although the vaccine remains powerfully effective against severe disease and death.

Experts were divided on the utility of booster shots so soon after vaccination began. Experience with other diseases indicates that older people and those with weak immune systems might benefit, but there is little hard evidence with the coronavirus.

“The problem here is, we’re just sort of going on immunological priors, rather than really great data to justify things one way or the other,” said Deepta Bhattacharya, an immunologist at the University of Arizona. “I totally understand the decision, but I think we have to acknowledge that there’s a wide range of uncertainty on what it’s going to do.”

Booster doses may help some people with weak immune systems, but others may show little improvement even after a third dose, and still others may not need a booster at all, scientists say.

While dozens of mostly wealthy countries, including the United States and most of Europe, have administered more than 100 doses per 100 people, many other nations remain below five per 100 — primarily in Africa, where cases have soared as the Delta variant spreads.

Doctors Without Borders said recently that it would be “unconscionable” to give booster doses in richer nations before people in poorer ones get their first doses.

“Wealthy governments shouldn’t be prioritizing giving third doses when much of the developing world hasn’t even yet had the chance to get their first Covid-19 shots,” Kate Elder, the senior vaccines policy adviser at Doctors Without Borders’ Access Campaign, said in a statement.

a so-called vector vaccine, like AstraZeneca or Johnson & Johnson.

It is the latest sign that governments are encouraging their citizens to mix and match vaccines in the hope of provoking a more protective immune response against Covid-19. Early results from a British vaccine study showed that volunteers produced high levels of antibodies and immune cells after getting one dose each of the Pfizer-BioNTech and AstraZeneca-Oxford shots.

The new German guidelines announced Monday also went a step further in encouraging parents to vaccinate children between 12 and 17, announcing that doctors and vaccination centers across the country would make the jab available to them before the start of the new school year.

Health ministers stopped short of making a formal recommendation for vaccinating children, but the move made plain their impatience with Germany’s Standing Committee on Vaccinations, which has so far refrained from guiding parents one way or the other, pending more data becoming available.

Vaccinating children “is one building block to allow a safe start into the new school year after the summer vacation,” Mr. Holetschek said.

Apoorva Mandavilli contributed reporting from New York, Benjamin Mueller from London, Aurelien Breeden from Paris, Gaia Pianigiani from Rome, Monika Pronczuk from Brussels, Raphael Minder from Madrid and Thomas Erdbrink from Amsterdam.

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Immunity to the Coronavirus May Persist for Years, Scientists Find

Immunity to the coronavirus lasts at least a year, possibly a lifetime, improving over time especially after vaccination, according to two new studies. The findings may help put to rest lingering fears that protection against the virus will be short-lived.

Together, the studies suggest that most people who have recovered from Covid-19 and who were later immunized will not need boosters. Vaccinated people who were never infected most likely will need the shots, however, as will a minority who were infected but did not produce a robust immune response.

Both reports looked at people who had been exposed to the coronavirus about a year earlier. Cells that retain a memory of the virus persist in the bone marrow and may churn out antibodies whenever needed, according to one of the studies, published on Monday in the journal Nature.

The other study, which is also under review for publication in Nature, found that these so-called memory B cells continue to mature and strengthen for at least 12 months after the initial infection.

other studies.

Some scientists have interpreted this decrease as a sign of waning immunity, but it is exactly what’s expected, other experts said. If blood contained high quantities of antibodies to every pathogen the body had ever encountered, it would quickly transform into a thick sludge.

Instead, blood levels of antibodies fall sharply following acute infection, while memory B cells remain quiescent in the bone marrow, ready to take action when needed.

landmark study in 2007 showed that antibodies in theory could survive decades, perhaps even well beyond the average life span, hinting at the long-term presence of memory B cells. But the new study offered a rare proof of their existence, Dr. Gommerman said.

Dr. Nussenzweig’s team looked at how memory B cells mature over time. The researchers analyzed blood from 63 people who had recovered from Covid-19 about a year earlier. The vast majority of the participants had mild symptoms, and 26 had also received at least one dose of either the Moderna or the Pfizer-BioNTech vaccine.

