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.

Though the number of epidemiologists in state health departments surged during the pandemic, the Centers for Disease Control and Prevention reported on Thursday that states were still far short of the public health workforces that officials said they needed.The report, based on a survey from early 2021 of state epidemiologists from all 50 states and the District of Columbia, reinforced broader concerns that public health workers across the country are less equipped to respond to a pandemic now than they were at the beginning of 2020.Even as the number of state epidemiologists grew, especially in fields like the Covid-19 response, staffing in general infectious diseases, chronic diseases, and maternal and child health declined, the C.D.C. said. Epidemiologists are responsible for tracking disease, developing responses, investigating health threats and studying public health services and health care.And although the number of state epidemiologists grew by 23 percent from 2017 to 2021, to a total of 4,136 positions, fewer states said in 2021 that they had the resources to fully monitor population health issues and to investigate and diagnose hazards that could affect people.In all, state officials said they needed another 2,196 epidemiologists to provide basic public health services, the survey found.“The Covid-19 response has strained the U.S. public health system,” the C.D.C. report said. “Workforce and capacity needs remain unmet.”The federal government has poured money into public health response activities during the pandemic, including $7.66 billion from the American Rescue Plan Act in 2021, the C.D.C. said. It is not clear how states’ public health needs or staffing levels had changed since the survey was conducted in 2021.The C.D.C. warned that public health needed predictable levels of funding over longer periods.Public health was often underfunded and neglected even before the pandemic. A combination of unpredictable funding, reduced authority to impose health orders and staffing problems have made the work of state and local public health departments all the more difficult.Funding for epidemiology was fragile, the C.D.C. reported. In 2021, epidemiology activities relied on federal money for 85 percent of their budgets, but states were unsure how long that money would last: 39 percent of the federal money was designated for Covid-19 work over limited time periods.The C.D.C. report tracked epidemiology positions only in state health departments, and not in other state agencies.Among the fields in which states reported being the shortest-staffed were genomics, mental health, oral health and occupational health.

Childhood environments shape people’s navigational skills, researchers reported. The findings one day may lead to better tests for early dementia.As a child in Chicago, Stephanie de Silva found that the city helped her get where she was going. Streets had directional names like “West” or “North,” and they often met at neat right angles. If all else failed, Lake Michigan could situate her.But when Ms. de Silva, 23, moved to London, where she now studies cognitive science, she suddenly could not navigate to a restaurant two blocks from home without a smartphone map. The streets were often crooked. Sometimes they seemed to lead nowhere.“I don’t think the cardinal directions exist here,” she said. “I’ve lived here for six months now, and I don’t know which direction I’m facing.”Scientists in Ms. de Silva’s lab at University College London, along with colleagues in Britain and France, have now arrived at an explanation: People who grow up in predictable, gridlike cities like Chicago or New York seem to struggle to navigate as easily as those who come from more rural areas or more intricate cities.Those findings, published in Nature on Wednesday, suggest that people’s childhood surroundings influence not only their health and well-being but also their ability to get around later in life. Much like language, navigation is a skill that appears to be most malleable when people’s brains are developing, the researchers concluded.The authors hope the findings eventually lead to navigation-based tests to help diagnose Alzheimer’s disease. Getting lost can sometimes occur earlier in the course of the illness than memory problems, they said.Researchers have developed virtual navigation tests for cognitive decline, but they can interpret the results only if they know what other factors influence people’s way-finding abilities.Among the forces shaping people’s navigation skills, the study suggested, was what kind of places they experienced as a child.