The debut of Omicron-specific vaccines raises a thorny question: How long should you wait between shots?An influential scientific panel on Thursday recommended updated coronavirus booster shots to the vast majority of Americans, adding a critical new tool to the country’s arsenal as it tries to blunt an expected wintertime surge of the virus.The decision, made by expert advisers to the Centers for Disease Control and Prevention, cleared the way for health workers to begin giving people the redesigned shots within days. And it marked a milestone in the fight against a rapidly shape-shifting pathogen: For the first time in the pandemic, manufacturers have capitalized on the potential of mRNA technology to begin distributing a Covid vaccine that perfectly matches the circulating strain of the virus, a feat that had long seemed improbable.For all their promise, the arrival of updated boosters also adds another wrinkle to what is the country’s most complicated vaccine. Vaccine recipients have already had to parse shifting eligibility rules and decide between brands. Now they face a weighty new question: how long to wait after their last vaccine dose or infection before seeking an updated booster.In authorizing the new boosters, federal regulators said on Wednesday that people needed to leave at least two months between doses. While several members of the expert panel expressed concern on Thursday that two months was too short, the C.D.C. pushed to endorse the same minimum interval.And yet immunologists said that waiting roughly four to six months after a last vaccine or infection would strengthen people’s response to an updated vaccine, even if those scientists also saw the rationale for giving Americans flexibility in choosing when to seek a new shot.The C.D.C. panel, the Advisory Committee on Immunization Practices, endorsed the updated boosters for all adults. People who have received a primary Covid vaccine series are eligible no matter how many additional doses they have gotten, part of an effort to stop counting doses and simplify guidelines, C.D.C. scientists said.The panel’s advice now goes to Dr. Rochelle Walensky, the C.D.C. director, who is expected to make a formal recommendation soon. Hundreds of thousands of updated vaccine doses were being delivered around the country on Thursday, the C.D.C. said. Providers nationwide are expected to have millions by Labor Day, the agency said, though some doctors have said they were told that initial supply would be small in their areas.The new boosters are designed to generate immune responses to the original version of the coronavirus and to BA.5, the Omicron subvariant that is now dominant. Pfizer and BioNTech’s shot was authorized for people as young as 12, and Moderna’s for those 18 and older.The C.D.C. advisory panel’s advice will now go to Dr. Rochelle Walensky, the C.D.C. director, who is expected to make a formal recommendation soon.Pool photo by Stefani ReynoldsExactly how much benefit the new boosters will offer over the existing shots is not clear. In a bid to catch up with the virus’s evolution, regulators authorized them while relying heavily on human trials of a different updated shot, one that was reformulated to defend against the BA.1 subvariant of Omicron.Moderna and Pfizer scientists on Thursday released their most detailed findings yet from studies of the latest BA.5 vaccines in mice. They found that those vaccines substantially increased immune responses to Omicron subvariants, including BA.5.Moderna also deliberately infected mice with the BA.5 virus, and found that a BA.5 booster offered significantly better protection against infection in the lungs than the original vaccine did. The company’s scientists argued that immune responses in mice had correlated with human immune responses in past studies.Immunologists said they were confident that the new shots were the best available Covid boosters, and that they would put something of a damper on the outbreak this fall and winter.Read More on the Coronavirus PandemicHeavy Toll: The average life expectancy of Americans fell precipitously in 2020 and 2021. The decline, largely driven by the pandemic, was particularly pronounced among Indigenous communities.Boosters: An influential panel of expert advisers to the Centers for Disease Control and Prevention recommended updated coronavirus booster shots to the vast majority of Americans, clearing the way for health workers to begin giving people the redesigned shots within days.Paxlovid Study: The Covid-19 medication Paxlovid reduced hospitalizations and deaths in older patients, but made no difference for patients under 65, new research from Israel found.Moderna’s Lawsuits: The vaccine manufacturer sued Pfizer and BioNTech, claiming that its rivals’ Covid-19 shot copied groundbreaking technology that Moderna had developed before the pandemic.Simply having updated vaccines by next week, rather than in mid-November, could save between roughly 7,500 and 18,000 lives by the spring, leading epidemiologists have estimated.“We’re still seeing just under 500 deaths per day, which is putting us at about four times the level of yearly deaths we’ll tolerate for influenza,” said E. John Wherry, an immunologist at the University of Pennsylvania. “That’s still an unacceptable level of death. I’m hopeful we’ll make a dent in that because of updated boosters.”The reformulated shots will deliver the biggest benefits for older and more vulnerable Americans, scientists said. While the existing shots offer more durable protection against hospital admission and death than they do against infections, their effectiveness does also wane over time against severe outcomes, scientists said. That makes updated boosters a potentially important aid in surviving an infection, they said.But it is often younger, healthier people who spread the virus to more vulnerable Americans, Dr. Wherry said. Even if those younger people are at relatively low risk of severe Covid themselves, scientists said, the updated boosters were likely to reduce their odds of getting infected in the first place and, in turn, spreading the virus to someone more vulnerable.The higher levels of immunity generated by an updated booster could also decrease the likelihood of developing long Covid, said Shane Crotty, a virologist at the La Jolla Institute for Immunology.“If people take these boosters, they’ll be better off, almost no matter what their situation is,” Dr. Crotty said. “If we can get as many people to take these boosters as possible, that will definitely reduce the number of cases this winter.”Success of the shots will depend largely how many Americans get them, as the country’s vaccine machinery has been scaled back since booster shots were last offered to all adults.Shannon Stapleton/ReutersBut other scientists have expressed concerns that not enough evidence had yet been generated showing that updated boosters were an upgrade to the existing offerings. They have also worried that updated vaccines would give recipients a false sense of security.“I understand that we need better vaccines,” Dr. Pablo Sánchez, a professor of pediatrics at The Ohio State University and member of the C.D.C. panel, said during Thursday’s meeting. But he said that he was struggling “to make a recommendation for a vaccine that has not been studied in humans,” even if those human studies were ongoing. He was the only one of the 14-member panel to vote against recommending the updated vaccines.Scientists said that serious adverse reactions had been very rare after booster doses, and that changes to the updated shots were so subtle that they would be highly unlikely to present new problems.The success of the shots will depend in large part on how many Americans get them. The country’s vaccine machinery has been scaled back since the United States last offered booster shots to all adults, a result in part of congressional resistance to more pandemic response spending.That has especially hurt efforts to give updated vaccines to marginalized and vulnerable Americans, some of the very people who face the greatest risks of being exposed to the virus and of becoming severely ill from it, epidemiologists said.Many Americans seeking an updated booster shot this fall will have passed the four-to-six-month period that immunologists recommend waiting between doses, C.D.C. data showed. Older adults were only made eligible for their last booster shots this spring, but uptake was highest five months ago, during the early period of the rollout in April. Younger people were made eligible for their most recent boosters almost a year ago, and most of them are more than six months past their last shot, the C.D.C. said.But millions of people have also recently gotten Covid. When it comes to the timing of future vaccine doses, those infections act like earlier vaccinations, scientists said, mitigating the benefits of getting a new booster shot shortly thereafter. Receiving a booster soon after an infection could also potentially raise the small risk, most prevalent in younger men, of post-vaccine heart problems, members of the C.D.C. panel said.Federal guidance indicates that people should wait at least until their Covid symptoms have resolved before getting a booster shot, and that they may want to wait a full three months after an infection.Some scientists said it was reasonable to want to give people flexibility about when to get an updated booster.People may want to strengthen their immune defenses before partaking in an unusually high-risk activity. Others may have weakened immune systems that limited their responses to an earlier shot. An approaching case surge, especially this winter, could raise the risk of waiting. And the C.D.C. has urged health providers to offer people flu and Covid vaccines at the same visit.Deepta Bhattacharya, an immunologist at the University of Arizona, said that C.D.C. recommendations could afford to space out doses of routine childhood vaccines at precisely the right intervals because children are unlikely to be exposed to those pathogens in the interim. That’s not the case for the coronavirus, which is circulating so widely that someone leaving a long gap between doses faces higher odds of getting infected in the meantime.“Honestly, that’s probably the more important factor to consider — what’s happening in the real world — rather than in an ideal world of immunological optimization,” Dr. Bhattacharya said.Still, he and other scientists said that waiting four to six months between doses would strengthen people’s response to the updated boosters. That would help people steer clear of a situation in which their antibody levels were so high when they received an updated shot that their immune systems partly cleared out the vaccine before building strong defenses against new Omicron subvariants.“You can really only boost the responses after you’ve gotten below your ceiling,” said Jenna Guthmiller, an immunologist at the University of Colorado. But, she said, “Without testing ourselves and figuring out what our ceilings are, which is very complex, the best option is honestly just to get the vaccine.”