So-called neutralizing antibodies, needed to prevent reinfection with the virus, remained unchanged between six and 12 months, while related but less important antibodies slowly disappeared, the team found.

confirming results from other studies; the shots also ramped up the body’s neutralizing ability by about 50-fold.

Senator Rand Paul, Republican of Kentucky, said on Sunday that he would not get a coronavirus vaccine because he had been infected in March of last year and was therefore immune.

But there is no guarantee that such immunity will be powerful enough to protect him for years, particularly given the emergence of variants of the coronavirus that can partially sidestep the body’s defenses.

The results of Dr. Nussenzweig’s study suggest that people who have recovered from Covid-19 and who have later been vaccinated will continue to have extremely high levels of protection against emerging variants, even without receiving a vaccine booster down the line.

“It kind of looks exactly like what we would hope a good memory B cell response would look like,” said Marion Pepper, an immunologist at the University of Washington in Seattle who was not involved in the new research.

The experts all agreed that immunity is likely to play out very differently in people who have never had Covid-19. Fighting a live virus is different from responding to a single viral protein introduced by a vaccine. And in those who had Covid-19, the initial immune response had time to mature over six to 12 months before being challenged by the vaccine.

“Those kinetics are different than someone who got immunized and then gets immunized again three weeks later,” Dr. Pepper said. “That’s not to say that they might not have as broad a response, but it could be very different.”

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Capri — First Choice of the Jet Set — Gets First Dibs on Vaccines

CAPRI, Italy — The ferry docked next to the blue “Capri a Covid Free Island” billboard and the residents and workers disembarked, carrying luggage and antibodies.

Among them was Mario Petraroli, 37, freshly vaccinated and ready for the grand reopening of the luxurious hotel where he works as director of marketing.

“The big day,” he said as he rode a funicular up above turquoise waters, terraced gardens dripping with lemons and winding cliff-side footpaths.

He reached the summit and stepped out onto a glamorous town famous for its Jackie O and J Lo sightings, exorbitantly priced Caprese salads, and reputation as a billionaire’s playground. Everyone around him — the shopkeepers unpacking the Pucci, Gucci and Missoni garments from plastic bags, the bartenders sliding ice into Spritzes, the carpenters hammering finishing touches on the underground Anema e Core Taverna dance club — had been vaccinated.

Mr. De Luca came to Capri’s famous piazzetta in the center of town to declare Mission Accomplished and to urge tourists to book their vacations on the islands.

Mr. Petraroli, the hotel marketing director, now crossed the same square, past copper-toned Capri enthusiasts who sipped and smoked, their faces pointed at the sun. He entered a warren of narrow streets, lined with Rolex outlets, brand name boutiques and Hangout, a popular pub in town owned by Simone Aversa.

Capri Tiberio Palace, which Kylie Jenner repaired to in a recent summer after, workers at the port told him, she felt unwell on her yacht.

The hotel is named for Tiberius, who ran the Roman Empire from Capri, throwing people off cliffs and training Caligula how to have a good time. Many here call him Capri’s first tourist.

Mr. Petraroli said modern hedonists were already calling, sending scouts to make sure that the vaccine situation, and vibe, is what they want.

“The real issue for them is once they are here, do they have something to do,” he said as workers carried an espresso machine and dusted the blinds.

Upstairs, Mr. Petraroli opened the Suite Bellevue, booked mostly by “sheikhs and sultans and very famous guys.” It leads to a terrace tiled with hand-painted ceramics, topped with a Jacuzzi plunge pool. Mr. Petraroli said the late basketball star Kobe Bryant had such a “special bond with our top suite” that he named his daughter Capri after staying there.

Outside the room, Alessandro De Simone, 23, dusted crystal decanters filled with cognac and whiskey. Mr. De Simone, who is also vaccinated, said none of his friends back home in Naples had been.

oldest cooperative of motorboat owners (“All our skippers and staff have been completely vaccinated!” reads their website) sped uninhibited around the island. He navigated through the island’s trademark Faraglione rock formations (“This is where Heidi Klum got married on a yacht”) and by La Fontelina beach club where three sunbathers, their knees bent and gleaming, laid under the cliff.

He lamented the “hysterical polemics about us getting vaccinated,” arguing that without a hospital, “if there was a cluster here, we had nothing to save our lives.”