“The environment matters,” said Hugo Spiers, a professor of cognitive neuroscience at University College London and one of the study’s lead authors. “The environment we’re exposed to has a knock-on effect, into the 70s, on cognition.”People in countries whose biggest cities were complex patchworks, like Barcelona, Spain, may have sharpened their navigational skills by dealing with chaotic street layouts, scientists suggested. Emilio Morenatti/Associated PressIt took a series of unlikely events — involving a cellphone company, a controversial YouTuber and a custom-made video game — to generate the large data set behind the study.In 2015, Michael Hornberger, who studies dementia at University of East Anglia in England, heard about a company that wanted to invest in dementia-related research.Having just attended a workshop about gaming in science, he proposed a video game that could help him figure out how people of different ages, genders and locations performed on navigation tasks. Such a game, he thought, could create benchmarks against which to assess patients who might be in the early stages of Alzheimer’s disease.To his surprise, the company — Deutsche Telekom, a major stakeholder in T-Mobile — funded his idea. Known as “Sea Hero Quest,” the smartphone game involved steering a boat to find sea creatures. To recruit players, the company launched an advertising campaign that included a video from PewDiePie, YouTube’s biggest star at the time, who was later penalized by the platform for using antisemitic language.The scientists had hoped that the game would draw 100,000 people in Western Europe. The participants would be testing their navigation skills while also providing basic demographic details, like whether they had grown up in or outside of cities.Instead, over 4.3 million people joined in, generating a global database of clues about people’s ability to get around. “We underestimated the gaming world,” Dr. Hornberger said. “It went beyond our wildest dreams.”For all its simplicity, the game has been shown to predict people’s ability to get around real places, including London and Paris. In recent years, the research team has used the resulting data to show that age gradually erodes people’s navigation skills and that gender inequality is a predictor of whether men will perform slightly better than women. The latest study addressed what its authors described as a more vexing question: Do cities, however grid-like, have the effect of honing people’s navigational skills by offering them a plethora of options for moving around? Or do people from more rural areas, where distances between places are long and paths are winding, develop superior navigation abilities?Ms. de Silva’s childhood in Chicago, a gridlike city, left her struggling to navigate her new home, London.Lyndon French for The New York TimesTo find out, the researchers studied game data from roughly 400,000 players from 38 countries. The effect was clear: People who reported growing up outside cities showed better navigation skills than those from within cities, even when the scientists adjusted for age, gender and education levels.The only situation in which people accustomed to more predictably arranged cities did better was on simpler levels of the video game.Players of varying nationalities performed differently. Urbanites from some places, like Spain, came very close to matching the navigation skills of their rural counterparts. In other nations, like the United States, people raised in cities were at a huge disadvantage.One explanation, the researchers suggested, was that in countries whose biggest cities were complex patchworks, like Spain, chaotic street layouts had sharpened navigation skills. By contrast, nations known for more predictable urban designs, like the United States, put people from outside cities at a bigger advantage.“If you grew up in a city like Chicago or Buenos Aires or Montreal — cities that are very grid-like — you don’t train as much your navigation skills as if you grew up in a more complex city, like London or Paris, where the streets are much more convoluted,” said Antoine Coutrot, a scientist at the French National Center for Scientific Research and another lead author of the paper.To address concerns that people from outside cities were only succeeding because the video game was set in nature, the study’s authors wrote that they replicated the findings in a smaller group of participants recruited to play a different game: “City Hero Quest,” with the same goals but a car in place of a boat.For that experiment, the researchers asked more detailed background questions, including what environment the participants currently lived in. As a result, they were able to learn that people’s present-day surroundings did not significantly affect their performance on the video games.“It really tells you that when your brain is developing, this is the key period,” Dr. Coutrot said. “It’s a bit like when you want to learn a new language.”The study speculated that more complex environments might help new neurons form in the hippocampus, a brain structure important in memory. The authors, though, emphasized that people still were able to develop navigation skills later in life.Some of the authors also noted that street layout was not the only factor making a city harder or easier to navigate. Visible landmarks can be important but are harder to quantify for research purposes than a street network.London’s finanical district and Canary Wharf, a confusing jumble.Hannah Mckay/ReutersThe sea creature game also steered clear of specific questions about people’s locations, professions or how they got around, part of an effort to assuage privacy concerns and keep the science from intruding on the gaming.That hid potentially relevant elements of someone’s upbringing from the research team, even as some commentators remained skeptical of the project on privacy grounds. Among the unknowns was how the Global Positioning System had changed people’s navigational experiences, though Dr. Spiers noted that younger participants produced results similar to those of older people.Outside scientists said that the range and number of participants were far greater than usual.