Reversing a decision made just two months ago, the Centers for Disease Control and Prevention is expected to recommend on Tuesday that people vaccinated for the coronavirus resume wearing masks indoors under certain circumstances. The change follows reports of rising breakthrough infections with the Delta variant of the virus in people who were fully immunized. But the new guidance would mark a sharp turnabout from the agency’s position since May that vaccinated people do not need to wear masks in most indoor spaces. As recently as last week, an agency spokesman said that the C.D.C. had no plans to change its guidance, unless there were a significant change in the science. Federal officials met on Sunday night to review new evidence that may have prompted the reversal, CNN reported on Tuesday.“I think that’s great,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. Based on what scientists are learning about the Delta’s variant’s ability to cause breakthrough infections, she said, “this is a move in the right direction.”The C.D.C.’s initial guidance in May said people fully protected from the coronavirus could go mask-free indoors, but recommended that unvaccinated people still wear masks. Those recommendations drew sharp criticism from some experts, who said it was premature given the vast swaths of unvaccinated people in the country.Dr. Rochelle Walensky, the C.D.C.’s director, at the time pointed to two scientific findings as significant factors. Few vaccinated people become infected with the virus, and transmission seems rarer still, she noted; and the vaccines appear to be effective against all known variants of the coronavirus.A day after the announcement, the agency released results from a large study showing that the mRNA vaccines made by Pfizer-BioNTech and Moderna were 94 percent effective in preventing symptomatic illness in those who got two doses, and 82 percent effective in those who had received one dose.But those data, and the C.D.C.’s decision, were based on infections of previous versions of the virus before the Delta variant began sweeping through the country. Reports of clusters of infections among fully immunized people have suggested that the variant may be able to break through the vaccine barrier more often than previous iterations of the virus.

What to Know: Breakthrough InfectionsApoorva MandavilliReporting on the coronavirusBreakthrough infections may be more common with the highly contagious Delta variant than with other versions of the virus. But the data are still unclear.There is some evidence that people infected with the variant carry about 1,000-fold more virus in their bodies than those infected with previous variants, and may remain contagious for longer. If that’s true even of vaccinated people, then some of them may be able to spread the virus to others.

There are almost as many reasons for vaccine hesitancy and refusal as there are unvaccinated Americans. But this problem, not the variant, lies at the root of rising infection rates.After an all too brief respite, the United States is again at a crossroads in the pandemic. The number of infections has ticked up — slowly at first, then swiftly — to 51,000 cases per day, on average, more than four times the rate a month ago. The country may again see overflowing hospitals, exhausted health care workers and thousands of needless deaths.The more contagious Delta variant may be getting the blame, but fueling its rise is an older, more familiar foe: vaccine hesitancy and refusal, long pervasive in the United States. Were a wider swath of the population vaccinated, there would be no resurgence — of the Delta variant, or Alpha variant, or any other version of the coronavirus.While mild breakthrough infections may be more common than once thought, the vaccines effectively prevent severe illness and death. Yet nearly half of the population remains unvaccinated and unprotected. About 30 percent of adults have not received even a single dose, and the percentage is much higher in some parts of the country.America is one of the few countries with enough vaccines at its disposal to protect every resident — and yet it has highest rates of vaccine hesitance or refusal of any nation except Russia.Public health experts have fruitlessly warned for months that the virus — any version of it — would resurge if the country did not vaccinate enough of the population quickly enough. Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health, predicted in January that Florida might have a rough summer. Now one in five new infections nationwide is in Florida.True, the speed and ferocity with which the Delta variant is tearing through Asia, Europe, Africa and now North America has taken many experts by surprise. It now accounts for about 83 percent of the infections in the United States.But Delta is by no means the wickedest variant out there. Gamma and Lambda are waiting in the wings, and who knows what frightful versions are already flourishing undetected in the far corners of the world, perhaps even here in America.Every infected person, anywhere in the world, offers the coronavirus another opportunity to morph into a new variant. The more infections there are globally, the more likely new variants will arise.The United States will be vulnerable to every one of them until it can immunize millions of people who now refuse to get the vaccine, are still persuadable but hesitant, or have not yet gained access. The unvaccinated will set the country on fire over and over again.And they will not be the only ones who are singed. Vaccinated people will be protected from severe illness and death, but there may be other consequences. Already in some communities, they are being asked to wear masks indoors. If the numbers continue to soar, the restrictions that divided the country before may return. Workplaces may need to close again, and schools, too.And some number of vaccinated people will become infected. Breakthrough infections were expected to be vanishingly rare with the original virus, but recent data suggest they may be less so with the Delta variant. It is roughly twice as contagious as the original coronavirus, and some early evidence hints that people infected with the variant carry the virus in much higher amounts.