Updated Covid vaccines, expected soon after Labor Day, were designed to thwart Omicron variants. But money to distribute them has dried up.Long past the point when pollsters said there were no more Americans willing to be vaccinated against the coronavirus, Coral Garner kept finding them.An organizer of mobile clinics for the Minnesota Department of Health, she arranged to provide vaccines and booster shots to people who had resisted them, setting up in a retrofitted city bus outside a Nigerian church, a Hmong senior center, a Somali mall and dozens of other sites.But even as the United States now prepares for a critical campaign to deliver Omicron-specific booster shots, Ms. Garner’s job no longer exists. In June, her contract position was canceled because the state said funding had dried up.At the very moment a better coronavirus vaccine is expected to finally become available, America’s vaccination program is feeling the effects of a long period of retreat.Local programs to bring shots to the places where Americans gather and the institutions they trust have folded, a consequence in some cases of congressional resistance to more pandemic response spending.The same local health department workers responsible for Covid and flu shots this fall have also, without new staffing, been juggling a monkeypox outbreak and childhood immunization deficits that have left some places susceptible to polio.And some state health officials, citing weak demand for vaccines and increased survival rates of late, said in interviews that they had stopped aggressively pushing coronavirus shots.With the virus killing far fewer people than it once did and many Americans reverting to their prepandemic ways, the country’s no-expenses-spared attitude to saving lives has evolved into a response that has put a greater onus on individuals to protect themselves. In keeping with that approach, many health officials believe the vaccine machinery is in place to meet what they expect, lamentably, to be tepid demand this fall.Tess Holman, held her daughter, Margot Holman, 1, as she received the Covid vaccine at the Mall of America in Bloomington, Minn., in June.Jenn Ackerman for The New York TimesBut others are worried that the country is surrendering a decisive opportunity to stoke that demand and restore the more robust vaccination efforts that lifted last year’s initial rollout.“We are watching the dismantling of the hyperlocal infrastructure that actually brought needles to arms in the most vulnerable communities in the country,” said Stephen Thomas, the director of the Center for Health Equity at the University of Maryland. “To this day, vaccine uptake in the United States is embarrassing.”The Biden administration said some 70,000 sites were prepared to vaccinate people this fall. While 60 percent of those are pharmacies, they also include doctor’s offices, community health centers and rural health clinics.States can also seek money from the Federal Emergency Management Agency for certain vaccination-related expenses, like setting up sites, buying equipment and offering translation or transportation services.Read More on the Coronavirus PandemicHeavy Toll: The average life expectancy of Americans fell precipitously in 2020 and 2021. The decline, largely driven by the pandemic, was particularly pronounced among Indigenous communities.Boosters: An influential panel of expert advisers to the Centers for Disease Control and Prevention recommended updated coronavirus booster shots to the vast majority of Americans, clearing the way for health workers to begin giving people the redesigned shots within days.Paxlovid Study: The Covid-19 medication Paxlovid reduced hospitalizations and deaths in older patients, but made no difference for patients under 65, new research from Israel found.Moderna’s Lawsuits: The vaccine manufacturer sued Pfizer and BioNTech, claiming that its rivals’ Covid-19 shot copied groundbreaking technology that Moderna had developed before the pandemic.Having shifted much of the rollout to private sites, though, states have been promised FEMA reimbursements on a relatively modest $550 million in vaccination spending so far this year. Last year, that figure was $8.5 billion.And while providers are supposed to vaccinate everyone for free, with or without insurance, the federal government ran out of money this spring to offer reimbursements for shots for uninsured people, making it more difficult for them to receive boosters.Sonya Bernstein, a senior policy adviser for the White House Covid response team, said federal spending to support vaccination efforts was being held back by a stalemate in Congress over the administration’s request for billions of dollars in additional pandemic aid. Republicans have said that additional coronavirus spending could be covered with funding already approved by Congress, an assertion that some state health officials say is false.“We are working with less because Congress has not provided us with that funding,” Ms. Bernstein said. “But that has not gotten in the way of our preparations. We’re working day in and day out to make sure states and our partners have the resources and support they need.”The United States is leaning ever more heavily on vaccines to defend against the virus at a time when health officials are pulling back on other preventive measures, like masking, distancing and quarantining.The fall vaccination campaign, which is expected to begin soon after Labor Day, could be crucial. Many Americans have gone months since their last Covid vaccine or infection, allowing immune defenses to wane. More indoor gatherings are on the horizon, and epidemiologists are predicting roughly 100,000 to 165,000 additional Covid deaths by the spring.And, for the first time, the government has bought vaccines that were reformulated in response to the virus’s evolution. Manufacturers may finally have gained on the pathogen: The Omicron subvariant that the updated shots were designed to protect against remains dominant in the United States.But, at the same time, the vaccination campaign is lagging. While two-thirds of Americans have completed the primary vaccine series, only about one-third have received boosters. The country’s per capita booster coverage trails that of some 70 other nations, according to Our World in Data.Some are worried that the country is surrendering a decisive opportunity to stoke demand for and restore the more robust vaccination efforts that lifted last year’s initial rollout.Hannah Beier for The New York TimesPartly as a result, scientists said, Americans this year have died from Covid at a rate 80 percent higher than Canadians and 30 percent higher than residents of the European Union.“We have criticisms of the way the initial vaccine rollout happened, but there was really a very significant effort to get everyone vaccinated,” said Elizabeth Wrigley-Field, a University of Minnesota sociologist, who mentioned mandates, financial rewards and large events. “None of that really exists with boosters.”With Covid deaths having plateaued around 480 a day, policymakers are grappling with whether renewed investments are needed. Some states believe they are not.In Alabama, where one-fifth of residents are boosted, Dr. Burnestine Taylor, the state’s medical officer for disease control and prevention, said officials had pared back health department clinics and become more reliant on pharmacies as demand dropped. The decision to receive additional shots, she said, now fell to individuals.