He moored the boat back at the dock where more ferries brought a trickle of tourists, but also returning residents. Dario Portale, a local greengrocer, and his family, were among them.

The day after getting their shot, the couple left for Milan, in the country’s hard hit region of Lombardy, to introduce their 10-month-old son to his mother. She is 62, works in a post office and is not vaccinated.

“She’s still waiting,” Mr. Portale said.

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Variant First Detected in India is Forcing UK to Speed Up Delivery of Second Doses

LONDON — Prime Minister Boris Johnson of Britain said on Friday that vaccination protocols would be changed to swiftly deliver second doses to people over 50-years-old to combat the spread of a coronavirus variant first detected in India, a warning sign for countries that are easing restrictions even though their own vaccination campaigns are incomplete.

“We believe this variant is more transmissible than the previous ones,” Mr. Johnson said. What remained unclear, he said, was by how much. The infectiousness of the variant first detected in India remains the subject of intense study and some leading experts have said it is too early to assess its transmissibility.

If it proves significantly more transmissible, he said, “we face some hard choices.” He added that there was no evidence that the variant was more likely to cause serious illness and death, and there was no evidence to suggest vaccines were less effective against the variant in preventing serious illness and death.

While he said the country would not delay plans to ease restrictions on Monday, he warned that the spread of the variant could force the government to change course.

official statistics.

The extent to which the variant has spread globally is unclear, because most countries lack the genomic surveillance capabilities employed in England.

That surveillance capability has allowed health officials in Britain to spot the rise of concerning variants more quickly than other nations, offering an early warning system of sorts as a variant seen in one nation almost invariably pops up in others.

Most cases detected in Britain are in northwestern England. The focus has been on Bolton, a town of nearly 200,000 that has one of the country’s highest rates of infection and where health officials have warned of widespread community transmission of the variant. Some cases have also been reported in London. The rapid spread of the variant has led officials to debate speeding up dosing schedules and opening up access to shots in hot spots to younger age groups.

Much is unknown about the new variant, but scientists fear it may have driven the rise of cases in India and could fuel outbreaks in neighboring countries.

Dr. Maria Van Kerkhove, the technical lead of the World Health Organization’s coronavirus response, said a study of a limited number of patients, which had not yet been peer-reviewed, suggested that antibodies from vaccines or infections with other variants might not be quite as effective against B.1.617. The agency said, however, that vaccines were likely to remain potent enough to provide protection from serious illness and death.

British officials have said the variant appears to be more contagious than the B.1.1.7 variant, which was detected last year in Kent, southeast of London and swept across Britain in the winter, forcing the country into one of the world’s longest national lockdowns. The B.1.1.7 variant has now been found in countries around the world.

The B.1.617 variant has been found in virus samples from 44 countries and was designated a variant of concern by the W.H.O. this week, which means there is some evidence that it could have an impact on diagnostics, treatments or vaccines and needs to be closely monitored.

said on Twitter.

Britain briefly reopened its economy at the end of last year, only to abruptly impose new restrictions that remained in place for months as it fought a deadly wave of infections.

In an attempt to offer at least partial protection to as many people as quickly as possible, Britain spaced injections between doses for two-stage coronavirus vaccines up to 12 weeks after the first vaccines were approved in December. That was far longer than the three- or four-week interval employed by most other countries.

Mr. Johnson said that those older than 50 will now be able to get second doses after eight weeks.

“It is more important than ever that people get the additional protection of a second dose,” he said.

The speedy rollout saved at least 11,700 lives and prevented 33,000 people from becoming seriously ill in England, according to research released by Public Health England on Friday.

Infections, serious illness and deaths have plummeted across Britain. Only 17 deaths were reported on Friday.

But the vaccination campaign has slowed down since last month because of supply shortages and the need to start distributing second doses. The number of daily first doses on average last month was 113,000, far below the average of 350,000 daily doses administered in March.

Only those over 38-years-old are currently eligible for vaccination.

It remains unclear whether the country has the vaccine supplies on hand to move rapidly to surge more into communities around the country to speed up vaccinating younger age groups.

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U.K. May Change Vaccine Protocol to Tackle India Covid Variant

The British authorities said on Friday that they are considering changing vaccination protocols and reintroducing local lockdowns to stem the spread of a coronavirus variant first detected in India, a warning sign for countries that are easing restrictions even though their own vaccination campaigns are incomplete.