“The larger the force of infection that comes from the pandemic in unvaccinated populations, the more breakthrough infections there will be,” Dr. Hanage said.Most breakthrough infections produce few to no symptoms, but some may prompt illness in vaccinated people serious enough to lay them up in bed, miss work — and put their children or older relatives at risk. Some cases may lead to long Covid, scientists now fear — a poorly defined syndrome in which symptoms seem to persist months.This grim redux has a glaringly obvious solution: shots in arms. But short of a federal mandate — or a patchwork of mandates by municipalities, hospitals, colleges and businesses — it is hard to see how enough Americans will be immunized to form a buttress against the virus.After a brisk vaccination campaign in the spring, the pace has slowed to about 537,000 doses per day, according to data gathered by The New York Times. Some responsibility for the lag lies with the frank refusal of conservative leaders — often Republicans — to champion the vaccines.But misinformation, an epidemic all its own on social media, emanates from all parts the cultural spectrum, and there is no single reason so many Americans remain unvaccinated. It is a Hydra-headed problem.Of the 39 percent of adults who are unvaccinated, about half say they are completely unwilling. But even within that group, some say they would comply if required to do so..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-w739ur{margin:0 auto 5px;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-w739ur{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-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-uf1ume{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;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.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;}Some are hesitant and may come around with the right persuasion from people they trust, while still others plan to be inoculated but say they have just not had the chance.Politics is a driver for only some of these people, noted Dr. Richard Besser, a former director of the Centers for Disease Control and Prevention. In New Jersey, where he lives, the rates vary drastically because of socioeconomic factors. In mostly white Princeton, 75 percent of adults are immunized, versus 45 percent in Trenton, just 14 miles away, which is heavily Black and Latino.“Both are strong Democratic areas, so it’s really important to break things down and to address the issues that are impeding vaccination progress in each segment of the unvaccinated population,” Dr. Besser said.Still, there is no doubt that the political divide is playing a role in rising infection rates. From the start, vaccinations in counties that voted for Donald J. Trump lagged those in counties that voted for Joseph R. Biden, and the gap has only widened — from two percentage points in April to nearly 12 points now, according to one recent poll by the Kaiser Family Foundation.Nationwide, 86 percent of Democrats have had at least one shot, compared with 52 percent of Republicans, according to another poll. Even the national goal of having 70 percent of adults vaccinated by July 4 somehow became “Biden’s goal,” said Dr. Nahid Bhadelia, director of the Center for Emerging Infectious Diseases Policy and Research at Boston University.“All of a sudden, even getting out of pandemic became a left versus right issue.”Fewer than half of House Republicans are vaccinated as of May, compared with 100 percent of congressional Democrats. For months, some Republican lawmakers including Senators Ron Johnson of Wisconsin and Rand Paul of Kentucky, and conservative news commentators like Tucker Carlson, have voiced their skepticism of vaccines, loudly and insistently.Lately, as infections rise in conservative precincts, a few Republican leaders have begun championing vaccination. Senator Mitch McConnell of Kentucky, the minority leader, who survived polio as a child, has worn masks and has urged that everyone be immunized. Senator Mitt Romney, Republican of Utah, said in an interview on Wednesday that “the politicization of vaccination is an outrage and frankly moronic.”All of these leaders, and many more, will need to repeat vaccine affirmations often enough to persuade millions of people to overcome their hesitation. The Delta variant is thriving amid American discord. The vaccines are the remedy not just for this variant, but all those yet to come.

“Can we not think and act on two pandemics at a time?” asked one epidemiologist.The Biden administration has not yet nominated a leader for the President’s Emergency Plan for Aids Relief, a $7 billion program that sets priorities for AIDS care worldwide — leaving countries that receive funding from the program without guidance during a pandemic that is particularly dire for those with H.I.V.Pepfar is led by a global AIDS coordinator, a cabinet-level position that was last held by Dr. Deborah Birx. Dr. Birx served from April 2014 to February 2020, when she left to join the White House coronavirus task force. Dr. Angeli Achrekar, a deputy, has acted as PEPFAR’s interim leader since President Biden took office.Global health experts sharply criticized the delay in nominating a permanent chief. “Can we not think and act on two pandemics at a time?,” asked Gregg Gonsalves, a longtime H.I.V. activist and an epidemiologist at the Yale School of Public Health.PEPFAR was launched in 2003 by President George W. Bush and has had bipartisan support ever since. Funds distributed by PEPFAR are used to support prevention and treatment programs, including offering voluntary male circumcision, as well as testing for H.I.V. and providing antiretroviral therapy to people of all ages.It is widely regarded as the most successful global health program. Since its inception, the U.S. government has invested more than $85 billion in more than 60 countries, saving an estimated 20 million lives.