“At this point, we’re not doing a hard push,” Dr. Taylor said. “It’s a personal decision.”Even some more proactive efforts have run into a wall of complacency. In Camden County, N.J., health workers have visited community events and knocked on doors, but they have not encountered as many takers as they had hoped, said Paschal Nwako, the health officer there.But other health workers said that they were still winning converts, if fewer than last year, including those who had been confused about boosters or unable to find clinics with evening availability.In Madison, Wis., Aaron Perry, a former police officer, said that $100 stipends provided by the state have helped draw a dozen or so booster recipients to his health clinic every Friday, many from Black barbershops where he also runs health screening centers.In San Bernardino, Calif., Jacinda Abdul-Mutakabbir, a pharmacist at Loma Linda University, said clinics late this spring could still attract as many as 30 first dose recipients.And in Bismarck, N.D., Renae Moch, the public health director, said organizations like food pantries and homeless shelters still wanted to host regular clinics. But with surge staffing over, workers exhausted and positions harder to fill, she said she could only hold monthly pop-up clinics at a limited number of sites — and none in September, when back-to-school immunizations would consume the staff.Of the hundreds of barbershops nationally that once hosted vaccination events, nine out of 10 are struggling to keep offering shots, said Dr. Thomas, of the University of Maryland, who has helped organize them. In some cases, he said, hospitals or pharmacies that eagerly used to send doses or staff have reported not having the money to partner with barbershops again or being concerned that small turnouts will not make it worth their while.Mike Brown cuts the hair of Michael Venable at the The Shop Spa, a barbershop outside of Washington, D.C. Michael A. McCoy for The New York Times“The health care providers lost interest in us,” said Mike Brown, a barber outside Washington, D.C. “But I don’t think now is the time to give up the fight. People are still dying.”For poorer Americans, the decrease in public vaccination sites could reduce the number who receive shots this fall, experts said.In New York, Emily Gerteis, who arranges shots for people living on the street or in shelters at the Center for Urban Community Services, recalled convincing a patient this summer to be vaccinated. But when she suggested a pharmacy, the patient refused, preferring to hold out for city clinics and their $100 incentives, Ms. Gerteis said.The problem was that those offerings no longer existed. The patient was not vaccinated.“A year ago, there was all this money for advertising, and they were throwing money at vaccines,” said Dr. Zeke McKinney, a physician in Minneapolis who had helped to organize vaccinations at his local barbershop until funding dried up. “Now, it’s like nobody cares.”The White House is still seeking more funding from Congress, which it says is also needed to produce tests and develop next-generation Covid vaccines.For now, some health officials said they were prepared to rev back up shuttered sites in the event demand surged, even if their workers were increasingly depleted. On some days recently, Dr. Mysheika Roberts, the health commissioner in Columbus, Ohio, said she needed to divert two-thirds of her Covid vaccination specialists to monkeypox clinics.“It’s a bit overwhelming for some of our staff members,” she said.Experts said that restoring health workforces and maintaining vaccine outreach could help break a boom-and-bust cycle in public health spending that has especially hurt marginalized Americans. Early investments, for example, helped narrow racial gaps in primary series vaccination rates. But in the booster rollout, considerable racial disparities have re-emerged.The relaxation of federal Covid guidance and the reluctance to incentivize booster shots has not made it any easier to persuade people of the benefits of additional doses, some health officials said.“There’s a lot of messaging from federal sources that things are good and we’re back to normal,” said Dr. Clay Marsh, West Virginia’s Covid czar. “It’s mixed messaging.”Dr. Mysheika Roberts has had to divert two-thirds of her Covid vaccination specialists to monkeypox clinics recently.Maddie McGarvey for The New York TimesHealth experts encouraged making Covid shots a routine part of people’s medical care, including by enlisting more primary care doctors in the rollout. More creative marketing could also help generate demand, said Dr. Kevin Schulman, a Stanford University professor.One example, he said, would be a campaign framed around protecting older relatives at fall or winter holiday gatherings. Despite the scientific uncertainties, he also said the time had to come to promise Americans that they would not be asked back for further Covid vaccines for at least a year — and that, when they were, it would be for an “annual Covid vaccine,” rather than a “booster.”“Marketers spend huge amounts of time trying to figure these things out,” Dr. Schulman said. “Unfortunately, we just haven’t seen effort devoted in that direction.”Ms. Bernstein, the White House adviser, said the administration was regularly surveying people about booster shots and using the results to inform messages it suggested to on-the-ground partners.Ben Weston, Milwaukee County’s chief health policy adviser, said the nation’s underfunded booster campaign had hurt the same vulnerable and often nonwhite residents who have long struggled to gain access to good medical care.“It’s putting up barriers,” he said, “particularly for populations that are more susceptible to those barriers.”

At the peak of the Omicron wave, Covid killed Black Americans in rural areas at a rate roughly 34 percent higher than it did white people.The coronavirus pandemic walloped rural America last year, precipitating a surge of deaths among white residents as the virus inflamed longstanding health deficits there.But across the small towns and farmlands, new research has found, Covid killed Black and Hispanic people at considerably higher rates than it did their white neighbors. Even at the end of the pandemic’s second year, in February 2022, overstretched health systems, poverty, chronic illnesses and lower vaccination rates were forcing nonwhite people to bear the burden of the virus.Black and Hispanic people in rural areas suffered an exceptionally high toll, dying at far higher rates than in cities during that second year of the pandemic.In towns and cities of every size, racial gaps in Covid deaths have narrowed. That has been especially true recently, when major gains in populationwide immunity have tempered the kind of pressure on health systems that appears to hurt nonwhite Americans the most.With coronavirus deaths climbing, though, and health officials bracing for an even deadlier winter, scientists warned that efforts so far to close the racial gap in vaccination rates had not been enough to insulate nonwhite people from the ravages of major Covid waves.Nowhere were those difficulties more pronounced than in rural areas. Black, Hispanic and Native American people in those places recorded the deadliest second year of the pandemic of any large racial or ethnic groups anywhere in the United States, according to the new research, which was led by Andrew Stokes, an assistant professor of global health at Boston University.In those communities, the Biden administration’s reassurances that every Covid death is now preventable jar with the difficulties of obtaining medical care.Rural pharmacies are often few and far between, making it difficult for poorer and less mobile residents to receive critical antiviral pills.Doctors said that some Black patients, especially those who are uninsured or far from hospitals, wait too long before seeking help to benefit from new treatments.And Black and Hispanic people have received booster shots at lower rates, a consequence of what some physicians describe as a lack of awareness stemming from cutbacks to public messaging, especially in conservative states.Read More on the Coronavirus PandemicYoung Children: In a new survey, a majority of American parents said they would refuse the coronavirus vaccine for their children, citing concern with the shot’s side effects.Periods: Nearly half of the participants in a recent study reported a heavier flow while menstruating after receiving the Covid vaccine. But experts said there was no cause for alarm.Boosters: The U.S. government has greenlit new vaccines to defend against the latest Omicron variants. But will the shots arrive too late?The Rise of BA.5: As the Omicron subvariant leads to a surge of new infections across the country, New Yorkers seem less inclined to get themselves into high alert mode again.“The national vibe is that everyone should now be in a position to do what they need to protect themselves from the virus,” said Bobby Jenkins, the mayor of Cuthbert, Ga., a mostly Black town whose only hospital closed six months into the pandemic. “But not everyone’s in a position to do that yet.”Racial disparities in Covid deaths have narrowed for several reasons, scientists said. The early vaccine rollout prioritized older Americans, who are disproportionately white. But over the last year, primary vaccinations for Black and Hispanic people climbed at roughly double the pace of white rates.The rate for Hispanic people, 54 percent, now exceeds that for white people, which is 50 percent. The Black vaccination rate, 43 percent, still lags, but the gap has diminished.Patients waited for Covid testing at the North Sunflower Medical Center in Ruleville, Miss.Rory Doyle for The New York TimesThe virus also infected and killed Black and Hispanic people at such greater rates in the pandemic’s first year — at one point in 2020, Black rural dwellers were dying at roughly six times the rate of white dwellers — that it may have had fewer targets by Year 2.Those changes have been so profound that among the oldest Americans, white Covid death rates have recently exceeded those of Black people, according to Centers for Disease Control and Prevention data.But the shrinking of the racial gap is partly because of a worsening of the pandemic for white people rather than serious advances for Black or Hispanic Americans. White Covid death rates climbed by 35 percent from the first to second year of the pandemic, the C.D.C. found. Over that period, death rates fell by only 1 percent in Hispanic people and 6 percent in Black people.