Mixing and Matching Covid VaccinesApoorva MandavilliReporting on the coronavirusWhen am I considered fully vaccinated?People who have received two mRNA vaccine doses or a single J. & J. dose should still consider themselves fully vaccinated.Research indicates that, with the exception of adults over 65, the vaccines remain highly protective against severe illness and death in the vast majority of people.

As the latest wave of the virus ebbs in most parts of the United States, advisers to the Centers for Disease Control and Prevention will meet on Thursday to evaluate which Americans need extra doses of the coronavirus vaccines made by Moderna and Johnson & Johnson.They will also discuss the so-called mix-and-match strategy — whether people fully immunized with one company’s vaccine should be allowed to switch to a different one for their booster shot.On Wednesday, the Food and Drug Administration authorized booster shots for millions of people who received the Moderna and Johnson & Johnson vaccines, just as it did for recipients of Pfizer-BioNTech shots last month. The F.D.A. also gave the green light for people eligible for boosters to get an extra dose of a different brand from the one they first received.But in practice who will get the shots and when depends greatly on the C.D.C.’s final guidance. Though the agency’s recommendations do not bind state and local officials, they hold great sway in the medical community.Regardless of the tenor of the discussion, the final recommendations from the C.D.C.’s panel — the Advisory Committee on Immunization Practices — are unlikely to hold surprises. The committee is expected to endorse additional doses of the vaccines for many Americans and to strive to bring the country closer to fulfilling President Biden’s promise to provide boosters to all adults.It will not happen without some misgivings, however.Some of the C.D.C.’s advisers last month voiced strong reservations about a booster of the Pfizer-BioNTech vaccine, saying the science did not support additional shots for anyone other than adults over 65. A majority voted to recommend booster shots for people with certain medical conditions that increase the risk of Covid-19.The committee did not support boosters for people whose jobs expose them to the virus — but in a highly unusual move, Dr. Rochelle P. Walensky, the C.D.C.’s director, overturned their decision.“It is going to be a difficult meeting, because I suspect that the science won’t have changed much,” said Dr. Sarah S. Long, a pediatrician and infectious diseases expert at Drexel University in Philadelphia. “I can see making it clear that we still don’t think this is the right approach, and Dr. Walensky would have to override it.”A similar dynamic played out on the expert panel advising the F.D.A. at a meeting last week. Several advisers to the F.D.A. criticized the paucity of evidence supporting a Johnson & Johnson booster and expressed discomfort at having to decide on the booster before the F.D.A. had carefully vetted the data.Still, they unanimously voted in favor of a second Johnson & Johnson shot for adults who had received that vaccine because it was clear that two doses would be more protective than one, according to one member of the panel, Dr. Paul A. Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia..css-1kpebx{margin:0 auto;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-1kpebx{font-size:1.25rem;line-height:1.4375rem;}}.css-1gtxqqv{margin-bottom:0;}.css-k59gj9{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;width:100%;}.css-1e2usoh{font-family:inherit;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;border-top:1px solid #ccc;padding:10px 0px 10px 0px;background-color:#fff;}.css-1jz6h6z{font-family:inherit;font-weight:bold;font-size:1rem;line-height:1.5rem;text-align:left;}.css-1t412wb{box-sizing:border-box;margin:8px 15px 0px 15px;cursor:pointer;}.css-hhzar2{-webkit-transition:-webkit-transform ease 0.5s;-webkit-transition:transform ease 0.5s;transition:transform ease 0.5s;}.css-t54hv4{-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-1r2j9qz{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-e1ipqs{font-size:1rem;line-height:1.5rem;padding:0px 30px 0px 0px;}.css-e1ipqs a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;}.css-e1ipqs a:hover{-webkit-text-decoration:none;text-decoration:none;}.css-1o76pdf{visibility:show;height:100%;padding-bottom:20px;}.css-1sw9s96{visibility:hidden;height:0px;}.css-1in8jot{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;font-family:’nyt-franklin’,arial,helvetica,sans-serif;text-align:left;}@media (min-width:740px){.css-1in8jot{padding:20px;width:100%;}}.css-1in8jot:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1in8jot{border:none;padding:10px 0 0;border-top:2px solid #121212;}What to Know About Covid-19 Booster ShotsThe F.D.A. has authorized booster shots for millions of recipients of the Pfizer-BioNTech, Moderna and Johnson & Johnson vaccines. Pfizer and Moderna recipients who are eligible for a booster include people 65 and older, and younger adults at high risk of severe Covid-19 because of medical conditions or where they work. Eligible Pfizer and Moderna recipients can get a booster at least six months after their second dose. All Johnson & Johnson recipients will be eligible for a second shot at least two months after the first.Yes. The F.D.A. has updated its authorizations to allow medical providers to boost people with a different vaccine than the one they initially received, a strategy known as “mix and match.” Whether you received Moderna, Johnson & Johnson or Pfizer-BioNTech, you may receive a booster of any other vaccine. Regulators have not recommended any one vaccine over another as a booster. They have also remained silent on whether it is preferable to stick with the same vaccine when possible.The C.D.C. has said the conditions that qualify a person for a booster shot include: hypertension and heart disease; diabetes or obesity; cancer or blood disorders; weakened immune system; chronic lung, kidney or liver disease; dementia and certain disabilities. Pregnant women and current and former smokers are also eligible.The F.D.A. authorized boosters for workers whose jobs put them at high risk of exposure to potentially infectious people. The C.D.C. says that group includes: emergency medical workers; education workers; food and agriculture workers; manufacturing workers; corrections workers; U.S. Postal Service workers; public transit workers; grocery store workers.Yes. The C.D.C. says the Covid vaccine may be administered without regard to the timing of other vaccines, and many pharmacy sites are allowing people to schedule a flu shot at the same time as a booster dose.