“PEPFAR is an example of what can be done when you combine diplomacy and global health,” said Dr. Carlos del Rio, an infectious-disease expert at Emory University in Atlanta and chair of PEPFAR’s scientific advisory board. “Throughout Africa, they love and they respect the U.S. because of PEPFAR.”Dr. Angeli Achrekar, the acting U.S. Global AIDS Coordinator.U.S. Department of StateLast week, a group of more than 50 advocacy organizations sent a letter to Mr. Biden, urging him to “immediately appoint a bold, creative, and qualified” leader for PEPFAR. “This is unacceptable, particularly during a time of the dueling pandemics of H.I.V. and Covid-19,” they wrote.The White House did not respond to a request for comment.The coronavirus pandemic has disrupted access to H.I.V. prevention, diagnosis and treatment, as well as supply chains for condoms, lubricants and antiretroviral drugs, according to a recent report from UNAIDS.And the pandemic has reversed hard-gained progress on ending H.I.V., including a 23 percent annual decrease in new infections since 2010.The inertia on naming a leader is particularly damaging “when more leadership, ambition and governance is sorely needed to guide global efforts to make up lost ground on the H.I.V. response,” said Suraj Madoori, a director of the Treatment Action Group, an advocacy organization based in New York.A new study released last week showed that people living with H.I.V. have a heightened risk of serious illness and death from Covid-19. The coronavirus pandemic could also benefit from the health care infrastructure set up to provide services for H.I.V., experts noted.“There’s a lot that can happen now, using the PEPFAR structure to confront Covid in those countries,” Dr. del Rio said.“Not leveraging the Pepfar infrastructure — I think it’s crazy, it’s a huge missed opportunity,” he added. “This administration has been around for six months. Why have we not appointed them?”Dr. del Rio said PEPFAR’s chief has been noticeably absent from global conversations, including a recent U.N. resolution to end AIDS by 2030, and efforts to enable PEPFAR sites to respond to the coronavirus pandemic. It’s also important for PEPFAR’s chief to speak up for the program when budget dollars are allocated, Dr. del Rio added: “I almost feel like the program is basically at a standstill.”The absence of a U.S. voice is also having ripple effects on many issues in African countries, said Richard Lusimbo, a program manager at Pan Africa ILGA in Uganda. Core programs for key populations like L.G.B.T.Q. people have been cut in several countries since the start of the Biden administration. In Ivory Coast, for example, the budget for key population services was cut by half.In Kenya, a dispute between its government and the U.S. Agency for International Development has led to a shortage of antiretroviral drugs. A permanent PEPFAR leader with political power would have been able to resolve that dispute, Mr. Lusimbo said.Mr. Biden named Samantha Power to lead USAID on Jan. 13, even before he took office. And last week, the White House announced nominees for seven other positions.For weeks, the H.I.V. community has heard that the administration is considering five widely known global health experts to lead PEPFAR: Shannon Hader, Charles Holmes, Chris Beyrer, Vanessa Kerry and Paul Farmer. But no candidate has emerged as the front-runner.“Unfortunately, we are watching as global support for the Covid-19 response in Africa is missing, the AIDS response is being weakened, and it is not clear who the U.S. government’s leader is on this,” Mr. Lusimbo said. “Does the administration not understand that, for our communities, the AIDS response and the Covid-19 response are critically interlinked?”

Many who received the shot may need to consider boosters, the authors said. But federal health officials do not recommend second doses. The coronavirus vaccine made by Johnson & Johnson is much less effective against the Delta and Lambda variants than against the original virus, according to a new study posted online on Tuesday. The findings add to evidence that the 13 million people inoculated with the J.&J. vaccine may need to receive a second dose — ideally of one of the mRNA vaccines made by Pfizer-BioNTech or Moderna, the authors said. But the conclusions are at odds with those from smaller studies published by Johnson & Johnson earlier this month suggesting that a single dose of the vaccine is effective against the variant even eight months after inoculation. The new study has not yet been peer reviewed nor published in a scientific journal, and relied on laboratory experiments. But it is consistent with observations that a single dose of the AstraZeneca vaccine — which has a similar architecture to the J.&J. vaccine — shows only about 33 percent efficacy against symptomatic disease caused by the Delta variant.“The message that we wanted to give was not that people shouldn’t get the J.&J. vaccine, but we hope that in the future, it will be boosted with either another dose of J.&J. or a boost with Pfizer or Moderna,” said Nathaniel Landau, a virologist at N.Y.U.’s Grossman School of Medicine, who led the study. Other experts said the results are what they would have expected, because all of the vaccines seem to work better when given in two doses. “I have always thought, and often said, that the J.&J. vaccine is a two-dose vaccine,” said John Moore, a virologist at Weill Cornell Medicine in New York. Dr. Moore pointed to several studies in monkeys and people that have shown greater efficacy with two doses of the J.&J. vaccine, compared with one dose. He said the new study was particularly credible because it was published by a team with no ties to any of the vaccine manufacturers. But the data from the new study “do not speak to the full nature of immune protection,” said Seema Kumar, a spokeswoman for J.&J. Studies sponsored by the company indicate that the vaccine “generated strong, persistent activity against the rapidly spreading Delta variant,” she said.The Delta variant is the most contagious version yet of the coronavirus. It accounts for 83 percent of infections in the United States, Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said at a Senate hearing on Tuesday. The variant may also be mainly responsible for a recent rise in infections: Although they are still low relative to last winter, cases are rising in all 50 states, and hospitalizations are increasing in nearly all of them. In the two weeks ending on Tuesday, the nation averaged 268 deaths per day.Experts said the results align with what they would have expected: All of the vaccines seem to work better when given in two doses.Bryan Anselm for The New York TimesDelta may cause more breakthrough infections than earlier forms of the virus, but more than 99 percent of the hospitalizations and deaths are occurring among unvaccinated people. Rates of immunization in the country have stalled, with just under 60 percent of adults fully protected against the virus. Several studies have suggested that the mRNA vaccines made by Pfizer-BioNTech and Moderna will maintain their efficacy against the coronavirus, including all variants identified so far. One recent study showed, for example, that the vaccines trigger a persistent immune reaction in the body that may protect against the coronavirus for years.But evidence on the J.