The numbers of cases involving the variant, known as B.1.617, rose from 520 last week to 1,313 cases this week in Britain, according to official statistics.

The extent to which the variant has spread globally is unclear, because most countries lack the genomic surveillance capabilities employed in England.

That surveillance capability has allowed health officials in Britain to spot the rise of concerning variants more quickly than other nations, offering an early warning system of sorts as a variant seen in one nation almost invariably pops up in others.

might not be quite as effective against B.1.617. The agency said, however, that vaccines were likely to remain potent enough to provide protection from serious illness and death.

British officials have said the variant appears to be more contagious than one detected last year in Kent, southeast of London, which swept across Britain in the winter, forcing the country into one of the world’s longest national lockdowns. The British variant has now been found in countries around the world.

The variant first detected in India has been found in virus sample from 44 countries, the W.H.O. said this week.

The U.N. agency has designated the B.1.617 variant as a variant of concern.

Christina Pagel, a member of a group of scientists advising the government, known as SAGE, said postponing next week’s reopening would avoid “risking more uncertainty, more damaging closures and longer recovery from a worse situation.”

“We need to learn from previous experience,” Dr. Pagel, the director of the Clinical Operational Research Unit at University College London, said on Twitter.

Britain briefly reopened its economy at the end of last year, only to abruptly impose new restrictions that remained in place for months as it fought a deadly wave of infections.

In an attempt to offer at least partial protection to as many people as quickly as possible, Britain spaced injections between doses for two-stage coronavirus vaccines up to 12 weeks after the first vaccines were approved in December. That was far longer than the three- or four-week interval employed by most other countries.

The speedy rollout saved at least 11,700 lives and prevented 33,000 people from becoming seriously ill in England, according to research released by Public Health England on Friday.

But the campaign has slowed down since last month because of supply shortages and the need to start distributing second doses. The number of daily first doses on average last month was 113,000, far below the average of 350,000 daily doses administered in March.

Only those over 38-years-old are currently eligible for vaccination.

Officials suggested Friday that the spread of the B1.617 variant may force a shift in strategy: In areas where the variant is spreading, they may move up the second doses in order to provide stronger protection and allow younger people — at the moment, only those who are at least 38-years-old are eligible to be vaccinated — in multigenerational households to be inoculated.

But it was unclear whether the country had the vaccine supplies on hand to move rapidly.

Mr. Zahawi, the vaccines minister, said Britain would “flex the vaccine program according to the clinical advice.” He also urged people to regularly use free P.C.R. tests that have been available since last month, and to “isolate, isolate, isolate” if they test positive for the coronavirus.

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Covid Vaccines Protect Pregnant Women, Study Confirms

The vaccines produced similar responses in all three groups of women, eliciting both antibody and T-cell responses against the coronavirus, the scientists found. Of particular note, experts said, was the fact that the shots produced high levels of neutralizing antibodies, which can prevent the virus from entering cells, in both pregnant and nonpregnant women.

“Clearly, the vaccines were working in these people,” said Akiko Iwasaki, an immunologist at Yale University who was not involved in the research. “These levels are expected to be quite protective.”

The researchers also found neutralizing antibodies in the breast milk of vaccinated mothers and in umbilical cord blood collected from infants at delivery. “Vaccination of pregnant people and lactating people actually leads to transfer of some immunity to their newborns and lactating infants,” said Dr. Ai-ris Y. Collier, a physician-scientist at Beth Israel who is the first author of the paper.

The results are “really encouraging,” Dr. Iwasaki said. “There is this added benefit of conferring protective antibodies to the newborn and the fetus, which is all the more reason to get vaccinated.”

The scientists also measured the women’s immune responses to two variants of concern: B.1.1.7, which was first identified in Britain, and B.1.351, which was first identified in South Africa. All three groups of women produced antibody and T-cell responses to both variants after vaccination, although their antibody responses were weaker against the variants, especially B.1.351, than against the original strain of the virus, according to the study.

“These women developed immune responses to the variants, although the asterisk is that the antibody responses were reduced several-fold,” said Dr. Dan Barouch, a study author and virologist at Beth Israel. (Dr. Barouch and his colleagues developed the Johnson & Johnson vaccine, which was not included in this study.)