“It’s not a movement toward equity,” said Alicia Riley, a sociologist at the University of California, Santa Cruz. “It’s that white people started getting and dying of Covid more.”The national picture has also disguised a shift that has as much to do with geography as with race, Dr. Stokes said. As the brunt of the pandemic shifted in late 2020 from big cities to rural areas, which have a bigger share of white residents, the national tally of Covid deaths naturally came to include more white people.But within rural areas, Covid deaths were apportioned at greater rates to nonwhite people, just as they were in big cities and in small or medium ones, Dr. Stokes’s team found. He used C.D.C. counts of Covid death certificates up until February, avoiding more recent and potentially incomplete data, and took into account the older age of the white population.At the worst of the Omicron wave this winter, Black and Hispanic death rates exceeded those of white people in towns and cities of every size, just as they had at the peak of every previous virus outbreak.Black death rates at this winter’s peak were greater than those of white people by 34 percent in rural areas, 40 percent in small or medium cities and 57 percent in big cities and their suburbs. The racial gap was so large in cities because white urbanites have died from Covid at vastly lower rates than white people in rural areas for most of the pandemic.

President Biden is 79, and Americans his age and older have made up larger and larger shares of those dying from Covid in recent months. The virus has taken advantage of falling immunity caused by long delays since older people’s last vaccinations, and the Omicron variant has evolved a growing ability to skirt the body’s defenses.Covid has been killing substantially fewer Americans of all ages this summer than it did during the peak of the wintertime Omicron wave. Still, older people remain at significantly higher risk.As of early June, four times as many Americans aged 75 to 84 were dying each week from the virus, compared with people two decades younger, according to data from the Centers for Disease Control and Prevention. (Those death counts are provisional, the C.D.C. cautioned, because they were based on death certificates and did not account for all deaths in those age groups.)That is an even bigger age gap than existed at the peak of the Omicron wave this winter. Then, the number of people aged 75 to 84 killed by Covid each week was twice as high as the number aged 55 to 64.The president received a second booster shot in late March, significantly reducing his risk of severe illness. This spring, people aged 50 and older who had received a single booster were dying from Covid at four times the rate of those with two booster doses, the C.D.C. has reported.In 2022, Covid deaths, though always concentrated in older people, have skewed toward older people more than they did at any point since vaccines became widely available. Many older people were vaccinated early in 2021, and among those who have not yet received a booster shot, immune defenses generated by the shots have significantly waned.In contrast, middle-aged Americans, who suffered a large share of pandemic deaths last summer and fall, are benefiting from greater stores of immune protection from both vaccination and prior infections.While Covid deaths remain far lower than in the winter, they are climbing again among older people as the immune-evasive Omicron subvariant known as BA.5 causes more infections, according to the latest C.D.C. data. From early May to early June, the number of Americans aged 75 to 84 dying from Covid each week increased by nearly 50 percent.

The emergence of highly contagious Omicron subvariants this spring in the United States appeared to reduce the protection offered by vaccines against Covid hospitalizations, scientists from the Centers for Disease Control and Prevention reported on Friday.But first and second booster shots helped to shore up people’s defenses, the agency found. The additional shots raised people’s levels of protection against those Omicron subvariants and restored some of the protection that was lost as time passed since their last shots.“Booster doses should be obtained immediately when persons become eligible,” the C.D.C. scientists wrote.The findings, however, came with a notable caveat: Measurements of vaccine effectiveness have been complicated by the number of unvaccinated people who have been infected by the virus, especially during the wintertime surge of Omicron cases.Those prior infections give people some protection against Covid. As a result, in studies like the C.D.C.’s that compare outcomes from the virus in vaccinated and unvaccinated people, the vaccines appear less protective than they really are.The C.D.C. scientists used what little information they had about patients’ infection histories to try to account for those difficulties. Using data from hospitals across 10 states, the agency’s scientists studied some 58,000 hospitalizations with a diagnosis of Covid-like illness from mid-December through mid-June. The study focused on adults with healthy immune systems.By late April, subvariants of Omicron known as BA.2 and BA.2.12.1 had outcompeted the version of Omicron that spread across the country over the winter.After those subvariants became dominant, the Pfizer-BioNTech and Moderna vaccines were less effective in keeping people from being admitted to the hospital with Covid than they had been during the wintertime Omicron wave, the study found.Two doses of the vaccines were 24 percent effective against hospitalizations after the variants took over, compared with 61 percent during the period when the original version of Omicron dominated. (Those figures held for people who had been given their second dose at least five months earlier.)That decline probably partly resulted from the ability of the subvariants to evade people’s immune defenses from the vaccine, and partly from unvaccinated people drawing some protection from earlier infections.A booster dose helped considerably, even though the benefits of those additional doses appeared to wane over time. Once the subvariants became the main sources of infection, a third dose of Pfizer or Moderna raised the vaccines’ effectiveness against hospitalization to 69 percent initially, and 52 percent after four months or more.The third doses had been even more protective during the wintertime surge of the original version of Omicron.Second boosters were authorized in late March for people 50 and older with healthy immune systems, and those additional boosters appeared to help people weather the subvariant surge, the C.D.C. found.At least a week after a fourth dose, the vaccines were 80 percent effective against hospitalizations with Covid, the agency said. That was a considerable lift from the 55 percent effectiveness offered by three doses after four months in that age group.It was not clear how quickly the protection offered by that fourth dose would diminish. The study also did not measure the vaccines’ performance against BA.5, the latest Omicron subvariant, which appears to be driving a fresh surge of cases and hospitalizations. That subvariant has become dominant among new U.S. cases and appears to be the most evasive form of Omicron to spread in the country.With hospital admissions rising, federal health officials have urged eligible people to get booster doses as soon as possible, saying that those shots would not prevent people from getting an additional dose of an updated, variant-specific vaccine in the fall or winter. The latest results reinforced the need for booster shots, the agency said.“Given recent increases in deaths and hospitalizations associated with the BA.5 variant,” the C.D.C. said on Friday, “everyone should stay up to date with recommended Covid-19 vaccinations, including additional booster doses for those who are moderately to severely immunocompromised and adults over 50.”