&J. vaccine has been limited, because it was rolled out later than the mRNA vaccines. Most studies of effectiveness of the coronavirus vaccines were conducted at medical centers and hospitals that relied on samples from staff members who received the mRNA vaccines.The J.&J. vaccine has also been dogged by reports of blood clots and a rare neurological syndrome, as well as problems with contamination at a manufacturing plant in Baltimore.Small studies published by researchers affiliated with J.&J. suggested that the vaccine was only slightly less effective against the Delta variant than against the original virus, and that antibodies stimulated by the vaccine grew in strength over eight months.Dr. Landau’s team would probably have seen a similar increase in the vaccine’s potency if they had looked at the data over time, said Dr. Dan Barouch, a virologist at Beth Israel Deaconess Medical Center in Boston. The data on the J.&J. vaccine’s strength against the Delta variant at Day 29 is not much different from those reported in his own study, Dr. Barouch said. “Fundamentally I don’t see that there’s any discordance,” he said. “The question is that of kinetics, it’s not just magnitude, because immune responses are not static over time.” The new study also did not consider other components of immune defense, he added.Dr. Landau and his colleagues looked at blood samples taken from 17 people who had been immunized with two doses of an mRNA vaccine and 10 people with one dose of the J.&J. vaccine. The J.&J. vaccine started out with a lower efficacy than the mRNA vaccines and showed a bigger drop in efficacy against the Delta and Lambda variants. “The lower baseline means that what’s left to counter Delta is very weak,” Dr. Moore said. “That is a substantial concern.”Very few vaccines are given as a single dose, because the second dose is needed to amp up antibody levels, noted Akiko Iwasaki, an immunologist at Yale University. People who were inoculated with the J.&J. vaccine “are relying on that primary response to maintain high levels of antibodies, which is difficult, especially against the variants,” she said.Boosting immunity with a second dose should raise the antibody levels high enough to counter the variants, she said. Turning to an mRNA vaccine for the second shot, rather than another J.&J. shot, may be better: Several studies have shown that combining one dose of the AstraZeneca vaccine with a dose of the Pfizer-BioNTech or Moderna vaccines kicks up the immune response more effectively than two doses of AstraZeneca. The Food and Drug Administration has said “Americans who have been fully vaccinated do not need a booster shot at this time,” and the agency is unlikely to change its recommendations based on laboratory studies. But the new data should prompt the F.D.A. to revisit its recommendations, Dr. Landau said: “I hope that they read our paper and think about it.”

The Delta variant of the coronavirus can evade antibodies that target certain parts of the virus, according to a new study published on Thursday in Nature. The findings provide an explanation for diminished effectiveness of the vaccines against Delta, compared with other variants.The variant, first identified in India, is believed to be about 60 percent more contagious than Alpha, the version of the virus that thrashed Britain and much of Europe earlier this year, and perhaps twice as contagious as the original coronavirus. The Delta variant is now driving outbreaks among unvaccinated populations in countries like Malaysia, Portugal, Indonesia and Australia.Delta is also now the dominant variant in the United States. Infections in the country had plateaued at their lowest levels since early in the pandemic, though the numbers may be rising. Still, hospitalizations and deaths related to the virus have continued a steep plunge. That’s partly because of relatively high vaccination rates: 48 percent of Americans are fully vaccinated, and 55 percent have received at least one dose.But the new study found that Delta was barely sensitive to one dose of vaccine, confirming previous research that suggested that the variant can partly evade the immune system — although to a lesser degree than Beta, the variant first identified in South Africa.French researchers tested how well antibodies produced by natural infection and by coronavirus vaccines neutralize the Alpha, Beta and Delta variants, as well as a reference variant similar to the original version of the virus.The researchers looked at blood samples from 103 people who had been infected with the coronavirus. Delta was much less sensitive than Alpha to samples from unvaccinated people in this group, the study found.One dose of vaccine significantly boosted the sensitivity, suggesting that people who have recovered from Covid-19 still need to be vaccinated to fend off some variants.The team also analyzed samples from 59 people after they had received the first and second doses of the AstraZeneca or Pfizer-BioNTech vaccines.Blood samples from just 10 percent of people immunized with one dose of the AstraZeneca or the Pfizer-BioNTech vaccines were able to neutralize the Delta and Beta variants in laboratory experiments. But a second dose boosted that number to 95 percent. There was no major difference in the levels of antibodies elicited by the two vaccines.“A single dose of Pfizer or AstraZeneca was either poorly or not at all efficient against Beta and Delta variants,” the researchers concluded. Data from Israel and Britain broadly support this finding, although those studies suggest that one dose of vaccine is still enough to prevent hospitalization or death from the virus.The Delta variant also did not respond to bamlanivimab, the monoclonal antibody made by Eli Lilly, according to the new study. Fortunately, three other monoclonal antibodies tested in the study retained their effectiveness against the variant.In April, citing the rise of variants resistant to bamlanivimab, the U.S. Food and Drug Administration revoked the emergency use authorization for its use as a single treatment in treating Covid-19 patients.