“Lots of different nations are represented, and lots of different types of geographical landscapes are represented,” said Amber Watts, an associate professor of psychology at the University of Kansas who has studied neighborhood layout and cognition but was not involved in the study.Whether the cognitive benefits of more unpredictable city designs were worth the cost of making places more complicated to navigate — including for people already struggling with impairments — was less clear.“Does this mean we should design environments that should be more cognitively challenging?” Dr. Watts said. “If I went to an urban planner and said make it as confusing as possible to get around a city, that’s probably not going to sell well.”Paolo Santi, a research scientist at the Massachusetts Institute of Technology’s Senseable City Lab who was not part of the “Sea Hero Quest” team, said that the results called to mind how he would give directions to tourists in the Italian cities where he grew up.If directions in Manhattan were sometimes as simple as down and over a few blocks, directions in Italian cities had to be more forgiving of grid-minded tourists.“Rather than telling you something you’ll forget, I say to just remember the first part, and when you get there, there are plenty of people to ask again,” he said.Of a place like New York, he said, “On the one hand, you can say the city’s designed well because it’s simplified for the main task, which is getting around. On the other hand, if we don’t challenge ourselves, in a sense we do not fully exploit the potential of our brains.”

Given the surge of Omicron cases in Asia, the new research lends urgency to vaccination campaigns in China and Hong Kong.Two doses of China’s Sinovac vaccine offered older people only a moderately high level of protection against severe disease and death from Covid-19, but a third dose significantly bolstered their defenses, according to a new study by scientists in Hong Kong.The study, based on patients infected during the current devastating Omicron wave in Hong Kong, serves as a cautionary note for mainland China, where Sinovac is a pillar of the country’s vaccination program. Many older people there have yet to receive booster shots.For people 60 and older, two Sinovac doses were 72 percent effective against severe or fatal Covid-19 and 77 percent effective against Covid-related death, the study found. Those levels of protection were lower than those provided by two Pfizer-BioNTech doses. The same study found they were 90 percent effective against severe or fatal Covid and 92 percent effective against death among Hong Kong residents of the same age group.A Sinovac booster shot helped considerably, proving to be 98 percent effective against severe or fatal Covid among people at least 60 years old, the study found.Yanzhong Huang, a global health expert at the Council on Foreign Relations, said that the results highlighted the urgency for mainland China to accelerate its lagging booster campaign. “There’s a lot of work for the government to do to make sure this segment of the population receives the booster shots,” he said.The study’s authors, who are scientists at the University of Hong Kong, noted that the city’s booster program began just recently, making it difficult to determine how long protection from a third dose would last.Because people with underlying health conditions in Hong Kong were more likely to resist getting vaccinated, they said, it was also possible that those who chose to be vaccinated or boosted were healthier in the first place, inflating estimates of how protective the vaccines initially were.Sinovac, a private Chinese company that makes the vaccine, is one of two manufacturers of Covid shots available in China. Vaccines using mRNA technology, like those made by Pfizer-BioNTech and Moderna, are not available there.The new study highlights the potential consequences for China, which has relied heavily on Sinovac and is battling its biggest Covid outbreak in two years. More than 87 percent of China’s population has been vaccinated. But just over half of people 80 and older have had two shots, and less than 20 percent of people in that age group have received a booster, Zeng Yixin, a vice minister of the National Health Commission, said recently.The new study from Hong Kong received funding from the Chinese Center for Disease Control and Prevention as part of what one of the study’s co-authors described this week as an effort to understand the