The government has greenlit new vaccines to defend against the latest Omicron variants. But the shots won’t arrive until the fall, and cases are rising now.Roseann Renouf, 77, has grown tired of the current generation of coronavirus shots. Having “never been one for a lot of vaccination,” she decided to forgo the latest round of boosters after watching vaccinated friends contract Covid-19, even though the doses offer a critical extra layer of protection.“It’s just taking another same booster,” Ms. Renouf, a retired nurse anesthetist from Fort Worth, said. “They haven’t done anything different with them to cover new variants.”But her gripe about the Covid vaccines may soon be settled. American regulators committed last week to updating the 2020 vaccine recipes for this fall’s booster campaign with new formulas meant to defend against the ultra-contagious Omicron subvariants, offering Ms. Renouf and other holdouts a fresh reason to change their minds.The Biden administration is betting that the new cocktails, the centerpiece of an effort to drastically speed up vaccine development, might appeal to the half of inoculated Americans who have so far spurned booster shots, a key constituency in the fight against future Covid waves.Vaccine updates are becoming more urgent by the day, many scientists said. The most evasive forms of Omicron yet, known as BA.4 and BA.5, appear to be driving a fresh surge of cases across much of the United States. The same subvariants have sent hospital admissions climbing in Britain, France, Portugal, Belgium and Israel.Covid deaths in the United States, which had been hovering for months near their lowest levels of the pandemic, are rising again. In the worst case, epidemiologists have predicted some 200,000 Covid deaths in the United States within the next year.“We’re hoping that we can convince people to go get that booster,” said Dr. Peter Marks, who oversees the vaccines office at the Food and Drug Administration, “and help mature their immune response and help prevent another wave.”Many scientists believe that updated boosters will be critical for diversifying people’s immune defenses as subvariants eat away at the protection offered by vaccines. Catching up with a virus that has been so rapidly mutating may be impossible, they said. But it was far better to be only a few months, rather than a couple of years, behind the pathogen.“Omicron is so different that, to me, it seems pretty clear we’re starting to run out of ground in terms of how well these vaccines protect against symptomatic infections,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “It’s very important that we update the shots.”Now, the question is whether those modified boosters will arrive in time. In a bid to match the latest forms of the virus, the F.D.A. asked vaccine manufacturers to tailor their new shots to the BA.4 and BA.5 subvariants, rather than to the original version of Omicron from last winter.Virologists said that a subvariant vaccine would generate not only the strongest immune defenses against current versions of the virus, but also the type of broad antibody response that will help protect against whatever form of the virus emerges in the months ahead.But building a fall booster campaign around vaccines at the forefront of the virus’s evolution could also come at a cost. Pfizer and Moderna said that they could deliver subvariant vaccine doses no earlier than October. Some F.D.A. advisers warned in a public meeting last week that the timeline could be slowed even further by any number of routine delays.Scientists said vaccine updates were becoming more urgent by the day, with the BA.4 and BA.5 subvariants driving a fresh surge of cases.Saul Martinez for The New York TimesIn contrast, a vaccine targeting the original version of Omicron is closer at hand: Moderna and Pfizer have already started making doses tailored to the original form of Omicron, and Moderna said that it could start supplying them this summer. Whether the benefits of a newer subvariant vaccine outweigh the drawbacks of having to wait longer depends on when exactly it arrives and how much havoc the virus wreaks before then, scientists said.They said that having some form of an updated vaccine by the fall was crucial.“I would lean toward thinking BA.4, BA.5 is a good choice unless it dramatically extends the timeline,” said Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle, voicing support for the subvariant vaccine. “If using BA.4, BA.5 only modestly extends the timeline, I think it’s a good choice.”The updated shots will test the public’s openness to an accelerated vaccine program that is reminiscent of the way annual flu shots are formulated, but that is entirely new when it comes to the coronavirus.The original Covid vaccines had to withstand slow and laborious testing: Volunteers took the shots and then went about their lives while researchers tracked who got sick. But there is now ample evidence that the shots are safe. And any tweaks to the recipe could be wasted if scientists were to spend the better part of a year testing them.Instead, vaccine manufacturers have been studying volunteers’ blood samples in the lab to gauge their immune responses to a booster that is tailored to the first version of Omicron. The subvariant boosters have so far been through lighter testing: Pfizer has studied only how they have affected antibody responses in mice.The F.D.A. said that it would not require clinical trial data for the subvariant boosters before authorization and would rely instead on studies of boosters targeting the original version of Omicron. Some scientists said that authorizing modified vaccines without time-consuming human studies was essential to keeping up.“It just seems dangerous to overly bureaucratize the rollout of an updated vaccine,” said Jeremy Kamil, a virologist at Louisiana State University Health Shreveport. Moving too slowly, he said, would risk leaving older and other vulnerable people exposed to a pathogen that looks different than what the original vaccines had prepared them for.“If a bank robber’s grown a beard and dyed their hair,” he said, “it’s going to help your response to know what they look like today rather than when they were 14 years old.”Some government vaccine advisers have said that regulators had not yet proven that updated boosters protected substantially better than existing boosters against severe Covid. Others have expressed concern that reformulating vaccines would undercut confidence in the vaccination program.For some booster-shy Americans, though, the fact that the current offerings had grown dated was the source of their apathy.“It probably helps a bit, the booster, but not to the extent of going to the trouble of getting it,” said Cherry Alena, a retired medical secretary in her 70s from Northern California, whose last Covid vaccine was 16 months ago. “It’s not specifically formulated for the thing that’s going around.”A modified shot would appeal to her, she said, because “it gives you specific immunity against the specific thing.”Gaps in booster coverage have left the United States more exposed to deaths during Omicron waves. More than half of vaccinated Americans have not received a booster. Three-quarters of those eligible for a second booster have not gotten one.This spring, people age 50 and older who had received a single booster were dying from Covid at four times the rate of those with two booster doses, according to the Centers for Disease Control and Prevention.A Pfizer-BioNTech Covid vaccine manufacturing plant in Puurs, Belgium. Both Pfizer and Moderna have said that they could deliver subvariant vaccine doses no sooner than October.Pfizer, via Associated PressThere are no certainties when it comes to forecasting the pathogen’s evolution. Come winter, the virus could take an unexpected turn away from the Omicron branch of the evolutionary tree. And whereas flu viruses typically turn over in the course of years, new coronavirus variants can emerge and then start stampeding across the world within months.But scientists said it was reassuring that the updated boosters — which would also contain a component of the original formulation — appeared to generate strong immune responses to many different versions of the virus. And for now, signs are pointing toward this winter’s virus being a descendant of Omicron.“The more time that passes, the more likely it is that anything new is going to emerge from Omicron,” said Trevor Bedford, an evolutionary biologist at the Fred Hutchinson Cancer Center.Even though the coronavirus evolves faster than the flu, Dr. Bedford said, mRNA technology enables Covid shots to be modified more quickly, too. Decisions about the composition of an autumn flu vaccine are typically made in February, he noted; this fall’s coronavirus vaccines are not being decided upon until early summer.And scientists have a wider window into what coronavirus strains are spreading and how quickly. “With SARS-CoV-2, we have 12 million genomes,” Dr. Bedford said of the virus. “For flu, we have collected 250,000 over decades.”The F.D.A.’s decision to give its blessing to updated vaccines may have ripple effects across the globe, setting Moderna and Pfizer on the path to making those shots. But some countries may choose boosters targeting the earlier version of Omicron because they will be ready sooner.Some F.D.A. advisers also said that a vaccine made for the original strain by a third company, Novavax, held promise as an Omicron-targeted booster. That shot is not yet authorized for use.Scientists said they were eager for a clearer picture of how updated vaccine candidates would be chosen in the future and how quickly they could be made. Some also pressed for closer cooperation between American regulators and the World Health Organization, which supports updating the vaccines but with the original version of Omicron, not its latest subvariants, as a different way of broadening immune responses.The ultimate goal, many scientists said, was to compress the time between when the next immune-dodging variant emerges and when people can be vaccinated against it.“We’re now seven months out from when we first detected Omicron,” said Dr. Michael Z. Lin, a professor of neurobiology at Stanford who has tracked the regulatory process. “We need a rapid way for strain selection, and it needs to be more rapid than what we’ve done so far.”Among those likely to line up for a modified vaccine is Randi Plevy, 57, of New York. Having been vaccinated, and then infected twice, she held off on getting a booster shot.“Why am I getting a booster if it’s not going to protect me against what’s out there?” she said. “If they can demonstrate you’re getting ahead of the curve, and ‘Here is the latest and greatest that is going to protect you from the next strain,’ I think that’d be really attractive to a lot of people.”