“There was a value to having that as a one-dose vaccine when it first came out,” Dr. Offit said. “But I think it was always on the road to being a two-dose vaccine.”He and others suggested that people who got the Johnson & Johnson vaccine might be better served by getting a second dose of the Pfizer-BioNTech or Moderna vaccines. The F.D.A. on Wednesday updated its authorization of all three vaccines, which will allow the C.D.C.’s advisory committee to recommend this mix-and-match strategy when it meets on Thursday.At last week’s meeting, the F.D.A. advisers were shown evidence indicating that switching between two vaccine types might produce a richer immune response than boosting with the same vaccine.San Francisco has offered Pfizer-BioNTech or Moderna shots to Johnson & Johnson recipients since August, and many Americans who initially received the Johnson & Johnson vaccine have sought out booster doses of those vaccines on their own before federal agencies had endorsed them.But the F.D.A. advisers could not vote to recommend that people who initially got the Johnson & Johnson vaccine follow it up with one of the mRNA vaccines because the F.D.A. did not give them that option.“The formal vote was on the J.-&-J. booster following the J.-&-J. vaccine,” said Dr. Stanley Perlman, an immunologist and coronavirus expert at the University of Iowa. But “everybody knew that the data would show that the heterologous boost would be better.” (Heterologous refers to a shot of a different vaccine.)

The finding upends conventional wisdom regarding coughing, long thought to be the main route of transmission.Upending centuries of medical dogma, a team of South African researchers has found that breathing may be a bigger contributor to the spread of tuberculosis than coughing, the signature symptom.As much as 90 percent of TB bacteria released from an infected person may be carried in tiny droplets, called aerosols, that are expelled when a person exhales deeply, the researchers estimated. The findings were presented on Tuesday at a scientific conference held online.The report echoes an important finding of the Covid pandemic: The coronavirus, too, spreads in aerosols carried aloft, particularly in indoor spaces — a route of transmission that was widely underappreciated as the pandemic began to unfold.TB is caused by a bacterium called Mycobacterium tuberculosis, which usually attacks the lungs. It is the world’s deadliest infectious disease after Covid-19, claiming more than 1.5 million lives last year — the first increase in a decade, according to a report published last week by the World Health Organization.As the Covid pandemic disrupted access to health care and supply chains around the globe, 5.8 million people were diagnosed with TB in 2020. But the W.H.O. estimates that about 10 million people were infected. Many may unwittingly be spreading the disease to others.“Our model would suggest that, actually, aerosol generation and TB generation can happen independent of symptoms,” said Ryan Dinkele, a graduate student at the University of Cape Town who presented the results.The finding helps explain why tightly packed indoor spaces, like prisons, often are breeding grounds for TB, as they are for Covid. And the research suggests that some of the methods used to limit coronavirus transmission — masks, open windows or doors, and being outdoors as much as possible — are important in curtailing TB.“Those of us who are TB people look at Covid and say, ‘Wow, it’s just a sped up version of TB,’” said Dr. Robert Horsburgh, an epidemiologist at Boston University who was not involved in the work.Researchers previously believed that most TB transmission occurred when an infected person coughed, spraying droplets containing the bacteria onto others. Some bacteria were thought to be released when a person breathed, but much less than by coughing.The new finding does not change that understanding: A single cough can expel more bacteria than a single breath. But if an infected person breathes 22,000 times per day while coughing up to 500 times, then coughing accounts for as little as 7 percent of the total bacteria emitted by an infected patient, Mr. Dinkele said.On a crowded bus or at school or work, where people sit in confined spaces for hours, “just simply breathing would contribute more infectious aerosols than coughing would,” Mr. Dinkele said.In so-called tidal breathing, inhalation opens up tiny air sacs in the lungs, and then exhalation carries the bacteria from the lungs via aerosols. Because of their smaller size, aerosols released by tidal breathing can stay afloat in the air for longer and travel further than droplets emitted by a cough.As with Covid, some TB patients spread the disease to many people — and may release a lot of bacteria — while others infect few people around them. But even if 90 percent of the bacteria expelled by an infected person were carried in aerosols, this route of transmission wouldn’t necessarily account for 90 percent of new cases, cautioned Dr. Silvia S. Chiang, who studies the disease at Brown University.Still, experts said, the finding does suggest that physicians shouldn’t wait for TB patients to arrive at clinics with a severe cough and weight loss, the telltale symptoms.“We just need to screen the entire population, just like you would do if you’re looking for a lot of Covid,” Dr. Horsburgh said.The discovery came about in large part because of technology developed by Dr. Robin Wood, an emeritus professor of medicine at the University of Cape Town in South Africa. The apparatus can collect aerosols from infected people and identify bacteria within them.Diagnosis and treatment of TB has changed very little in decades. “It was time to start using modern, state-of-the-art technology to approach an old disease,” Dr. Wood said. With some tweaks, the system could also be used to study other diseases, including Covid, he added.TB has been around for millenniums, and its cause known for nearly 150 years.“And yet, we’re still finding out new things about such a fundamental part of its biology,” Mr. Dinkele said. “It’s humbling to realize that we need to be so careful when it comes to a dogmatic approach in a field.”