comparative effectiveness of vaccines. It was posted online as a preprint, but has not yet been vetted by peer scientists for publication in a scientific journal.Sinovac’s vaccine performed similarly to Pfizer’s among younger people, even without a booster dose, the study found. In people younger than 60, two Sinovac doses were roughly 92 percent effective against severe or fatal Covid, whereas two Pfizer doses were about 95 percent effective.Coronavirus Vaccine TrackerA look at all the vaccines that have reached trials in humans.Neither vaccine provided very much protection against mild or moderate Covid, though Pfizer’s offered more than Sinovac’s and a booster dose considerably lifted levels of protection. During the latest wave, people in Hong Kong have largely been infected by the subvariant of Omicron known as BA.2. Like other versions of Omicron, BA.2 has infected many vaccinated people.The Hong Kong wave is killing people at a rate exceeding that of almost any country since the coronavirus emerged — a result, in large part, of low vaccination rates among older residents. Almost 90 percent of people who died during the latest wave were not fully immunized, suggesting that getting shots to the most vulnerable is more important than the particular brand of vaccine.Like Hong Kong, mainland China had largely succeeded in tamping down transmission of the virus before Omicron, leaving its population with very little immunity from previous infections.Beyond China, Sinovac vaccines have also been critical in protecting people against severe Covid, especially in poorer countries. The vaccine is being used in 49 countries, including in South America and Africa.But concerns about the protection it offered had already prompted the World Health Organization to recommend in October that recipients 60 and older get a third dose.Dr. Andrew Morris, an infectious disease specialist at Sinai Health and University Health Network in Toronto, who was not involved with the Hong Kong study, said that the results fit with lab studies suggesting that Sinovac generated lower levels of neutralizing antibodies than mRNA vaccines, like Pfizer’s.“I think what we’ll see is in countries that have relied heavily on Sinovac, if they don’t have boosting — especially with an mRNA booster, or even with Sinovac — they’re probably going to struggle with high rates of infection with this latest BA.2 wave,” he said.Dr. Morris said that the results in Hong Kong, like those from other vaccine studies, were also highly dependent on how long it had been since people were administered the shots. Protection tends to weaken over time.The results from the latest study about the effectiveness of third Sinovac doses might be taken as an encouraging sign by Chinese leaders that Chinese-made vaccines could remain the focus of their immunization campaign, said Dr. Huang, of the Council on Foreign Relations.“Now, for the Chinese leaders, they don’t need to face a strong pressure to approve BioNTech’s vaccine,” he said.

As the death toll from Covid-19 surges in Hong Kong, some scientists said that, in the era of Omicron, vaccinating as many older people as possible should be a top priority.Hong Kong, which was once one of the world’s most formidable redoubts of “zero Covid,” has offered scientists a case study about the threat Omicron poses in an entirely different setting: a dense city where people were not only largely untouched by previous infections, but also where the oldest and most vulnerable residents were largely unvaccinated.Several critical lessons emerged, health experts said.With two Omicron subvariants circulating, including the highly contagious BA.2 subvariant, vaccinating a broad swath of the population remained important, scientists said. But inoculating as many older people as possible had become far and away the most critical thing to do.That message, they said, was most pressing for China, where vaccinations in older age groups also appear to be lagging and there is little immunity from earlier infections.But it was relevant again in the United States, too, where subpar vaccination and booster rates among older people have left scientists concerned about a potential surge of BA.2 cases. Partly because so many more Americans have been infected and killed by the coronavirus during earlier waves, scientists do not expect the United States to face as serious a situation in the coming months as Hong Kong.Hong Kong, which along with mainland China had been among the last holdouts of a strategy of tight restrictions and border controls to eradicate the virus, was left vulnerable by how few of its residents had any immunity from prior infections: Before the Omicron surge, scientists estimated that only 1 percent of Hong Kong’s population had contracted the virus.Less than one-quarter of people aged 80 and over in Hong Kong had been given two doses of a vaccine before Omicron surged, compared with more than 90 percent of people in Singapore and New Zealand.The city has now vaccinated 39 percent of residents aged 80 and above, despite having inoculated almost two-thirds of 12- to 19-year-olds.