Most Americans now carry some immune protection, experts said, whether from vaccines, infection or both.For two years, the coronavirus killed Americans on a brutal, predictable schedule: A few weeks after infections climbed so did deaths, cutting an unforgiving path across the country.But that pattern appears to have changed. Nearly three months since an ultra-contagious set of new Omicron variants launched a springtime resurgence of cases, people are nonetheless dying from Covid at a rate close to the lowest of the pandemic.The spread of the virus and the number of deaths in its wake, two measures that were once yoked together, have diverged more than ever before, epidemiologists said. Deaths have ticked up slowly in the northeastern United States, where the latest wave began, and are likely to do the same nationally as the surge pushes across the South and West. But the country remains better fortified against Covid deaths than earlier in the pandemic, scientists said.Because so many Americans have now been vaccinated or infected or both, they said, the number of people whose immune systems are entirely unprepared for the virus has significantly dwindled.“In previous waves, there were still substantial pockets of people who had not been vaccinated or exposed to the virus, and so were at the same risk of dying as people at the beginning of the pandemic,” said Dr. David Dowdy, an epidemiologist at Johns Hopkins Bloomberg School of Public Health. “Those pockets don’t exist anymore.”That turn in the pandemic has nevertheless left many Americans behind.Older people make up a larger share of Covid deaths than they did last year. The virus continues to kill unvaccinated people at much higher rates than vaccinated people, despite many unvaccinated people having some protection from prior infections. And those with weakened immune systems also face greater risks.Covid is still killing an average of 314 people daily, one-tenth the number who were dying every day in January 2021, but, even so, an awful toll. At that rate, the virus is killing more than twice as many Americans every day as suicide or car crashes are. And many of those who survive the virus are debilitated, some of them for long after their infections.With the country’s resources for fighting the virus drying up and many Americans forgoing booster shots, the decoupling of cases and deaths may not last. Immunity will wane and a more evasive variant could cut into people’s residual protection against severe disease.Elmhurst Hospital Center in Queens, N.Y., was considered the epicenter of the virus at the start of the pandemic.Dave Sanders for The New York TimesThe Rose River Memorial in Los Angeles, created in 2021 by the artist Marcos Lutyens, honored lives taken by Covid.Isadora Kosofsky for The New York Times“As the time since people got vaccinated becomes longer and longer, the efficacy of the immune response will be lessened,” said Dr. Abraar Karan, an infectious disease physician at Stanford University. “We can be caught off guard later this year.”The link between Covid cases and deaths started weakening over the winter, scientists said, but the sheer volume of Americans getting infected meant that fatalities still soared.This spring, Covid has been killing fewer Americans daily than during any period except the summer of 2021. The country is now recording 10 times as many cases as it was at that time, indicating that a smaller share of cases are ending in death.By some estimates, the case fatality rate — the share of recorded Covid cases that prove deadly — is one-third lower than it was last summer and one-quarter lower than it was in December. Recorded cases always understate actual infection levels, and the prevalence of at-home testing these days has made that especially true.To account for those problems, Dr. Dowdy looked at the proportion of reported test results that are positive, a figure known as test positivity. That measure, too, is imperfect, but it reflects the enormous numbers of Americans who recently contracted the virus; some scientists estimate that the current wave of cases is the second-largest of the pandemic.By his rough calculations, Dr. Dowdy estimated that the ratio of deaths to test positivity fell threefold from the early days of the pandemic to January 2022, and fourfold from January 2022 to this spring.“What we’re seeing is that the average case of Covid-19 is becoming much milder,” he said.That is a better reflection of gains in immunity than it is of any intrinsic weakening of the virus, scientists said. Government estimates of the share of Americans who have contracted the virus jumped from one-third in December 2021 to well over one-half two months later.The country paid a staggering price: Some 200,000 people were killed by Covid this winter and large numbers beyond that were seriously sickened. But those who survived infections emerged with immune systems that had learned to better deal with the virus.Relatives hold the hand of Carmen Evelia Toro, 74, as she dies from Covid in Queens in April 2020.Victor J. Blue for The New York Times“Our level of community immunity heading into this wave was much higher than it’s ever been due to the combination of infection and vaccination,” said Dr. Joe Gerald, an associate professor of public health at the University of Arizona. “A lot of people who weren’t vaccinated, and were infection-naive — most of them were infected with Omicron over the period from January to early March.”In the Northeast, where the Omicron subvariants first took hold this spring, deaths climbed as cases surged. In New York, the daily average of Covid deaths rose from eight in April to about 24 in mid-June. Daily deaths in New England increased from five to a peak of 34 over the same period.But across the United States, where cases have been climbing since early April, deaths have remained roughly level. In each previous wave, national Covid deaths surged several weeks after cases did.“I think it’s somewhat reassuring that deaths didn’t really spike as they had during earlier points of the pandemic,” said Jennifer Nuzzo, an epidemiologist at Brown University.Virginia Pitzer, an epidemiologist at the Yale School of Public Health, said that shift stemmed in part from a growing share of cases occurring in people who were fully vaccinated, previously infected or both. In Arizona, for instance, the share of Covid cases being recorded in vaccinated people grew to 60 percent in April from 25 percent five months earlier.In a country as large as the United States, every Covid wave is also a collection of staggered regional surges, complicating national trends. In early May, for example, continued declines in Covid deaths in the South and the West from the wintertime Omicron wave might have helped to obscure rising mortality levels in the Northeast.Some states have also moved from reporting Covid deaths daily to doing so weekly, and they have only slowly caught up from holiday reporting breaks, causing more frequent daily swings in the data.And some states said that so many residents had died from Covid this winter that it took them weeks to report all of those deaths publicly. That, too, could have affected the national death curve.“Our surveillance system in the U.S. is not as strong as it should be or could be,” Dr. Gerald said, “and it does make it more difficult for us to understand the pace and trajectory of the outbreak.”There are a number of possible reasons that Covid deaths have not fallen even further. With infection levels so high and few precautions being taken, the virus is inevitably reaching people who are more vulnerable because of their vaccine status, age or underlying conditions. And even as some people gain immune protection during the pandemic, others become more susceptible to bad outcomes as they age or develop weakened immune systems.Hospital admissions are still climbing nationally, making it likely that increases in deaths will gradually follow, epidemiologists said.Shannon Stapleton/ReutersThe country’s stagnant booster campaign has also left many older people at a long distance from their last shot and so vulnerable to the effects of waning immunity.“Overall, the people who’ve been coming through with Covid are much, much less sick than they were even this winter,” said Dr. Megan Ranney, an emergency physician at Brown University. “It feels like almost a different disease for folks, with the exception of people who are really old, who are unvaccinated or who are immunosuppressed.”Disparities in access to booster shots and antiviral pills have also put some Americans at higher risk. Black and Hispanic people eligible for boosters have received the shots at lower rates than white people have, reflecting what some epidemiologists describe as limited efforts in some states to put boosters within easy reach. Patients who do not have primary care doctors, or who live far from pharmacies, can also struggle to get antiviral pills.The number of hospitalized Covid patients is still climbing nationally, making it likely that increases in deaths will gradually follow, epidemiologists said. It is unclear how hard the wave will hit less-vaccinated regions, like the South, where immunity from past infections has also grown.“Unfortunately, vaccination rates in many southern states are among the lowest in the country,” said Jason Salemi, a professor of epidemiology at the University of South Florida. “But there is certainly a lot of immunity built up through prior infection.”Even as fewer cases turn deadly, the unprecedented number of infections this winter and spring has created significant problems of its own. In the United States, one in five adult survivors of Covid under 65 has dealt with some version of long Covid, a recent study found. Many people have missed work, including doctors, whose absences this spring have periodically strained hospitals that already had staffing problems.Dr. Karan, of Stanford, said that he had lingering symptoms from a January bout with Covid until April. A month later, he was infected again. As of last week, he said, with the subvariant surge hitting California, his team of five doctors at one of the hospitals where he works had been reduced to two because of Covid absences, forcing delays to consultations for some patients.In the Northeast, where cases have been falling for several weeks, Dr. Ranney said that Covid patients had generally been spending less time in the hospital during the latest wave.They had also been presenting differently, she said. In previous surges, patients’ most pressing difficulties tended to be the direct result of Covid, like low oxygen levels or severe pneumonia. This spring, she said, more patients needed care because Covid had exacerbated underlying conditions, like diabetes or heart trouble.“This wave feels qualitatively and quantitatively different,” Dr. Ranney said. “We’re not seeing our I.C.U. get filled up with patients who are gasping for breath or who are on death’s door.”Sarah Cahalan