Deaths from tuberculosis, the world’s biggest infectious disease killer until the Covid-19 pandemic arrived, have increased for the first time in more than a decade, totaling more than 1.5 million people in 2020. That trend is expected to worsen in 2021 and 2022, according to a report released on Thursday by the World Health Organization.The report confirmed the warnings from the W.H.O. and other global health organizations that the Covid-19 pandemic would reverse years of progress against other infectious diseases, including TB, H.I.V. and malaria.“This is alarming news that must serve as a global wake-up call to the urgent need for investments and innovation to close the gaps in diagnosis, treatment and care for the millions of people affected by this ancient but preventable and treatable disease,” Dr. Tedros Adhanom Ghebreyesus, the W.H.O.’s director general, said in a statement.Reported diagnoses of TB also dropped sharply, to 5.8 million cases in 2020 from 7.1 million in 2019, suggesting that many more cases than before are going undiagnosed and untreated — a trend that is likely to have a long-term effect on TB deaths. And only 2.8 million people were given preventive treatment for TB in 2020, a 21 percent decrease from 2019.In many poor countries, health care workers, funds and testing equipment that would normally be dedicated to TB were redirected to cope with Covid-19, according to the W.H.O. report. Lockdowns and disruptions in supply chains also interrupted access to treatment and care.At the same time, global funding for TB has fallen to $5.3 billion from $5.8 billion, less than half of what’s needed, according to the W.H.O. report.There were some glimmers of good news amid the sobering statistics. In the Russian Federation, the incidence of TB fell by 6 percent a year between 2010 and 2020, and the W.H.O. European Region overall exceeded the 2020 goal with a decrease of 25 percent.

So-called natural immunity varies from patient to patient, scientists say. Immunization is still the best choice after recovering from the disease. When Jonathan Isaac, a prominent basketball player for the Orlando Magic, explained why he chose not to be vaccinated against the coronavirus, he tapped into a dispute that has been simmering for months: Do people who have had Covid-19, as Mr. Isaac said he has, really need the vaccine?That question has thrust tortuous immunological concepts into a national debate on vaccine mandates, with politicians, athletes, law professors and psychiatrists weighing in on the relative strength of so-called natural immunity versus the protection afforded by vaccines.But the answer, like nearly everything about the virus, is complicated.While many people who have recovered from Covid-19 may emerge relatively unscathed from a second encounter with the virus, the strength and durability of their immunity depends on their age, health status and severity of initial infection.“That’s the thing with natural infection — you can be on the very low end of that or very high end, depending on what kind of disease you developed,” said Akiko Iwasaki, an immunologist at Yale University.Those with powerful natural immunity may be protected from reinfection for up to a year. But even they should not skip the vaccine, experts said. For starters, boosting their immunity with a vaccine is likely to give them long-lasting protection against all the variants.“If you’ve gotten the infection and then you’ve been vaccinated, you’ve got superpowers,” said Jennifer Gommerman, an immunologist at the University of Toronto.Without that boost, antibodies from an infection will wane, leaving Covid-recovered people vulnerable to reinfection and mild illness with variants — and perhaps liable to spread the virus to others.This is the same argument for giving boosters to people who are fully vaccinated, said Michel Nussenzweig, an immunologist at Rockefeller University in New York. “After a certain period of time, you’re either going to get boosted or you’re going to get infected,” he said.How immunity from infection and from vaccination compare is difficult to parse. Dozens of studies have delved into the debate, and have drawn contradictory conclusions.Some consistent patterns have emerged: Two doses of an mRNA vaccine produce more antibodies, and more reliably, than an infection with the coronavirus does. But the antibodies from prior infection are more diverse, capable of fending off a wider range of variants, than those produced by vaccines.Studies touting the durability and strength of natural immunity are hobbled by one crucial flaw. They are, by definition, assessing the responses only of people who survived Covid-19. The road to natural immunity is perilous and uncertain, Dr. Nussenzweig said. A 78-year-old man received a booster shot in the Bronx last month.Dave Sanders for The New York TimesOnly 85 percent to 90 percent of people who test positive for the virus and recover have detectable antibodies to begin with. The strength and durability of the response is variable.For example, while the immunity gained from vaccines and infection is comparable among younger people, two doses of the mRNA vaccines protected adults older than 65 better than a prior infection did.Research published by Dr. Iwasaki’s team in May showed a stepwise increase in the level of antibodies with rising severity of infection. About 43 percent of recovered people had no detectable neutralizing antibodies — the kind needed to prevent reinfection — according to one study. The antibodies drop to undetectable levels after about two months in about 30 percent of people who recover.Other researchers may find different results depending on the severity of illness in the participants, said Fikadu Tafesse, an immunologist at Oregon Health & Science University.“If your cohort is just only hospitalized individuals, I think the chance of having a detectable antibody is higher,” Dr. Tafesse said.In terms of the quality of the antibodies, it makes sense that invasion by a live virus would produce a broader immune response than would injecting the single protein encoded in the vaccines, he and others said.The virus would stimulate defenses in the nose and throat — exactly where they are needed to prevent a second infection — while the vaccines produce antibodies mainly in the blood.