Scientists were critical of the announcement, pointing to evidence that the current two-dose regimen is powerfully effective against the coronavirus.Pfizer and BioNTech announced on Thursday that they were developing a version of the coronavirus vaccine that targets Delta, a highly contagious variant that has spread to nearly 100 countries. The companies expect to begin clinical trials of the vaccine in August.Pfizer and BioNTech also reported promising results from studies of people who received a third dose of the original vaccine. A booster given six months after the second dose of the vaccine increases the potency of antibodies against the original virus and the Beta variant by five- to tenfold, the companies said.Vaccine efficacy may decline six months after immunization, the companies said in a news release, and booster doses may be needed to fend off virus variants.The data have not been published, nor peer-reviewed. The vaccine makers said they expected to submit their findings to the Food and Drug Administration in the coming weeks, a step toward gaining authorization for booster shots.But the companies’ assertions contradict other research, and several experts pushed back against the claim that boosters will be needed.“There’s really no indication for a third booster or a third dose of an mRNA vaccine, given the variants that we have circulating at this time,” said Dr. Céline Gounder, an infectious disease specialist at Bellevue Hospital Center in New York. “In fact, many of us question whether you will ever need boosters.”Federal agencies also sounded a dubious note on Thursday night. Generally, Americans who have been fully vaccinated do not need a booster shot at this time, the F.D.A. and the Centers for Disease Control and Prevention said in a joint statement.“We are prepared for booster doses if and when the science demonstrates that they are needed,” the agencies said.The Delta variant, first identified in India, is believed to be about 60 percent more contagious than Alpha, the version of the virus that tore through Britain and much of Europe earlier this year, and perhaps twice as contagious as the original coronavirus.The Delta variant is now driving outbreaks among unvaccinated populations in countries like Malaysia, Portugal, Indonesia and Australia. Delta is also now the dominant variant in the United States, the C.D.C. reported this week.Until recently, infections in the United States had plateaued at their lowest levels since early in the pandemic. Hospitalizations and deaths related to the virus have continued to decline, but new infections may be rising.It’s not yet clear to what extent the variant is responsible; a slowing vaccination drive and swift reopenings also are playing roles.Citing data from Israel, Pfizer and BioNTech suggested their vaccine’s efficacy “in preventing both infection and symptomatic disease has declined six months post-vaccination.” Noting the rise of Delta and other variants, the companies said that “a third dose may be needed within 6 to 12 months after full vaccination.”Health officials in Israel have estimated that full vaccination with the Pfizer-BioNTech offers only 64 percent efficacy against the Delta variant. (Efficacy against the original virus is greater than 90 percent.)But Israel’s estimates have been contradicted by a number of other studies finding that the vaccine is highly effective at preventing infection — against all variants. One recent study showed, for example, that the mRNA vaccines like Pfizer’s trigger a persistent immune reaction in the body that may protect against the coronavirus for years.“Pfizer looks opportunistic by hanging an announcement on the back of very early and undigested data from Israel,” said John Moore, a virologist at Weill Cornell Medicine in New York. “When the time is right for using boosters here, the decision isn’t theirs to make.”The companies described their plan to develop a new vaccine against Delta as a sort of backup effort in case boosters of the original vaccine should fail. The new vaccine will target the entirety of the spike protein, as opposed to one part, and the first batch has already been produced.The Delta variant does pose challenges to the immune system. In the journal Nature, French researchers reported on Thursday new evidence that the Delta variant can partly sidestep the body’s immune response because of alterations to the spike protein on its surface that make it more difficult for antibodies to attack.The team analyzed blood samples from 59 people after they had received the first and second doses of the vaccines. Blood samples from just 10 percent of people immunized with one dose of the AstraZeneca or the Pfizer-BioNTech vaccines were able to neutralize the Delta and Beta variants in laboratory experiments.“A single dose of Pfizer or AstraZeneca was either poorly or not at all efficient against Beta and Delta variants,” the researchers concluded. Data from Israel and Britain broadly support this finding, although those studies also suggested that one dose of vaccine was still enough to prevent hospitalization or death from the virus.But a second dose boosted efficacy to 95 percent. There was no major difference in the levels of antibodies elicited by the two vaccines.“If you receive two doses of an mRNA vaccine, you are very well protected against severe disease, hospitalization and death with respect to any of the variants,” Dr. Gounder said.The researchers also looked at blood samples from 103 people who had been infected with the coronavirus. Delta was much less sensitive than Alpha to samples from unvaccinated people in this group, the study found.One dose of vaccine significantly boosted the sensitivity, suggesting that people who have recovered from Covid-19 still need to be vaccinated to fend off some variants.Taken together, the results suggest that two doses of vaccine are powerfully protective against all variants, as is one dose for people who have recovered from Covid-19 and have some natural immunity.Some experts also questioned discussions about boosters for Americans while much the world is yet to receive even a single dose.“It’s impossible to ignore the global situation,” said Natalie Dean, a biostatistician at Emory University in Atlanta. “It’s hard for me to imagine getting a third dose when there are frontline workers treating Covid patients who still haven’t been vaccinated.”Every unvaccinated person offers the virus additional opportunities to mutate into dangerous variants, Dr. Gounder noted.“If we’re worried about variants,” she said, “our best protection is to get the rest of the world vaccinated, not to hoard more doses to give third doses of mRNA vaccines to people here in the U.S.”