“Overall, it’s good news,” he added. “And it increases the data that suggests that there is a substantial benefit for pregnant women to be vaccinated.”

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A Coronavirus Variant First Found in India is Now Officially a ‘Variant of Concern’

Amid a deepening crisis in India, the World Health Organization announced Monday that it had designated the B.1.617 variant, which has been growing more common in the country, as a variant of concern. Scientists still don’t know much about the variant yet, but they are worried that it may be helping to fuel the rise in the nation’s coronavirus infections, which experts say are likely undercounted.

“There is increased transmissibility demonstrated by some preliminary studies” of the variant, said Dr. Maria Van Kerkhove, the technical lead of the W.H.O.’s coronavirus response.

Dr. Van Kerkhove also said that a study of a limited number of patients, which had not yet been peer reviewed, suggested that antibodies from vaccines or infections with other variants might not be quite as effective against B.1.617. However, the agency said that vaccines will likely remain potent enough to provide protection against B.1.617.

More details will be released in a report on Tuesday, Dr. Van Kerkhove said.

The variant was first detected in India at the end of 2020 but became more common in the country starting in March. It has since been found in 32 countries including the United States and the United Kingdom. The W.H.O.’s announcement comes as growing numbers of medical experts are adding their voices to a chorus of condemnation of the Indian government’s response and calling for nationwide restrictions to try to limit the horrifying death toll.

the official figures are already staggering — more than 350,000 new infections daily this month and nearly 250,000 total deaths — some experts say that the numbers are a vast undercount and estimate that India is on pace to suffer more than one million deaths by August.

Initially, the W.H.O. classified B.1.617 as a “variant of interest,” because it had certain mutations that have been linked to higher transmission and the potential to evade vaccines. At a news conference on Monday, agency officials announced they were elevated it to a higher level.

Other variants of concern include B.1.1.7, which was first identified in the United Kingdom, and P.1., which was originally detected in Brazil.

But experts caution that it’s not yet clear just how much of a factor B.1.617 has played in the catastrophic rise in cases in India. They point to a perfect storm of public health blunders, such as permitting massive political rallies and religious festivals in recent months.

“I am concerned about 617 — I think we have to keep a very close eye on it,” said Kristian Andersen, a virologist at Scripps Research Institute in La Jolla, California. But he cautioned that relatively few variant samples are being analyzed in India, making it hard to know just how dangerous B.1.617 is. “We really, really need better data out of India,” he said.

editorial published on Saturday in The Lancet, a medical journal, said that Mr. Modi “seemed more intent on removing criticism” on social media than “trying to control the pandemic.”

“India squandered its early successes in controlling Covid-19,” the editorial said.

The medical journal also cited an estimate by the Institute for Health Metrics and Evaluation that projected that India would witness a total of more than a million coronavirus deaths by August — far higher than government figures would suggest.

On May 2, for example, the institute said that total deaths were actually about 642,000, about three times higher than the government’s own number for that date, just over 217,000.

Referring to the possibility that there may actually be a million victims by August, the Lancet editorial said, “If that outcome were to happen, Modi’s government would be responsible for presiding over a self-inflicted national catastrophe.”

wrote in a tweet on Sunday that it was likely that between two to five million people were being infected every day and that India’s “true” coronavirus death toll was “closer to 25,000 deaths” each day.

He based his own calculations, he wrote, on the number of cremations taking place in the country.

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New studies suggest that vaccines can protect against some variants and severe Covid cases.

Several new studies released on Wednesday offered encouraging news about the ability of widely used vaccines to protect against severe Covid-19 cases, including illness caused by some dangerous variants.

Two published studies found that the Pfizer-BioNTech vaccine was extraordinarily effective against severe disease caused by two variants, including the dominant one in the United States. And the results of an early-stage trial of the Moderna vaccine — though not published or vetted by scientists — suggested that a single dose given as a booster was effective against variants first identified in South Africa and Brazil, the company said.

The emergence of new variants, and whether vaccines are effective against them, is a subject of continued concern as a variant first detected in India, called B.1.617, spreads across the country. There is also a risk that further variants will arise there as the country’s outbreak grows, experts say. Another worrisome variant, P.1, is wreaking havoc across South America.