Covid has surged in a number of Asian countries that had once held the virus at bay. Vaccination levels have largely determined how deadly those waves would be.The first time the Omicron variant breached Hong Kong’s coronavirus defenses, in late 2021, the city stamped it out, cementing its status as one of the world’s most formidable redoubts of “zero Covid.”But a few weeks later, Omicron came to the metropolis again, this time causing an outbreak among cleaners at a public-housing estate that spiraled out of control. The conflagration of resulting cases is now killing people at a rate exceeding that of almost any country since the coronavirus emerged.Over the entire pandemic, Hong Kong’s death toll per capita, once far lower than those of Western nations, is no longer exceptional. A month ago, Americans had died from Covid at 90 times the rate of people in Hong Kong. By Monday, the cumulative American toll was three and a half times as high.As the United States braces for its own, less punishing rise in cases, and mainland China battles its biggest outbreak in two years, scientists have looked to Hong Kong for clues about the threat Omicron poses in an entirely different setting: a dense city where people were not only largely untouched by previous infections, but whose oldest and most vulnerable residents were also largely unvaccinated.Several critical lessons emerged, health experts said.In the era of Omicron and its even more infectious subvariant, BA.2, vaccinating a broad swath of the population remained important, scientists said. But inoculating as many older people as possible had become far and away the top priority.That message, they said, was most pressing for China, where vaccinations in older age groups also appear to be lagging and there is little immunity from earlier infections.But it was relevant again in the United States, too, where subpar vaccination and booster rates among older people have left scientists concerned about a potential surge of BA.2 cases. Partly because so many more Americans have been infected and killed by the coronavirus during earlier waves, scientists do not expect the United States to face as serious a situation in the coming months as Hong Kong.Hong Kong’s dreadful outbreak also signals the perils of trying to eliminate the virus without a plan for what would come next, health experts said. Omicron’s high transmissibility, they said, made outbreaks almost inevitable.Hong Kong, which along with mainland China had been among the last holdouts of a strategy of tight restrictions and border controls to eradicate the virus, was left vulnerable by how few of its residents had any immunity from prior infections: Before the Omicron surge, scientists estimated that only 1 percent of Hong Kong’s population had contracted the virus.A temporary isolation site to house patients with Covid-19 on Tsing Yi Island in Hong Kong.Billy H.C. Kwok for The New York TimesThose low levels of immunity can leave places vulnerable to waves of cases, as more contagious variants sneak in or restrictions are lifted. But governments can still prepare for those waves, said Dr. Gabriel Leung, the dean of medicine at the University of Hong Kong.Less than one-quarter of people aged 80 and over in Hong Kong had been given two doses of a vaccine before Omicron surged, compared with more than 90 percent of people in Singapore and New Zealand.Because of the number of unvaccinated older people in China, scientists said, it might also have some difficulty lifting “zero Covid” restrictions. More than 87 percent of China’s population have been vaccinated. But just over half of people 80 and older have had two shots, and less than 20 percent of people in that age group have received a booster, Zeng Yixin, a vice minister of the National Health Commission, said on Friday.“I don’t think it’s quite ready for the transition,” Dr. Leung said.A number of Asian and Pacific countries had largely kept the virus at bay for two years, only to face Omicron outbreaks because the virus was so contagious and their populations had avoided earlier infections. But high vaccination rates, including among older people, have helped many of those countries avoid more devastating surges.In South Korea, for example, where 87 percent of people are vaccinated and 63 percent have booster shots, the cumulative death toll per capita is one-tenth of America’s, even though South Korea has recorded more than three-quarters as many cases as the United States over the entire pandemic.Health experts said that Hong Kong’s difficulties vaccinating older people resulted from a combination of complacency, given the city’s earlier success in containing the virus, and unfounded fears that older people and those in poor health faced particular risks from vaccines.The city has now vaccinated 39 percent of residents aged 80 and above, despite having inoculated almost two-thirds of 12- to 19-year-olds.Many people in Hong Kong have been given the Chinese vaccine Sinovac, which appears to offer relatively little protection from Omicron infections but a better defense against severe disease. Scientists noted that almost 90 percent of people who died during the latest wave were not fully vaccinated, suggesting that getting shots to the most vulnerable is more important than the particular brand.