Las células inmunes siguen combatiendo al coronavirus meses después de la inoculación, reportaron los investigadores.Las vacunas fabricadas por Pfizer-BioNTech y Moderna generan una reacción inmunitaria persistente en el organismo que puede proteger contra el coronavirus durante años, informó el lunes un grupo de científicos.Los hallazgos se suman a la creciente evidencia de que la mayoría de las personas inmunizadas con las vacunas de ARNm podrían no necesitar refuerzos, siempre y cuando el virus y sus variantes no evolucionen mucho más allá de sus formas actuales, algo que no está garantizado. Es posible que las personas que se recuperaron de COVID-19 antes de ser vacunadas no necesiten refuerzos incluso si el virus realiza una transformación significativa.“Es una buena señal de lo durable que es nuestra inmunidad proveniente de esta vacuna”, dijo Ali Ellebedy, inmunólogo de la Universidad de Washington en Saint Louis que dirigió el estudio, publicado en la revista Nature.El estudio no incluyó a la vacuna fabricada por Johnson & Johnson, sin embargo, el doctor Ellebedy dijo que esperaba que la respuesta inmunitaria sea menos durable que la que producen las vacunas de ARN mensajero.Ellebedy y sus colegas reportaron el mes pasado que en quienes sobreviven a la COVID-19, las células inmunitarias que reconocen al virus permanecen inactivas (durmientes) en la médula ósea durante al menos ocho meses después de la infección. Un estudio de otro equipo indicó que las llamadas células B de memoria siguen madurando y fortaleciéndose durante al menos un año tras la infección.Los investigadores, con fundamento en esos hallazgos, sugirieron que la inmunidad podría durar por años y tal vez toda la vida en las personas que contrajeron el coronavirus y luego fueron vacunadas. Pero no quedó muy claro si es posible conseguir un efecto tan duradero solo con la vacunación.El equipo de Ellebedy buscó resolver esa pregunta al examinar la fuente de las células de memoria: los nódulos linfáticos, donde las células inmunitarias se entrenan para reconocer y combatir el virus.Después de una infección o la vacunación, se forma una estructura especializada llamada centro germinal en los ganglios linfáticos. Esta estructura es una suerte de escuela de élite para las células B, un campo de entrenamiento donde se vuelven cada vez más sofisticadas y aprenden a reconocer un conjunto diverso de secuencias genéticas virales.Es más probable que estas células logren frustrar a las variantes del virus que puedan surgir si disponen de más tiempo y rango para practicar.“Todos se enfocan siempre en la evolución del virus; esto muestra que las células B están haciendo lo mismo”, dijo Marion Pepper, inmunóloga de la Universidad de Washington en Seattle. “Y va a proteger contra la evolución en curso del virus, lo cual es realmente alentador”.Después de contraer el coronavirus, se forma el centro germinal en los pulmones. Pero después de la vacunación, la educación de las células sucede en los nódulos linfáticos de las axilas, al alcance de los investigadores.Ellebedy y sus colegas reclutaron a 41 personas, incluidas ocho con antecedentes de infección por el virus, que fueron inmunizadas con dos dosis de la vacuna Pfizer-BioNTech. De 14 de estas personas, el equipo extrajo muestras de los ganglios linfáticos a las tres, cuatro, cinco, siete y 15 semanas después de la primera dosis.Ese laborioso trabajo es lo que hace que este sea un “estudio heróico”, comentó Akiko Iwasaki, inmunólogo de Yale. “Este tipo de análisis cuidadoso de series de tiempo en humanos es muy difícil de realizar”.El equipo de Ellebedy encontró que a las 15 semanas de recibir la primera dosis de la vacuna, el centro germinal seguía altamente activo en los 14 participantes y que la cantidad de células de memoria capaces de reconocer al coronavirus no había disminuido.“Que las reacciones continuaran casi cuatro meses después de la vacunación, es una señal muy muy buena”, comentó Ellebedy. Los centro germinales suelen tener su máxima expresión una o dos semanas después de la inmunización y luego declinan.“Por lo general no suele quedar mucho después de cuatro a seis semanas”, dijo Deepta Bhattacharya, inmunólogo de la Universidad de Arizona. Pero los centros germinales estimulados por las vacunas de ARNm “siguen activas a los meses y no declinan mucho en la mayoría de las personas”.Bhattacharya indicó que la mayor parte de lo que los científicos saben sobre la persistencia de los centros germinales proviene de la investigación con animales. El nuevo estudio es el primero en mostrar lo que sucede en las personas después de la vacunación.Los resultados sugieren que una gran mayoría de las personas vacunadas estarán protegidas a largo plazo, al menos contra las variantes de coronavirus existentes. Pero los adultos mayores, las personas con sistemas inmunitarios débiles y aquellos que toman medicamentos inmunosupresores puede que necesiten refuerzos; Es posible que las personas que sobrevivieron a la COVID-19 y luego fueron inmunizadas nunca los necesiten.Es difícil predecir con exactitud cuánto durará la protección de las vacunas de ARN mensajero. Si no existieran las variantes que esquivan a la inmunidad, esta podría durar en teoría toda la vida. Pero el virus claramente sigue evolucionando.“Cualquier necesidad de un refuerzo sería a causa de una variante, no de un declive de la inmunidad”, dijo Bhattacharya. “No veo que eso llegue a suceder”.Las personas que se infectaron con el coronavirus y luego se inmunizaron experimentan un aumento importante en sus niveles de anticuerpos, muy probablemente porque sus células B de memoria, que producen anticuerpos, tuvieron muchos meses para evolucionar antes de la vacunación.La buena noticia: la vacuna de refuerzo probablemente tendrá el mismo efecto que una infección previa en personas inmunizadas, dijo Ellebedy. “Si les brindas otra oportunidad de participar, responderán de forma masiva”, dijo, refiriéndose a las células B de memoria.En lo que respecta a reforzar el sistema inmunológico, la vacunación es “probablemente mejor” que recuperarse de la infección real, dijo. Otros estudios han sugerido que el repertorio de células B de memoria que se producen después de la vacunación es más diverso que el generado por la infección, lo que sugiere que las vacunas protegen mejor contra variantes que la inmunidad natural por sí sola.Ellebedy dijo que los resultados también sugieren signos de una reacción inmunitaria persistente producidos por las vacunas de ARNm, en lugar de por aquellas más tradicionales, como la de Johnson & Johnson.Pero esa es una comparación injusta, ya que la vacuna de Johnson & Johnson se administra en una sola dosis, dijo Iwasaki: “Es probable que si la J & J tuviera una segunda dosis, podría inducir el mismo tipo de respuesta”.Apoorva Mandavilli es reportera del Times y se enfoca en ciencia y salud global. En 2019 ganó el premio Victor Cohn a la Excelencia en Reportaje sobre Ciencias Médicas. @apoorva_nyc