“That will give you an edge in terms of resisting a subsequent infection,” Dr. Gommerman said.Fragments of the virus may also persist in the body for weeks after infection, which gives the immune system more time to learn to fight it, while the proteins carried by the vaccine quickly exit the body.Several studies have now shown that reinfections, at least with the earlier versions of the virus, are rare.At the Cleveland Clinic, none of 1,359 health care workers who remained unvaccinated after having Covid-19 tested positive for the virus over many months, noted Dr. Nabin Shrestha, an infectious disease physician at the clinic. But the findings must be interpreted with caution, he acknowledged. The clinic tested only people who were visibly ill, and may have missed reinfections that did not produce symptoms. The participants were 39 years old on average, so the results may not apply to older adults, who would be more likely to become infected again.Most studies have also tracked people for only about a year, Dr. Shrestha noted. “The important question is, how long does it protect, because we’re not under any illusions that this will be a lifelong protection,” he said.It’s also unclear how well immunity after infection protects against the newer variants. Most studies ended before the Delta variant became dominant, and more recent research is patchy..css-1kpebx{margin:0 auto;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-1kpebx{font-size:1.25rem;line-height:1.4375rem;}}.css-1gtxqqv{margin-bottom:0;}.css-k59gj9{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;width:100%;}.css-1e2usoh{font-family:inherit;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;border-top:1px solid #ccc;padding:10px 0px 10px 0px;background-color:#fff;}.css-1jz6h6z{font-family:inherit;font-weight:bold;font-size:1rem;line-height:1.5rem;text-align:left;}.css-1t412wb{box-sizing:border-box;margin:8px 15px 0px 15px;cursor:pointer;}.css-hhzar2{-webkit-transition:-webkit-transform ease 0.5s;-webkit-transition:transform ease 0.5s;transition:transform ease 0.5s;}.css-t54hv4{-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-1r2j9qz{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-e1ipqs{font-size:1rem;line-height:1.5rem;padding:0px 30px 0px 0px;}.css-e1ipqs a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;}.css-e1ipqs a:hover{-webkit-text-decoration:none;text-decoration:none;}.css-1o76pdf{visibility:show;height:100%;padding-bottom:20px;}.css-1sw9s96{visibility:hidden;height:0px;}.css-1in8jot{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;font-family:’nyt-franklin’,arial,helvetica,sans-serif;text-align:left;}@media (min-width:740px){.css-1in8jot{padding:20px;width:100%;}}.css-1in8jot:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1in8jot{border:none;padding:10px 0 0;border-top:2px solid #121212;}What to Know About Covid-19 Booster ShotsThe F.D.A. authorized booster shots for a select group of people who received their second doses of the Pfizer-BioNTech vaccine at least six months ago. That group includes: Pfizer recipients who are 65 or older or who live in long-term care facilities; adults who are at high risk of severe Covid-19 because of an underlying medical condition; health care workers and others whose jobs put them at risk. People with weakened immune systems are eligible for a third dose of either Pfizer or Moderna four weeks after the second shot.Regulators have not authorized booster shots for recipients of the Moderna and Johnson & Johnson vaccines yet, but an F.D.A. panel is scheduled to meet to weigh booster shots for adult recipients of the Moderna and Johnson & Johnson vaccines.The C.D.C. has said the conditions that qualify a person for a booster shot include: hypertension and heart disease; diabetes or obesity; cancer or blood disorders; weakened immune system; chronic lung, kidney or liver disease; dementia and certain disabilities. Pregnant women and current and former smokers are also eligible.The F.D.A. authorized boosters for workers whose jobs put them at high risk of exposure to potentially infectious people. The C.D.C. says that group includes: emergency medical workers; education workers; food and agriculture workers; manufacturing workers; corrections workers; U.S. Postal Service workers; public transit workers; grocery store workers.It is not recommended. For now, Pfizer vaccine recipients are advised to get a Pfizer booster shot, and Moderna and Johnson & Johnson recipients should wait until booster doses from those manufacturers are approved.Yes. The C.D.C. says the Covid vaccine may be administered without regard to the timing of other vaccines, and many pharmacy sites are allowing people to schedule a flu shot at the same time as a booster dose.The most widely cited study in favor of natural immunity’s potency against the Delta variant comes from Israel. A patient waited for a third dose of coronavirus vaccine at health clinic in Jerusalem in August.Ammar Awad/ReutersBreakthrough infections after vaccination were 13-fold more likely than reinfections in unvaccinated people, and symptomatic breakthrough infections 27-fold more likely than symptomatic reinfections, the study found.But experts cautioned against inferring from the results that natural immunity is superior to the protection from vaccines. The vaccinated group included many more people with conditions that would weaken their immune response, and they would be expected to have more breakthrough infections, noted Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.The study also did not account for people whose immune defenses may have been strengthened by a second exposure to the virus.For those lucky enough to have recovered from Covid-19, vaccination is still the ideal choice, experts said. It provides a massive boost in antibody levels and a near-impenetrable immune shield — perhaps even against future variants.“They are like rock stars on all the variants,” said Dr. Duane Wesemann, an immunologist at Harvard Medical School.Colorful graphs from Dr. Wesemann’s recent paper have been helpful for convincing Covid-recovered patients of the stark advantage even a single dose would offer them, he said.Regardless of the evolving understanding of natural immunity, on one point there is near-universal agreement among scientists. For people who were never infected, vaccines are much safer, and far less a gamble, than Covid-19.Many people who argue against vaccines cite the low mortality rates from Covid-19 among young people. But even seemingly mild cases of Covid-19 can result in long-term damage to the heart, kidneys and brain, or leave people feeling exhausted and unwell for weeks to months, Dr. Iwasaki said.“No one should try to acquire immunity through natural infection,” she said. “It’s just too dangerous.”