Researchers identified a protein in the fluid that could boost the cognition of aging animals — and might lead to future treatments for people.Five years ago, Tal Iram, a young neuroscientist at Stanford University, approached her supervisor with a daring proposal: She wanted to extract fluid from the brain cavities of young mice and to infuse it into the brains of older mice, testing whether the transfers could rejuvenate the aging rodents.Her supervisor, Tony Wyss-Coray, famously had shown that giving old animals blood from younger ones could counteract and even reverse some of the effects of aging. But the idea of testing that principle with cerebrospinal fluid, the hard-to-reach liquid that bathes the brain and spinal cord, struck him as such a daunting technical feat that trying it bordered on foolhardy.“When we discussed this initially, I said, ‘This is so difficult that I’m not sure this is going to work,’” Dr. Wyss-Coray said.Dr. Iram persevered, working for a year just to figure out how to collect the colorless liquid from mice. On Wednesday, she reported the tantalizing results in the journal Nature: A week of infusions of young cerebrospinal fluid improved the memories of older mice.The finding was the latest indication that making brains resistant to the unrelenting changes of older age might depend less on interfering with specific disease processes and more on trying to restore the brain’s environment to something closer to its youthful state.“It highlights this notion that cerebrospinal fluid could be used as a medium to manipulate the brain,” Dr. Iram said.Turning that insight into a treatment for humans, though, is a more formidable challenge, the authors of the study said. The earlier studies about how young blood can reverse some signs of aging have led to recent clinical trials in which blood donations from younger people were filtered and given to patients with Alzheimer’s or Parkinson’s disease.But exactly how successful those treatments might be, much less how widely they can be used, remains unclear, scientists said. And the difficulties of working with cerebrospinal fluid are steeper than those involved with blood. Infusing the fluid of a young human into an older patient is probably not possible; extracting the liquid generally requires a spinal tap, and scientists say that there are ethical questions about how to collect enough cerebrospinal fluid for infusions.While there are theoretically other ways of achieving similar benefits — such as delivering a critical protein in the fluid that the researchers identified or making a small molecule that mimics that protein — those approaches face their own challenges.Jeffery Haines, a biochemist who has studied cerebrospinal fluid and multiple sclerosis at Mount Sinai Medical Center in New York, said that the study had elegantly identified how certain ingredients in the fluid might promote memory. But he said the general public’s appetite for anti-aging drugs was outpacing the science.“In general, people are looking for the Holy Grail of aging, and they think there is going to be a magical factor that’s being secreted that’s just going to reverse this thing,” he said. “I don’t think it’s that simple.”Cerebrospinal fluid made for a logical target for researchers interested in aging. It nourishes brain cells, and its composition changes with age. Unlike blood, the fluid sits close to the brain.But for years, scientists saw the fluid largely as a way of recording changes associated with aging, rather than countering its effects. Tests of cerebrospinal fluid, for example, have helped to identify levels of abnormal proteins in patients with significant memory loss who went on to develop Alzheimer’s disease. Scientists knew that there were also health-promoting proteins in cerebrospinal fluid, but identifying their locations and precise effects seemed out of reach.For one thing, scientists said, it was difficult to track changes in the fluid, which the body continuously replenished. And collecting it from mice while avoiding contaminating the fluid with even trace amounts of their blood was extremely challenging.“The field has lagged decades behind other areas of neuroscience,” said Maria Lehtinen, who studies cerebrospinal fluid at Boston Children’s Hospital and is the co-author of a commentary in Nature about the new mouse study. “Largely this is because of the technical limitations in studying a fluid that’s deep inside the brain, and that turns over continuously.”Dr. Iram was undaunted. She set about taking the liquid from 10-week-old mice, cutting above their necks and drawing out fluid from a tiny cavity near the back of the brain while trying not to puncture any blood vessels or poke the brain itself.When she was successful, Dr. Iram said, the result was about 10 microliters of cerebrospinal fluid — roughly one-fifth of the size of a drop of water. To collect enough for infusions, she had to do the procedure on many hundreds of mice, taming the technical challenges that Dr. Wyss-Coray had warned of by sheer force of repetition.“I like doing these types of studies that require a lot of perseverance,” Dr. Iram said. “I just set on a goal, and I don’t stop.”To infuse the young cerebrospinal fluid into old mice, Dr. Iram drilled a tiny hole in their skulls and implanted a pump below the skin on their upper backs. For comparison, a separate group of old mice was infused with artificial cerebrospinal fluid.A few weeks later, the mice were exposed to cues — a tone and a flashing light — that they had earlier learned to associate with shocks to their feet. The animals that had received the young cerebrospinal fluid infusion tended to freeze for longer, suggesting that they had preserved stronger memories of the original foot shocks.“This is a very cool study that looks scientifically solid to me,” said Matt Kaeberlein, a biologist who studies aging at the University of Washington and was not involved in the research. “This adds to the growing body of evidence that it’s possible, perhaps surprisingly easy, to restore function in aged tissues by targeting the mechanisms of biological aging.”Dr. Iram tried to determine how the young cerebrospinal fluid was helping to preserve memory by analyzing the hippocampus, a portion of the brain dedicated to memory formation and storage. Treating the old mice with the fluid, she found, had a strong effect on cells that act as precursors to oligodendrocytes, which produce layers of fat known as myelin that insulate nerve fibers and ensure strong signal connections between neurons.The authors of the study homed in on a particular protein in the young cerebrospinal fluid that appeared involved in setting off the chain of events that led to stronger nerve insulation. Known as fibroblast growth factor 17, or FGF17, the protein could be infused into older cerebrospinal fluid and could partially replicate the effects of young fluid, the study found.Even more strikingly, blocking the protein in young mice appeared to impair their brain function, offering stronger evidence that FGF17 affects cognition and changes with age.The study strengthened the case that breakdowns in myelin formation were related to age-associated memory loss. That is something of a departure from the longstanding focus on the fatty insulation in the context of diseases like multiple sclerosis.Some scientists said that knowing one of the proteins responsible for the effects of young spinal fluid could open the door to potential treatments based on that protein. At the same time, recent technological advances have brought scientists closer to observing changes in cerebrospinal fluid in real time, helping them “peel back the layers of complexity and mystery surrounding this fluid,” Dr. Lehtinen said.Still, scientists cautioned that those treatments would not materialize anytime soon. Among the difficulties are understanding what other proteins might be involved and figuring out how to harness their effects without causing separate problems.But Dr. Wyss-Coray said that the study filled a critical gap in the understanding of how the brain’s environment changes as people age.“The question is, ‘How can you maintain cognitive health until you die? How can you make the brain resilient to this relentless degeneration of the body?’” he said, “and what a growing number of studies show is that as we learn more about the aging process itself, maybe we can slow down aspects of aging and maintain tissue integrity or even rejuvenate tissues.”