Close examinations of more than a dozen vaccinated people found that immune cells were still organizing to fight the coronavirus months after inoculation.The vaccines made by Pfizer-BioNTech and Moderna set off a persistent immune reaction in the body that may protect against the coronavirus for years, scientists reported on Monday.The findings add to growing evidence that most people immunized with the mRNA vaccines may not need boosters, so long as the virus and its variants do not evolve much beyond their current forms — which is not guaranteed. People who recovered from Covid-19 before being vaccinated may not need boosters even if the virus does make a significant transformation.“It’s a good sign for how durable our immunity is from this vaccine,” said Ali Ellebedy, an immunologist at Washington University in St. Louis who led the study, which was published in the journal Nature.The study did not consider the coronavirus vaccine made by Johnson & Johnson, but Dr. Ellebedy said he expected the immune response to be less durable than that produced by mRNA vaccines.Dr. Ellebedy and his colleagues reported last month that in people who survived Covid-19, immune cells that recognize the virus lie quiescent in the bone marrow for at least eight months after infection. A study by another team indicated that so-called memory B cells continue to mature and strengthen for at least a year after infection.Based on those findings, researchers suggested that immunity might last for years, possibly a lifetime, in people who were infected with the coronavirus and later vaccinated. But it was unclear whether vaccination alone might have a similarly long-lasting effect.Dr. Ellebedy’s team sought to address that question by looking at the source of memory cells: the lymph nodes, where immune cells train to recognize and fight the virus.After an infection or a vaccination, a specialized structure called the germinal center forms in lymph nodes. This structure is an elite school of sorts for B cells — a boot camp where they become increasingly sophisticated and learn to recognize a diverse set of viral genetic sequences.The broader the range and the longer these cells have to practice, the more likely they are to be able to thwart variants of the virus that may emerge.“Everyone always focuses on the virus evolving — this is showing that the B cells are doing the same thing,” said Marion Pepper, an immunologist at the University of Washington in Seattle. “And it’s going to be protective against ongoing evolution of the virus, which is really encouraging.”After infection with the coronavirus, the germinal center forms in the lungs. But after vaccination, the cells’ education takes place in lymph nodes in the armpits, within reach of researchers.Dr. Ellebedy and his colleagues recruited 41 people — including eight with a history of infection with the virus — who were immunized with two doses of the Pfizer-BioNTech vaccine. From 14 of these people, the team extracted samples from the lymph nodes at three, four, five, seven and 15 weeks after the first dose.That painstaking work makes this a “heroic study,” said Akiko Iwasaki, an immunologist at Yale. “This kind of careful time-course analysis in humans is very difficult to do.”Dr. Ellebedy’s team found that 15 weeks after the first dose of vaccine, the germinal center was still highly active in all 14 of the participants, and that the number of memory cells that recognized the coronavirus had not declined.“The fact that the reactions continued for almost four months after vaccination — that’s a very, very good sign,” Dr. Ellebedy said. Germinal centers typically peak one to two weeks after immunization, and then wane.“Usually by four to six weeks, there’s not much left,” said Deepta Bhattacharya, an immunologist at the University of Arizona. But germinal centers stimulated by the mRNA vaccines are “still going, months into it, and not a lot of decline in most people.”Dr. Bhattacharya noted that most of what scientists know about the persistence of germinal centers is based on animal research. The new study is the first to show what happens in people after vaccination.The results suggest that a vast majority of vaccinated people will be protected over the long term — at least, against the existing coronavirus variants. But older adults, people with weak immune systems and those who take drugs that suppress immunity may need boosters; people who survived Covid-19 and were later immunized may never need them at all.Exactly how long the protection from mRNA vaccines will last is hard to predict. In the absence of variants that sidestep immunity, in theory immunity could last a lifetime, experts said. But the virus is clearly evolving.“Anything that would actually require a booster would be variant-based, not based on waning of immunity,” Dr. Bhattacharya said. “I just don’t see that happening.”People who were infected with the coronavirus and then immunized see a major boost in their antibody levels, most likely because their memory B cells — which produce antibodies — had many months to evolve before vaccination.The good news: A booster vaccine will probably have the same effect as prior infection in immunized people, Dr. Ellebedy said. “If you give them another chance to engage, they will have a massive response,” he said, referring to memory B cells.In terms of bolstering the immune system, vaccination is “probably better” than recovering from the actual infection, he said. Other studies have suggested that the repertoire of memory B cells produced after vaccination is more diverse than that generated by infection, suggesting that the vaccines will protect better against variants than natural immunity alone.Dr. Ellebedy said the results also suggested that these signs of persistent immune reaction might be caused by mRNA vaccines alone, as opposed to those made by more traditional means, like Johnson & Johnson’s.But that is an unfair comparison, because the Johnson & Johnson vaccine is given as a single dose, Dr. Iwasaki said: “If the J & J had a booster, maybe it will induce this same kind of response.”