Myocarditis, a rare side effect, occurs mostly after the second dose. So in some countries, officials are trying out single doses for children.Even as parents in the United States wrestle with difficult questions over vaccinating their children against the coronavirus, families in other countries have been offered a novel option: giving children just one dose of the vaccine.Officials in Hong Kong as well as in Britain, Norway and other countries have recommended a single dose of the Pfizer-BioNTech vaccine for children ages 12 and older — providing partial protection from the virus, but without the potential harms occasionally observed after two doses. On Wednesday, Sweden and Denmark joined the ranks, announcing that adolescents should get only one jab of the Moderna vaccine.Health officials in those countries are particularly worried about increasing data suggesting that myocarditis, an inflammation of the heart, may be more common among adolescents and young adults after vaccination than had been thought.The risk remains very small, and significant only after the second dose of an mRNA vaccine. But the numbers have changed the risk-benefit calculus in countries where new infections are mostly lower than in the United States.Advisers to the Centers for Disease Control and Prevention reviewed data on myocarditis in June, and unanimously voted to recommend the vaccine for children ages 12 and older, saying the benefits far outweighed the risk.Agency research has estimated that for every million vaccinated boys ages 12 to 17 in the United States, the shots might cause a maximum of 70 myocarditis cases, but they would prevent 5,700 infections, 215 hospitalizations and two deaths. Studies have also shown that the risk of heart problems after Covid-19 is much higher than after vaccination.Myocarditis was among the concerns that led the Food and Drug Administration to ask vaccine makers this summer to increase the number of children in clinical trials. The issue is likely to be the focus of intense discussion when agency advisers meet next week to review the evidence for vaccinations of children ages 5 to 11.The latest analysis, which was published on Wednesday in The New England Journal of Medicine, found that the incidence of myocarditis after vaccination in Israel was highest among males aged 16 to 29. About 11 of every 100,000 males in that age group developed the condition a few days after being vaccinated, a rate higher than most earlier estimates. (The risk was negligible in females of any age.)Of the 54 cases identified in the study, one was severe enough to require ventilation. Another patient with a history of heart disease died of an unknown cause soon after discharge from the hospital.Of the 14 patients in the new study who showed heart abnormalities when they were admitted to the hospital, 10 still had some signs of problems when they were discharged. But when the patients were examined again a few weeks later, all five of those for whom results were available appeared to have fully recovered.A second study, which was also published in the journal, found that boys between 16 and 19 years of age had the highest incidence of myocarditis after the second dose — nine times as high when compared with unvaccinated boys of the same age during the same time.Health officials in other countries plan to revisit the one-dose strategy as more safety information becomes available, and they may choose to proceed with second shots. But the possibility of deferring the second jab has not received enough attention in the United States, said Dr. Walid Gellad, a drug safety expert at the University of Pittsburgh.“In the U.S., people have not wanted to talk about it, for unclear reasons,” Dr. Gellad said. “Parents who are hesitant may appreciate the fact that the risk of side effects is actually much lower for one dose than it is for two doses.”Serious side effects have primarily been seen in boys, so the dosing calculus should be different for boys and girls, he added.Of the 15 myocarditis cases recorded in a recent study, 13 occurred after the second dose. The chances of myocarditis after one dose were not any higher than usual for that population. Jacquelyn Martin/Associated PressIt is too early to know whether myocarditis might permanently weaken the hearts of some people after vaccination, said Dr. Jeremy Brown, an expert in respiratory diseases at University College London and a member of Britain’s vaccine advisory group.“That makes it very hard for us to make the absolute statement that it’s totally safe to give this vaccine,” Dr. Brown said. “We need some feel for what the long-term consequences of the myocarditis might be.”The urgency of fully vaccinating children with two doses has to be weighed for each country’s particular situation, experts said. In Britain, high rates of vaccination among older and high-risk adults have helped to keep hospitals mostly free of patients severely ill with Covid-19.