of a two-dose regimen before it is certain the third dose will cinch immunity is likely to encourage some parents to get their children the first two doses in hopes it will put them on the road to protection against the virus; others will want to wait until all the data are available.Evaluation of the Pfizer-BioNTech vaccine has proceeded in stages. First came the large trials of adults that delivered an efficacy of 95 percent, laying the groundwork for the vaccine’s swift authorization for Americans aged 16 and older.The companies then tested the vaccine in adolescents aged 12 to 15, but opted for a 10-microgram dose, a third of the dose for adults. In the youngest children, the companies tested three doses — 3, 10 and 30 micrograms — and chose the lowest dose because it seemed to be safe and yet strong enough to fend off the virus.Unlike the adult trials, the pediatric groups were too small to gauge efficacy by comparing the rate of infections in those who got the vaccine versus just saline water. The F.D.A. instead set antibody levels in people aged 16 to 25 as the benchmark the vaccine must meet in these children.This method, called immunobridging, is commonly used; it was the basis for the vaccine’s authorization in adolescents.As of Jan. 20, the trial had enrolled 1,570 children aged 6 months to 2 years, and 2,328 children aged 2 to 4, according to Pfizer. Roughly twice as many children in each group got the vaccine as received the placebo.In results Pfizer-BioNTech announced in December, children aged 2 to 4 did not produce as many antibodies as adolescents and young adults — meaning that the trial did not meet the bar the F.D.A. had set. The investigators decided to test a third dose in all the children.The Coronavirus Pandemic: Key Things to KnowCard 1 of 3Some mask mandates ending.

Among those under age 50, vaccination even without a booster protected strongly against hospitalization and death, according to new C.D.C. data.The Centers for Disease Control and Prevention on Thursday night published new data on the risks of hospitalization and death from Covid-19 among people who are unvaccinated and vaccinated, with or without booster doses.The agency recommends booster shots for Americans 12 and older. These are the first comprehensive data on the effectiveness of boosters by age in the United States.The figures confirm that booster doses are most beneficial to older adults, as the C.D.C. has previously reported. But the new numbers for younger Americans were less compelling. In those age groups, vaccination itself — two doses of the Moderna or Pfizer-BioNTech vaccines, or one dose of the Johnson & Johnson vaccine — decreased the risk of hospitalization and death so sharply that a booster shot did not seem to add much benefit.The data run only through the end of December, when the Omicron surge had just begun. Because the variant is so highly contagious, booster shots may have helped limit the variant’s spread through the population, an argument for boosters that would not be fully captured in the new research.Still, several recent studies have found that vaccination alone, without boosters, remained strongly protective against severe illness and death in most people, even after Omicron’s appearance.“I do not think these data support a universal booster rollout for everyone,” said Dr. Celine Gounder, an infectious disease expert and epidemiologist at Kaiser Health News.Instead, boosters seem most essential for older adults, she said, and those who have certain immune conditions or live in long-term care facilities. In younger Americans, it may have made sense to make booster shots available only to those with certain medical risks, she said.The advantages of booster shots in various age groups were hotly debated last fall, when the Delta variant was the primary form of the virus in the United States. But many scientists came to favor additional doses after the arrival of the highly contagious Omicron variant.“The effect of the booster can be seen in the data sets, but it’s far smaller than the effect of vaccination compared to not,” said John Moore, a virologist at Weill Cornell Medicine in New York. “The real problem is the carnage among the unvaccinated.”Unvaccinated people in every age group are at higher risk of infection, hospitalization and death than those who have been immunized, according to the C.D.C.’s data — a persistent trend ever since vaccines were introduced.As of Dec. 25, the rate of hospitalization among unvaccinated adults older than age 65 was 246 per 100,000 people. That rate dropped to 27.4 per 100,000 among people who were vaccinated without a booster dose, and to 4.9 among those who were vaccinated and received a booster.There were roughly 44 deaths per 100,000 