Black children who had not been immunized made up about a third of those hospitalized during the winter surge.Unvaccinated children from 5 to 11 years old were hospitalized with Covid at twice the rate of vaccinated children during the winter Omicron variant surge, the Centers for Disease Control and Prevention reported on Tuesday.The study was the latest to demonstrate that vaccines help keep children out of the hospital with Covid, despite the shots losing some of their potency at stopping infections from the Omicron variant.But the C.D.C. report, based on data from hospitals serving about 10 percent of the U.S. population across 14 states, also offered some of the strongest evidence to date that racial disparities in childhood vaccination might be leaving Black children more exposed to severe illness from Covid.Black children in the 5-to-11 age group accounted for about a third of unvaccinated children in the study, the largest of any racial group, and made up roughly a third of overall Covid-related hospitalizations within the age group.Estimates from 2020 based on census data suggest that Black children made up about 14 percent of U.S. residents from 5 to 11 years old. But it is not clear whether the areas covered in the C.D.C. study are representative of the country’s population, making it difficult to precisely measure any disparities.“Increasing vaccination coverage among children, particularly among racial and ethnic minority groups disproportionately affected by Covid-19, is critical to preventing Covid-19-associated hospitalization and severe outcomes,” the C.D.C. study said.The agency has not reported nationwide data on the race or ethnicity of vaccinated children, making it difficult for researchers to examine gaps in protection.Seven states and Washington, D.C., report race data for vaccinated children from 5 to 11. Black children were inoculated at lower rates than white children in most, but not all, of those states, an analysis by the Kaiser Family Foundation found this month. Asian children tended to have the highest vaccination rates, the analysis found, and Hispanic children were inoculated at rates lower than or similar to those of white children.Among all U.S. residents, Black people remain less likely than white people to be vaccinated, though the gap has grown smaller over the course of the vaccination campaign.Children are protected in far smaller numbers: Only about a third of children from 5 to 11 have at least one vaccine dose, the lowest rate of any age group. And the pace of vaccinations in that age group has slowed considerably in recent weeks.The C.D.C. study covered the period from mid-December to late February, during which about 400 children were hospitalized with Covid at the select hospitals participating in the study. Almost 90 percent of them were unvaccinated. The report said that roughly a third of the children had no underlying medical conditions and a fifth were admitted to an intensive care unit.Among the children who tested positive for the virus before or during their hospitalization, three-quarters of them were admitted primarily for Covid, rather than other illnesses, the C.D.C. said.The agency said that Omicron appeared to be causing less serious illness in children than the Delta variant, as was the case for adults, but that Omicron was so contagious and infecting so many children that they were hospitalized at higher rates during the Omicron surge.Infected children are far less likely to become seriously ill, compared with adults. But because the youngest children (under age 5) do not yet qualify for vaccination and older children are inoculated at much lower rates, children overall are somewhat less protected from the virus than adults.

The coronavirus remains new enough and its long-term effects unpredictable enough that measuring the threat posed by an infection is a thorny problem.Like it or not, the choose-your-own-adventure period of the pandemic is upon us.Mask mandates have fallen. Some free testing sites have closed. Whatever parts of the United States were still trying to collectively quell the pandemic have largely turned their focus away from community-wide advice.Now, even as case numbers begin to climb again and more infections go unreported, the onus has fallen on individual Americans to decide how much risk they and their neighbors face from the coronavirus — and what, if anything, to do about it.For many people, the threats posed by Covid have eased dramatically over the two years of the pandemic. Vaccines slash the risk of being hospitalized or dying. Powerful new antiviral pills can help keep vulnerable people from deteriorating.But not all Americans can count on the same protection. Millions of people with weakened immune systems do not benefit fully from vaccines. Two-thirds of Americans, and more than a third of those 65 and older, have not received the critical security of a booster shot, with the most worrisome rates among Black and Hispanic people. And patients who are poorer or live farther from doctors and pharmacies face steep barriers to getting antiviral pills.These vulnerabilities have made calculating the risks posed by the virus a fraught exercise. Federal health officials’ recent suggestion that most Americans could stop wearing masks because hospitalization numbers were low has created confusion in some quarters about whether the likelihood of being infected had changed, scientists said.“We’re doing a really terrible job of communicating risk,” said Katelyn Jetelina, an epidemiologist at the University of Texas Health Science Center at Houston. “I think that’s also why people are throwing their hands up in the air and saying, ‘Screw it.’ They’re desperate for some sort of guidance.”To fill that void, scientists are thinking anew about how to discuss Covid risks. Some researchers are working on tools to compare infection risks to the dangers of a wide range of activities, finding, for instance, that an average 43-year-old vaccinated last year is roughly as likely to be hospitalized from an infection as a bull rider is to be hospitalized after a ride. Others have studied when people could unmask indoors if the goal was not only to keep hospitals from being overrun but also to protect immunocompromised people.But many scientists said they also worried about this latest phase of the pandemic heaping too much of the burden on individuals to make choices about keeping themselves and others safe, especially while the tools for fighting Covid remained beyond some Americans’ reach.“As much as we wouldn’t like to believe it,” said Anne Sosin, who studies health equity at Dartmouth, “we still need a society-wide approach to the pandemic, especially to protect those who can’t benefit fully from vaccination.”Collective metricsWhile Covid is far from America’s only health threat, it remains one of its most significant. In March, even as deaths from the first Omicron surge plummeted, the virus was still the third-leading cause of death in the United States, behind only heart disease and cancer.Causes of DeathCovid has been among the top three causes of death in the United States for most of the last two years.