“The chance of getting severe Covid in a healthy 12- to 15-year-old is almost negligible,” Dr. Brown said. “Against that, you have to make sure that the vaccine that you’re giving is utterly safe.”Some experts have argued that immunizing children would help to sever chains of transmission and contain the virus. But immunizing children to protect others — when there may be a risk to the recipient, however small — was indefensible, Dr. Brown said.“You don’t vaccinate a 15-year-old to prevent them infecting other adults — that’s not morally, ethically the right thing to do,” he said.In Hong Kong, the argument for double-dosing adolescents is even weaker than in Britain, said Benjamin Cowling, an epidemiologist at the University of Hong Kong.Hong Kong has recorded only 213 deaths and just over 12,000 cases of Covid-19 since the start of the pandemic, with fewer than 10 cases per day since April. So the risk of myocarditis, however rare, outweighs the benefit of fully vaccinating adolescents, Dr. Cowling said.Clinical trials of the vaccine in children are not large enough to detect rare side effects like myocarditis, he added. “You’d only see it when it goes to the population level, and then it’s too late.” Whether to offer second doses to children “does need a careful consideration.”Vaccinations in the Tate Modern museum in London. High rates of vaccination among older and high-risk adults have helped to keep hospitals mostly free of patients severely ill with Covid-19. Tolga Akmen/Agence France-Presse — Getty ImagesBut the United States is not in same position as other countries, noted Dr. Jeffrey Duchin, an infectious diseases physician and a nonvoting member of the C.D.C.’s advisory group on vaccines..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-1kpebx{margin:0 auto;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-1kpebx{font-size:1.25rem;line-height:1.4375rem;}}.css-1gtxqqv{margin-bottom:0;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-19zsuqr{display:block;margin-bottom:0.9375rem;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}About 2,000 Americans are dying every day, and hospitals in many parts of the country are still packed. “We’ve had a significant impact on our pediatric population,” Dr. Duchin said.Nearly 900,000 children have been hospitalized with Covid-19 since the pandemic began, and about 520 have died. Some children have developed so-called long Covid-19, in which symptoms can persist for months, and more than 4,000 have been diagnosed with a dangerous condition called multisystem inflammatory syndrome in children.“All the data that we have so far suggests the disease itself is significantly worse than the vaccine side effects,” Dr. Duchin said. Given all of that, a small risk of myocarditis is well worth taking, he said, and two doses are justified.Dr. Duchin said he also had some concerns that one dose of the vaccine might not shield children against infection or illness — at least, not for long. “I just have not seen data that would suggest that one dose would have a durable and high level of protection,” he said.All of these concerns, as well as the data on myocarditis, should inform a national conversation about the wisdom of offering one dose versus two shots to adolescents, some experts said.“There hasn’t been enough discussion about the potential harms of vaccination, because everybody is very, very sensitive about hesitancy and doesn’t want to give any fuel to anti-vaccination campaigns,” Dr. Cowling said.In the United States, in particular, many public health experts have been reluctant to voice concerns about the vaccines, Dr. Gellad said: “No one wants to introduce any doubt that kids should be vaccinated.”“But I think there are ways to talk about it that will appeal to people who are hesitant.”Kristina Rogers, a 51-year-old mother of two in Oklahoma, said she would welcome the option to give her 12-year-old daughter only one dose of the vaccine.Ms. Rogers, who is fully immunized, worries that not enough was known about the vaccines’ long-term effects in children and said she wished there were more open discussion.Ms. Rogers has diabetes and developed chronic kidney disease after a severe bout with Covid last year. She lost her brother-in-law to Covid a year ago.But the two doses of vaccine left her feeling flattened and fatigued, as well, and she worried the shots might prove too much for her children. Her children wear masks to school and wash their hands regularly, but she and her husband are not yet ready to vaccinate them.“The last thing you want to do is mess with their ticker, man — that’s what makes them go,” she said. “I would be more up for the one dose, if that was an option.”