unvaccinated adults 65 and older. Vaccinations dropped that number to about 3.6 deaths per 100,000, one-twelfth as much. Booster shots reduced the rate further, to about 0.5 deaths per 100,000, a figure 90 times as small.But such risk comparisons were less useful in younger people, for whom the rate of severe outcomes was already low.Among adults 50 to 64, 73 unvaccinated adults per 100,000 were hospitalized, compared with nine per 100,000 among those who were vaccinated and just two per 100,000 among those who had also received a booster shot.Boosters made less of a difference in the number of Covid deaths in this age group. Vaccinations decreased the rate to 0.4 deaths per 100,000 from 8.26 per 100,000. With boosters, that number fell to 0.1 deaths per 100,000 people.“This is the difference between a relative risk reduction and an absolute risk reduction,” Dr. Gounder said. “If you’re starting off with a relatively low risk, and you further reduce that risk, in the big picture that may not be such a big impact.”The agency did not provide hospitalization numbers for adults 18 through 49, perhaps because the numbers were too small. Dr. Gounder added that such data “would be really helpful in guiding decisions about boosters.”The C.D.C. also did not release data for children age 12 and older, possibly because boosters have not been recommended for long enough in that age group to have generated meaningful numbers.The Coronavirus Pandemic: Key Things to KnowCard 1 of 4The state of the virus in the U.S.

With spotty immunity in the population and a churn of new variants, the coronavirus is likely to become a persistent but hopefully manageable threat.After a frenetic few weeks when the Omicron variant of the coronavirus seemed to infect everyone, including the vaccinated and boosted, the United States is finally seeing encouraging signs.As cases decline in some parts of the country, many have begun to hope that this surge is the last big battle with the virus — that because of its unique characteristics, the Omicron variant will usher Americans out of the pandemic.The variant spiked in South Africa and Britain, then fell off quickly. Twitter is agog over charts showing declining virus levels in sewage in Boston and San Francisco. On Monday, the top European regional official of the World Health Organization suggested that “Omicron offers plausible hope for stabilization and normalization.”“Things are looking good,” Dr. Anthony S. Fauci, the Biden administration’s top adviser on the pandemic, said on Sunday. “We don’t want to get overconfident, but they look like they’re going in the right direction right now.”What’s driving the optimism? The idea is that so many people are gaining immunity through vaccination or infection with Omicron that soon the coronavirus will be unable to find a foothold in our communities, and will disappear from our lives.But in interviews with more than a dozen epidemiologists, immunologists and evolutionary biologists, the course of the virus in the United States appeared more complicated — and a bit less rosy.By infecting so many people, Omicron undoubtedly brings us closer to the end of the pandemic, they said. The current surge in infections is falling back, and there is reason to hope that hospitalizations and deaths will follow.The path to normalcy may be short and direct, the goal just weeks away, and horrific surges may become a thing of the past. Or it may be long and bumpy, pockmarked with outbreaks over the coming months to years as the virus continues to find footing.In any case, it is not likely that the coronavirus will ever completely disappear, many scientists said, and herd immunity is now just a dream. The population’s immunity against the virus will be imperfect, for a variety of reasons.“Maybe there was a short while where we could have reached that goal,” said Shweta Bansal, an infectious disease modeler at Georgetown University. “But at this point, we are well beyond that.”Instead, the coronavirus seems likely to become endemic — a permanent part of American lives, a milder illness, like the flu, that people must learn to live with and manage.But the future also depends on a wild card: new variants. Omicron surfaced only at the end of November. Most researchers believe other variants are coming, because too little of the world is vaccinated. Eventually some may be both highly contagious and have a knack for short-circuiting the body’s immune defenses, lengthening the misery for everyone.