“The problem in Hong Kong is, we haven’t succeeded in vaccinating our most vulnerable population — the elderly, especially those staying in elderly-care homes,” said Dr. Siddharth Sridhar, a clinical virologist at the University of Hong Kong. “And as a result, we are in a very bad situation.”The United States has vaccinated many more of its older residents than Hong Kong but fewer than Western Europe and has seen a high death rate. And as immunity from early vaccinations wanes and booster shots become critical for shoring up protection against Omicron among older people, the United States finds itself exposed on that count, too. About 41 percent of people 65 and over have not received a booster shot.Unlike other parts of Asia that had gradually lifted restrictions in recent months, Hong Kong was not ready for its defenses to fail, scientists said.“From the government’s point of view, there was such a strong fixation on ‘zero Covid’ that as long as that worked, vaccination was not necessarily the priority,” said Ben Cowling, a professor of epidemiology at the University of Hong Kong.Many older residents and their families adopted the same view, public health experts said. If Hong Kong’s rigid social-distancing measures and careful border controls were going to keep the virus out anyway, the conventional thinking went, was getting a vaccine worth the trouble?“If you’re telling people that the disease is never going to get in, then there’s less of an incentive to go and get vaccinated,” said Dr. David Owens, a family doctor in Hong Kong. “To an extent, the messaging around elimination confounded the need to vaccinate.”People waiting to be tested for the coronavirus in Tsing Yi, which was placed under lockdown this month.Billy H.C. Kwok for The New York TimesDr. Cowling, of the University of Hong Kong, said that his city could have responded in one of two ways to signs that cases would surge: either double down on “zero Covid” through measures like building better quarantine facilities for overseas arrivals, or acknowledge that outbreaks are unavoidable and raise vaccination rates.“Zero Covid is a really good strategy if you can stay at zero,” Dr. Cowling said. “But as we found in Hong Kong, it doesn’t last forever.”Hong Kong eventually took steps to persuade older people to become vaccinated, once earlier inducements like vaccine passes proved ineffective. In January, the government announced that it would ban unvaccinated people from restaurants that serve dim sum, which are popular among older residents. But it was too late.With cases and deaths now declining, Hong Kong announced on Monday that it would lift certain restrictions.Singapore began abandoning “zero Covid” policies in the summer. Dr. Ooi Eng Eong, an infectious disease expert at Duke-National University of Singapore Medical School, said that it took a wave of the Delta variant to raise vaccination rates and disabuse people of the notion that they did not need protection.Now, cases in Singapore have surged, but deaths are relatively low.“It’s so much more transmissible that I think wearing a face mask and all — that helps but not to the extent that it has impacted the epidemiology,” Dr. Ooi said of Omicron. “The trends are really driven by vaccination.”Still, even after five or six waves of the pandemic, the reasons that some countries have succeeded while others have suffered remain unclear.Japan, for example, has tamped down on cases throughout the pandemic without resorting to full-fledged lockdowns, scientists said.The country benefited from its government sharing sound publichealth advice early in the pandemic. As much as residents tired of precautions, they largely took the advice seriously, said Taro Yamamoto, a professor at the Institute of Tropical Medicine at Nagasaki University.Roughly 80 percent of people in Japan have had their initial vaccine series. But even though the country is lagging in administering booster doses and had a surge of Omicron infections, death rates during Omicron have remained considerably lower than in nearby South Korea.“Partly it’s a mystery,” Professor Yamamoto said. “We cannot explain it all.”