The Alpha variant traveled more efficiently in small droplets, two new studies found. The Delta variant may have continued this evolution.Newer variants of the coronavirus like Alpha and Delta are highly contagious, infecting far more people than the original virus. Two new studies offer a possible explanation: The virus is evolving to spread more efficiently through air.The realization that the coronavirus is airborne indoors transformed efforts to contain the pandemic last year, igniting fiery debates about masks, social distancing and ventilation in public spaces.Most researchers now agree that the coronavirus is mostly transmitted through large droplets that quickly sink to the floor and through much smaller ones, called aerosols, that can float over longer distances indoors and settle directly into the lungs, where the virus is most harmful.The new studies don’t fundamentally change that view. But the findings signal the need for better masks in some situations, and indicate that the virus is changing in ways that make it more formidable.“This is not an Armageddon scenario,” said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases, who led one of the new studies. “It is like a modification of the virus to more efficient transmission, which is something I think we all kind of expected, and we now see it happening in real time.”Dr. Munster’s team showed that small aerosols traveled much longer distances than larger droplets and the Alpha variant was much more likely to cause new infections via aerosol transmission. The second study found that people infected with Alpha exhaled about 43 times more virus into tiny aerosols than those infected with older variants.The studies compared the Alpha variant with the original virus or other older variants. But the results may also explain why the Delta variant is so contagious — and why it displaced all other versions of the virus.“It really indicates that the virus is evolving to become more efficient at transmitting through the air,” said Linsey Marr, an expert in airborne viruses at Virginia Tech who was not involved in either study. “I wouldn’t be surprised if, with Delta, that factor were even higher.”The ultratransmissibility of the variants may come down to a mix of factors. It may be that lower doses of the variants are required for infection, or that the variants replicate faster, or that more of the variant virus is exhaled into aerosols — or all three.The Alpha variant proved to be twice as transmissible as the original virus, and the Delta variant has mutations that turbocharged its contagiousness even more. As the virus continues to change, newer variants may turn out to be even more transmissible, experts said.But the tools at our disposal all still work well to halt the spread. Even loosefitting cloth and surgical masks block about half of the fine aerosols containing virus, according to the study of people infected with variants, published this month in the journal Clinical Infectious Diseases.Still, at least in some crowded spaces, people may want to consider switching to more protective masks, said Don Milton, an aerosol expert at the University of Maryland who led the research.“Given that it seems to be evolving towards generating aerosols better, then we need better containment and better personal protection,” Dr. Milton said of the virus. “We are recommending people move to tighter-fitting masks.”To compare how different variants spread through the air, his team asked participants with mild or asymptomatic infections to recite the alphabet, sing “Happy Birthday” loudly or shout out the University of Maryland slogan, “Go Terps!”People infected with the Alpha variant had copious amounts of virus in their nose and throat, much more than those infected with the original virus. But even after adjusting for that difference, those infected with the variant released about 18 times as much virus into the smallest aerosols.But the researchers examined only four people infected with Alpha, and 45 with older variants. That could skew the observed differences between the variants, said Seema Lakdawala, a respiratory virus expert at the University of Pittsburgh, who was not involved in either new study.Infected people can pass the virus along to many, many others — or to none at all. How much virus they expel may depend on where in the respiratory tract it is replicating, the nature of the mucus in its environment, and what other microbes it may hitch a ride with..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-1kpebx{margin:0 auto;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-1kpebx{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-1kpebx{font-size:1.25rem;line-height:1.4375rem;}}.css-1gtxqqv{margin-bottom:0;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-19zsuqr{display:block;margin-bottom:0.9375rem;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}“We have really no idea why some individuals are superspreaders and others are not,” Dr. Lakdawala said. “There’s a lot of heterogeneity between individuals.”Data from a greater number of participants would be more convincing, but the two studies together do suggest that enhanced transport through aerosols at least partly contributes to the variant’s contagiousness, she said.Dr. Munster’s study did not involve people at all, but Syrian hamsters. Using the animals allowed the team to control the experimental conditions tightly and focus only on the movement of aerosols, Dr. Munster said.The researchers separated pairs of hamsters with tubes of different lengths that allowed airflow but no physical contact. They looked at how well the different variants traveled from infected “donor” hamsters to uninfected “sentinel” hamsters.When the cages were more than two meters apart, only the smallest aerosols — particles smaller than 5 microns — were shown to infect the sentinel hamsters. And the team found, as expected, that the Alpha variant outcompeted the original virus in infecting the sentinel hamsters.The results were posted on bioRxiv, a website that features papers before they have been published in a scientific journal.The researchers are now testing the Delta variant and expect to find that it is even more efficient, Dr. Munster said.Together, the new findings underscore the importance of masks for vaccinated people, especially in crowded spaces, experts said. Although people with breakthrough infections after vaccination are much less likely to spread the virus than unvaccinated people, the contagiousness of the variants raises the probability.With billions of people worldwide vaccinated, and billions still unvaccinated, the virus may still change in unexpected ways, Dr. Munster said: “There might be additional evolutionary pressures, shaping the evolutionary direction of this virus.”