More than 1,200 cases have been reported, mostly mild and more often in young men and boys. The benefits of vaccination still far outweigh the risks, experts said.The coronavirus vaccines made by Pfizer-BioNTech and Moderna may have caused heart problems in more than 1,200 Americans, including about 500 who were younger than age 30, according to data reported on Wednesday by researchers at the Centers for Disease Control and Prevention.Still, the benefits of immunization greatly outweighed the risks, and advisers to the C.D.C. strongly recommended vaccination for all Americans 12 and older.The heart problems reported are myocarditis, an inflammation of the heart muscle; and pericarditis, inflammation of the lining around the heart. The risk is higher after the second dose of an mRNA vaccine than after the first, the researchers reported, and much higher in men than in women.But overall, the side effect is very uncommon — just 12.6 cases per million second doses administered. The researchers estimated that out of a million second doses given to boys ages 12 to 17, the vaccines might cause a maximum of 70 myocarditis cases, but would prevent 5,700 infections, 2,215 hospitalizations and two deaths.Agency researchers presented the data to members of the Advisory Committee on Immunization Practices, which makes recommendations on vaccine use in the United States. (The scientists grouped pericarditis with myocarditis for reporting purposes.)Most cases were mild, with symptoms like fatigue, chest pain and disturbances in heart rhythm that quickly cleared up, the researchers said. Of the 484 cases reported in Americans under age 30, the C.D.C. has definitively linked 323 cases to vaccination. The rest remain under investigation.“These events are really very rare, extremely rare,” said Dr. Brian Feingold, an expert on heart inflammation in children at the UPMC Children’s Hospital of Pittsburgh. “That needs to be taken in context with illness and morbidity and mortality related to Covid.”Separately, more than a dozen federal and professional medical organizations said in a joint statement on Wednesday that myocarditis “is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination.”Federal researchers on Wednesday also presented early safety data regarding the six million doses of vaccines administered to children ages 12 to 15. The side effects — usually fatigue and pain at the injection site — were similar to those observed in young people ages 16 to 25.“To date, the Covid-19 vaccines authorized in the U.S. have demonstrated a high degree of safety,” said Dr. Matthew F. Daley, a senior investigator at Kaiser Permanente Colorado and a member of the advisory committee.The C.D.C. advisers met as the Biden administration publicly acknowledged that it expected to fall short of its goal of getting 70 percent of Americans at least partly vaccinated by July 4. The shortfall, officials said on Tuesday, resulted in part from reluctance among younger Americans to be immunized.About two out of every 100,000 people aged 15 to 18 — about two-thirds of them male — are hospitalized each year with myocarditis, according to data presented at the meeting. Patients with the most severe cases may require mechanical support, like a ventilator, or a heart transplant.Even those with mild symptoms must refrain from exercise for about six months after recovery. It’s unclear what typically causes the condition, or why it’s more common in young men than in women.The first cases of myocarditis linked to coronavirus vaccines were reported in Israel, mostly among young men aged 16 to 19. Israel recorded 148 cases from December to May, 95 percent of them mild.In the United States, too, myocarditis has been more common in men and boys: Up to 80 percent of cases diagnosed after the second dose were in males. There has also been a clear age difference, with the side effect clustered in individuals in their late teens and early 20s.About 318 million coronavirus vaccine doses have been administered in the United States as of June 21, and 150 million people are considered to be fully protected. Most of the myocarditis symptoms emerged within about four days of the first or second dose.“We have clear evidence here of onset for the vaccinated cases within the first week,” said Dr. Tom Shimabukuro, a vaccine expert at the C.D.C. who presented the new data. There is also a dose effect, he said, adding, “The rates are higher for both vaccines after dose two.”The vast majority of patients with the side effect fully recover, noted Dr. James de Lemos, a cardiologist at the University of Texas Southwestern Medical Center in Dallas, who reported one of the first cases in January. Covid-19 itself may cause heart problems in young people. A large study of collegiate athletes showed that 2.3 percent of those who had recovered from Covid-19 had heart abnormalities consistent with myocarditis.“It’s going to be manifold more common to get heart muscle inflammation from getting Covid than you would from getting a vaccine, even in young men,” Dr. de Lemos said.More than 4,000 children infected with the coronavirus developed multisystem inflammatory syndrome, which includes heart symptoms. Some children have also died, while none have died from vaccination, Dr. Feingold noted. “You can say no to the vaccine, but you’re assuming other risks.”The C.D.C. recommends vaccination for all Americans over age 12. But on Wednesday, officials suggested that anyone who develops myocarditis after the first dose should defer a second dose until they discuss the risks with a health care provider.The C.D.C.’s recommendations may influence decisions about whether to immunize children younger than 12 as vaccines become available for that age group. Some experts have questioned whether the benefits to children outweigh the potential risks, given the low odds of developing serious illness from the virus in young children.Still, the agency reported this month that the number of Covid-19-related hospitalizations among adolescents in the United States was about three times higher than hospitalizations linked to influenza over three recent flu seasons.The overall number of infections has steeply dropped since January, but as more adults have been vaccinated, the proportion of children in the total has risen. About one-third of new infections reported in May were in Americans aged 12 to 29, and 316 deaths have been recorded in this age group since April.Vaccination is becoming an even more urgent priority, given more contagious variants of the coronavirus now circulating in the United States, Dr. Paul Offit, a member of the Food and Drug Administration’s vaccine safety committee, said in an interview.“We are not close to being near where we need to be” in terms of the percentage of Americans who should be vaccinated, said Dr. Offit, who is also a pediatrician at the Children’s Hospital of Philadelphia. “And you’re going to head into winter when you’re going to have a generally undervaccinated population.”