In the Pfizer studies, which were based on real-world use of the vaccine in Qatar and Israel, the two variants of focus were B.1.1.7, first identified in Britain and now detected in over 100 countries, and B.1.351, first identified in South Africa. The studies showed that the vaccine can prevent some of the most severe outcomes from Covid-19, such as pneumonia and death, caused by those variants.

One of the Pfizer studies showed that the vaccine was 87 to 89.5 percent effective at preventing infection with B.1.1.7 among people who were at least two weeks past their second shot. It was 72.1 percent to 75 percent effective at preventing infection with B.1.351. The study was based on information about more than 200,000 people that was pulled from Qatar’s national Covid-19 databases from Feb. 1 to March 31.

Another study, conducted by researchers at Pfizer and at Israel’s Health Ministry, found that the vaccine was more than 95 percent effective at protecting against a coronavirus infection, hospitalization and death among fully vaccinated people 16 and older.

In the United States, experts now believe that attaining herd immunity is unlikely because of the spread of variants and hesitancy among some people in the country to be vaccinated. The variant that has caused the most alarm is B.1.1.7, which is about 60 percent more transmissible than original versions of the virus.

Moderna’s announcement was greeted cautiously, because the results of an early-stage trial have not been published or peer-reviewed. But the company said it was encouraged by results that suggested that a single booster shot of its vaccine would rapidly increase antibodies in vaccinated people, and that those antibodies were effective against the original form of the virus as well as the variants first identified in South Africa and Brazil.

A second booster specifically designed to counter the variant identified in South Africa produced an even stronger immune response, the company said.

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This New Covid Vaccine Could Bring Hope to the Unvaccinated World

In early 2020, dozens of scientific teams scrambled to make a vaccine for Covid-19. Some chose tried-and-true techniques, such as making vaccines from killed viruses. But a handful of companies bet on a riskier method, one that had never produced a licensed vaccine: deploying a genetic molecule called RNA.

The bet paid off. The first two vaccines to emerge successfully out of clinical trials, made by Pfizer-BioNTech and by Moderna, were both made of RNA. They both turned out to have efficacy rates about as good as a vaccine could get.

In the months that followed, those two RNA vaccines have provided protection to tens of millions of people in some 90 countries. But many parts of the world, including those with climbing death tolls, have had little access to them, in part because they require being kept in a deep freeze.

Now a third RNA vaccine may help meet that global need. A small German company called CureVac is on the cusp of announcing the results of its late-stage clinical trial. As early as next week, the world may learn whether its vaccine is safe and effective.

Novavax, a company based in Maryland whose vaccine uses coronavirus proteins, is expected to apply for U.S. authorization in the next few weeks. In India, the pharmaceutical company Biological E is testing another protein-based vaccine that was developed by researchers in Texas. In Brazil, Mexico, Thailand and Vietnam, researchers are starting trials for a Covid-19 shot that can be mass-produced in chicken eggs.

Vaccines experts are particularly curious to see CureVac’s results, because its shot has an important advantage over the other RNA vaccines from Moderna and Pfizer-BioNTech. While those two vaccines have to be kept in a deep freezer, CureVac’s vaccine stays stable in a refrigerator — meaning it could more easily deliver the newly discovered power of RNA vaccines to hard-hit parts of the world.

“It’s gone largely under the radar,” said Jacob Kirkegaard, a senior fellow at the Peterson Institute for International Economics in Washington, D.C. But now, he added, “they look pretty well positioned to clean up the global market.”

For CureVac’s co-founder, the biologist Ingmar Hoerr, the company’s Covid-19 vaccine trial is the culmination of a quarter-century’s worth of work with RNA, a molecule that helps turn DNA into the proteins that do the work of our cells. As a graduate student at the University of Tübingen in the 1990s, Dr. Hoerr injected RNA into mice and found that the animals could make the protein encoded by the molecules. He was surprised to find that the mice’s immune systems made antibodies against the new proteins.

only a few scientists in the world considered an RNA vaccine a serious possibility. But proponents thought it might change medicine. You could, in theory, craft an RNA molecule to immunize people against any virus. You might even be able to create an RNA vaccine to cure cancer, if you could make an RNA molecule that encoded a tumor protein.