Scientific advisers to the Centers for Disease Control and Prevention will take up a thorny challenge on Thursday: Who qualifies for the new Pfizer-BioNTech coronavirus booster and why?On Wednesday evening, the Food and Drug Administration authorized booster shots of the vaccine for people over 65 who received their second at least six months earlier. The agency also approved boosters for adult Pfizer-BioNTech recipients who are at high risk of severe Covid-19, or who are at risk of serious complications because of exposure to the virus in their jobs.Roughly 22 million Americans are at least six months past their second Pfizer dose, according to the C.D.C. About half are 65 or older.But who exactly risks becoming severely ill? What does it mean to be exposed on the job? Do teachers count as exposed, or just frontline health care workers? And what about Americans who got the Moderna and Johnson & Johnson shots?Those are questions scientists on the C.D.C. Advisory Committee on Immunization Practices have been debating, and their decisions will shape the federal government’s guidance.In its deliberations on Wednesday, the C.D.C.’s advisory committee zeroed in on unanswered questions.A third dose undoubtedly amps up antibody levels, the experts concluded. But it’s unclear so far how long that increase lasts, whether it translates to meaningful extra protection against severe disease, and whether it can significantly decrease transmission of the virus.Scientists on the committee also noted the paucity of safety data, especially among younger people. And several advisers said they believed the goal of the boosters should be to prevent severe illness, hospitalization and death, rather than stave off infection.“I don’t think there’s any hope that vaccines such as the ones we have will prevent infection after the first, maybe, couple weeks that you have those extraordinary immediate responses,” said Dr. Sarah Long, a pediatric infectious disease expert at Drexel University College of Medicine in Philadelphia.The advisers also wrestled with the practicalities of endorsing a booster shot of Pfizer’s vaccine, but not of Moderna or Johnson & Johnson’s. Recipients of those vaccines may hear that boosters are necessary — but they can’t have them yet.“That’s a big public health panic that we would like to avoid,” Dr. Long said.Moderna has applied for F.D.A. authorization of booster shots, but at half the dosage given in the first two.Mixing first shots of the Moderna vaccine with a Pfizer booster — or vice versa — is untested ground, and federal agencies are always reluctant to make moves that the evidence doesn’t explicitly support.Some global health experts have criticized the Biden administration for pushing booster shots when much of the world has yet to receive a first dose. But on Wednesday, Jen Psaki, the White House press secretary, argued that was a “false choice.”On Wednesday morning, President Biden said the United States would buy 500 million more doses of the Pfizer-BioNTech vaccine to donate worldwide, doubling up on a purchase in July.“We’re now donating three shots globally for every one shot we put in the arm of an American, and our view continues to be that we can do both,” Ms. Psaki said. “Our view also continues to be that frankly the rest of the world needs to step up and do more.”Sharon LaFraniere

Scientific advisers to the Centers for Disease Control and Prevention will take up a thorny challenge on Thursday: Who qualifies for the new Pfizer-BioNTech coronavirus booster and why?On Wednesday evening, the Food and Drug Administration authorized booster shots of the vaccine for people over 65 who received their second at least six months earlier. The agency also approved boosters for adult Pfizer-BioNTech recipients who are at high risk of severe Covid-19, or who are at risk of serious complications because of exposure to the virus in their jobs.Roughly 22 million Americans are at least six months past their second Pfizer dose, according to the C.D.C. About half are 65 or older.But who exactly risks becoming severely ill? What does it mean to be exposed on the job? Do teachers count as exposed, or just frontline health care workers? And what about Americans who got the Moderna and Johnson & Johnson shots?Those are questions scientists on the C.D.C. Advisory Committee on Immunization Practices have been debating, and their decisions will shape the federal government’s guidance.In its deliberations on Wednesday, the C.D.C.’s advisory committee zeroed in on unanswered questions.A third dose undoubtedly amps up antibody levels, the experts concluded. But it’s unclear so far how long that increase lasts, whether it translates to meaningful extra protection against severe disease, and whether it can significantly decrease transmission of the virus.Scientists on the committee also noted the paucity of safety data, especially among younger people. And several advisers said they believed the goal of the boosters should be to prevent severe illness, hospitalization and death, rather than stave off infection.“I don’t think there’s any hope that vaccines such as the ones we have will prevent infection after the first, maybe, couple weeks that you have those extraordinary immediate responses,” said Dr. Sarah Long, a pediatric infectious disease expert at Drexel University College of Medicine in Philadelphia.The advisers also wrestled with the practicalities of endorsing a booster shot of Pfizer’s vaccine, but not of Moderna or Johnson & Johnson’s. Recipients of those vaccines may hear that boosters are necessary — but they can’t have them yet.“That’s a big public health panic that we would like to avoid,” Dr. Long said.Moderna has applied for F.D.A. authorization of booster shots, but at half the dosage given in the first two.Mixing first shots of the Moderna vaccine with a Pfizer booster — or vice versa — is untested ground, and federal agencies are always reluctant to make moves that the evidence doesn’t explicitly support.Some global health experts have criticized the Biden administration for pushing booster shots when much of the world has yet to receive a first dose. But on Wednesday, Jen Psaki, the White House press secretary, argued that was a “false choice.”On Wednesday morning, President Biden said the United States would buy 500 million more doses of the Pfizer-BioNTech vaccine to donate worldwide, doubling up on a purchase in July.“We’re now donating three shots globally for every one shot we put in the arm of an American, and our view continues to be that we can do both,” Ms. Psaki said. “Our view also continues to be that frankly the rest of the world needs to step up and do more.”Sharon LaFraniere

The Latest on Kids and Covid VaccinesApoorva MandavilliReporting on the coronavirusChildren have a much lower risk of Covid-19 than adults, even when exposed to the Delta variant. Still, a small number of infected children develop a life-threatening condition called multi-system inflammatory syndrome in children, or MIS-C. Others may have lingering symptoms for months.And unvaccinated children, even if they do not become ill themselves, can spread the virus to family members, teachers and others they interact with regularly.