“This is a choose-your-own-adventure story, and the ending is not written yet,” said Anne Rimoin, an epidemiologist at the University of California, Los Angeles. “Nobody is going to be able to tell us what will happen.”Emergency room staff members treated a Covid patient at Pascagoula Hospital in Mississippi. Hospitalizations finally seem to be nearing a plateau, at roughly 156,000 per day, on average.Rory Doyle for The New York TimesAs of Wednesday, the United States was reporting more than 650,000 new cases daily, on average, down from more than 800,000 two weeks ago. Deaths continue to rise, at more than 2,300 per day, on average, but hospitalizations seem to be nearing a plateau, at about 155,000 per day, on average.In the best-case scenario, as those numbers fall, many Americans may soon be able to reclaim much of their prepandemic lives. Perhaps by the spring in the Northeast, and probably later in other regions, many Americans may go to work mask-free, send their children to school and socialize with family and friends without worry.Only those at high risk from Covid — because of their age, health status or occupation — would need regular boosters tailored to the latest variant.“If we could keep people out of the hospital and not get terribly ill, I think we could get back to normal basically with the tests and with vaccines,” said Michel Nussenzweig, an immunologist at Rockefeller University in New York.In the long run, many of us might experience a mild infection every few years, as with coronaviruses that cause the common cold, but would not become seriously ill.The idea of Omicron as the last stand of the coronavirus holds enormous appeal. It’s what everybody wants, every scientist hopes for. But to get there, Americans would need to be both lucky and smart.An endemic virus does not necessarily indicate a minor threat. Tuberculosis is endemic in India and other countries, and kills more than a million people each year. In African countries, measles is endemic. That virus constantly circulates at low levels and periodically triggers large outbreaks.Earlier in the pandemic, health officials estimated that for the coronavirus, vaccinating about 70 percent of the population might get us past the herd immunity threshold, meaning the coronavirus becomes a negligible threat.But the more contagious a variant, the higher the percentage of vaccinated people needed to reach the threshold. When the Alpha variant surfaced, scientists revised the level to 90 percent.By early last year, they acknowledged that the herd immunity goal was probably out of reach.Imperfect ImmunityEarly in the pandemic, some estimated that vaccinating about 70 percent of the population might lead to herd immunity. But the more contagious a variant, the higher the percentage of vaccinated people required to reach the threshold.Allison Zaucha for The New York TimesHow big a threat the coronavirus remains depends in part on the level of immunity that the country maintains over time. That’s a difficult assessment to make.There are still millions in the United States and elsewhere who have no protection from the virus and no plans to be immunized. Booster shots are needed to prevent Omicron infection, and only about half of eligible Americans have received them.Moreover, scientists know little about the strength or duration of immunity left by an Omicron infection, and they do know that the protection against infections conferred by vaccines wanes after a relatively short period. (The protection against hospitalization and death remains strong over a longer period.)If the population’s protection against the virus is weak or transient, as is possible, then Americans may continue to experience outbreaks large enough to flood hospitals for years. To contain them, people would have to line up for annual coronavirus shots, perhaps in the fall, as they do for flu shots.If the virus persists as an endemic threat, the number of people vulnerable to it will also change over time. Young people will age into higher risk groups or develop conditions that put them at risk, and babies will arrive without immunity.“Whether it’s because of evolution, whether it’s because of waning or whether it’s because of population turnover, we’ve got an influx of susceptibility which allows for future transmission,” said Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine.New VariantsJim Wilson/The New York TimesThe lack of widespread vaccination, in the United States and worldwide, coupled with the uncertainty regarding the strength of immunity left behind by Omicron, opens the door to the possibility of new variants. Someday, one of them may dodge immune defenses as well as, or even better than, Omicron does.“I consider Omicron an example of what endemic Covid-19 looks like,” said Kristian Andersen, a virologist at the Scripps Research Institute in San Diego. “But this doesn’t end with Omicron, because future variants will emerge.”The Coronavirus Pandemic: Key Things to KnowCard 1 of 5Omicron in retreat.