In 2001, Dr. Hoerr co-founded CureVac to chase the idea, but for the first few years the company struggled to survive. To keep the lights on, it took orders from other labs for custom-built RNA molecules. On the side, CureVac’s scientists tinkered with their own designs for RNA vaccines.

Over time, they found subtle tweaks to RNA vaccine molecules that caused cells to make more proteins. The more potent the RNA, the lower the dose they needed in vaccines.

CureVac’s researchers also figured out how to put the RNA molecules in fatty bubbles to protect them from destruction on their journey to cells. And perhaps most important, they used a form of RNA that could stay stable at relatively warm temperatures. Instead of requiring a deep freezer, CureVac’s vaccine could be refrigerated.

In time, other companies entered the RNA vaccine business as well: BioNTech in Germany in 2008, then Moderna in Boston in 2011. Their experiments began showing that these vaccines could protect animals against an assortment of viruses. In 2013, CureVac injected human volunteers with a rabies RNA vaccine, in the first clinical trial of the technology against an infectious disease.

For years, CureVac and other RNA vaccine companies toiled on perfecting their vaccines. CureVac’s first attempt at a rabies vaccine demonstrated it was safe, but it yielded a weak response from the immune system. The company has since retooled that vaccine, and the updated version has shown promise in early clinical studies. But other efforts ended in failure. In 2017, CureVac announced that its RNA vaccine against prostate cancer offered no benefits to patients.

$1 billion to move its operations to the United States. CureVac denied the reports, but the chief executive suddenly left, to be replaced by Dr. Haas.

CureVac’s researchers moved ahead with their limited resources, designing an RNA molecule encoding a protein found on the surface of the coronavirus, called spike. Experiments on hamsters showed that it could protect the animals from the virus.

Phase 3 trial, recruiting 40,000 volunteers in Europe and Latin America. The company will get its first look at the data when 56 volunteers develop Covid-19. If most of them are in the placebo group, and few in the vaccinated group, it will be proof that the vaccine works.

with a lawsuit.

In April, the European Union finally fixed this shortfall, negotiating with Pfizer and BioNTech to get 1.8 billion doses of their vaccine between now and 2023. That arrangement has left analysts wondering how much demand will be left for CureVac.

“They’re going to miss the boat on the major, advanced-economy markets,” said Dr. Kirkegaard. “The U.S., Europe and Japan are going to be largely vaccinated using these Moderna and Pfizer vaccines.”

Dr. Haas countered that most of the bloc’s doses from Pfizer-BioNTech won’t come until next year. “CureVac sees itself as a major player in ending the Covid-19 pandemic in Europe and elsewhere,” he said.

Ursula von der Leyen, president of the European Commission, said that if the CureVac vaccine worked, it would be in the mix, thanks to two advantages: It is an mRNA vaccine, and it was created in Europe. It is also possible that individual European nations will make side deals with the company.

Billions of other people in low- and middle-income countries have yet to receive a vaccine, and experts say that CureVac may meet some of their demand. “We still need a lot of vaccine globally,” said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai in New York. “I think a lot of people can benefit from it.”

The vaccines from Moderna and Pfizer-BioNTech are challenging to distribute in the developing world because of the equipment and power supply required to freeze these vaccines. CureVac’s RNA vaccine can stay stable for at least three months at 41 degrees Fahrenheit, and it can sit for 24 hours at room temperature before it is used.

“The stability is a real advantage,” Dr. Jackson said. C.E.P.I. is “in very active discussions” with CureVac, he said, about distributing the company’s vaccine through Covax, an initiative to distribute vaccines to low- and middle-income countries.

But CureVac is also designing a new generation of vaccines with a goal of eventually moving into markets in the United States and other wealthy nations. Because its potent RNA requires only a small dose, the company could potentially create vaccines for different variants and mix them in a single shot.

But such possibilities are meaningless until CureVac can prove that its vaccine works. Mary Warrell, a vaccine researcher at the University of Oxford, is reluctant to speculate about the fate of the vaccine before that milestone.

“Prediction during this pandemic has rarely been profitable,” she warned.

Matina Stevis-Gridneff contributed reporting.

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