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This more diminutive antibody can access tinier pockets and crevices on spike proteins — the proteins that allow viruses like the novel coronavirus to break into host cells and infect us — that human antibodies cannot. That can make it more effective in neutralizing viruses.

The llama’s antibody still forms a Y, but its arms are much shorter because it doesn’t have any light-chain proteins.

Humans produce only one kind of antibody, made of two types of protein chains — heavy and light — that together form a Y shape. Heavy-chain proteins span the entire Y, while light-chain proteins touch only the Y’s arms. Llamas, on the other hand, produce two types of antibodies

They can be linked or fused with other antibodies, including human antibodies, and remain stable despite those manipulations.

Llamas’ antibodies are also easily manipulated

researchers looked to llamas — and, specifically, Winter — to find a smaller llama antibody “that could broadly neutralize many different types of coronavirus,” Dr. McLellan said.

They injected Winter with spike proteins from the virus that caused the 2002-03 SARS epidemic as well as MERS, then tested a sample of her blood. And while they couldn’t isolate a single llama antibody that worked against both viruses, they found two potent antibodies that each fought separately against MERS and SARS.

They immediately realized that the smaller llama antibodies “that could neutralize SARS would very likely also recognize the Covid-19 virus,” Dr. Saelens said.

It did, the researchers found, effectively inhibiting the coronavirus in cell cultures.

While the treatment’s protection would be immediate, its effects wouldn’t be permanent, lasting only a month or two without additional injections.

This proactive approach is at least several months away, but the researchers are moving toward clinical trials. Additional studies may also be needed to verify the safety of injecting a llama’s antibodies into human patients.

Dr. Venturi said those tests were essentially “useless” because they did not describe whether a person had the right level of antibodies and because no one knew yet how long they lasted.

Results will take weeks more, but if confirmed, he said, it would show that people who test positive for an adequate level of neutralizing IgG antibodies do not pose a danger of contagion.

In theory, even those lucky few would not need to wait for two consecutive negative swab tests — which sometimes take months — before being released from quarantine and allowed back to work. “It would be like a negative test result,” he said.

In the meantime, though, Dr. Venturi said it was impossible to consider a work force made up only of people showing neutralizing antibodies.

Instead, he said, Italy’s reopening phase needs to rely on uninfected people, which he considered to be 90 percent of the population, and on social distancing and other protective measures.

“I did it to free myself of this doubt,” said Mr. Passaggio, who last month showed Covid-19 symptoms, including a low fever. Unable to get tested for the virus, as swabs are generally reserved for the hospitalized, he took precautions to avoid contaminating others, including staying isolated for weeks.

Last Tuesday he got the results: high levels of immunoglobulin G, or IgG, antibodies. Mr. Passaggio said he understood from doctors on television that it meant a degree of immunity, confidence he was not a danger to others and that he could possibly “go to work before someone else.”

Mr. Francese said that a month ago, 50 people in the town showed clear symptoms of the virus, but that the health authorities failed to test them. They got sick, and then their relatives got sick and cases kept building.

The town, out of desperation, tracked down serological tests offered by a Chinese factory that the mayor said had an 96 percent accuracy rate, and which he said Italy’s national research center had approved.

Robbio has already tested about half of its residents and found and isolated many positive cases, about half of whom had symptoms, he said. He also said that the tests had shown 12 percent had IgG antibodies.

He said he considered those people “immune and thus would be eligible” for an eventual immunity license, despite health experts’ doubts.

They worried that a vaccine would initially elude scientists, that weary citizens would abandon restrictions despite the risks, that the virus would be with us from now on.

Most experts believed that once the crisis was over, the nation and its economy would revive quickly. But there would be no escaping a period of intense pain.

Exactly how the pandemic will end depends in part on medical advances still to come. It will also depend on how individual Americans behave in the interim. If we scrupulously protect ourselves and our loved ones, more of us will live. If we underestimate the virus, it will find us.

More Americans may die than the White House admits.

The epidemiological model often cited by the White House, which was produced by the University of Washington’s Institute for Health Metrics and Evaluation, originally predicted 100,000 to 240,000 deaths by midsummer. Now that figure is 60,000.

The institute’s projection runs through Aug. 4, describing only the first wave of this epidemic. Without a vaccine, the virus is expected to circulate for years, and the death tally will rise over time.

Fatality rates depend heavily on how overwhelmed hospitals get and what percentage of cases are tested. China’s estimated death rate was 17 percent in the first week of January, when Wuhan was in chaos, according to a Center for Evidence-Based Medicine report, but only 0.7 percent by late February.

Various experts consulted by the Centers for Disease Control and Prevention in March predicted that the virus eventually could reach 48 percent to 65 percent of all Americans, with a fatality rate just under 1 percent, and would kill up to 1.7 million of them if nothing were done to stop the spread.

A model by researchers at Imperial College London cited by the president on March 30 predicted 2.2 million deaths in the United States by September under the same circumstances.

China has officially reported about 83,000 cases and 4,632 deaths, which is a fatality rate of over 5 percent. The Trump administration has questioned the figures but has not produced more accurate ones.

The tighter the restrictions, experts say, the fewer the deaths and the longer the periods between lockdowns. Most models assume states will eventually do widespread temperature checks, rapid testing and contact tracing, as is routine in Asia.

In this country, hospitals in several cities, including New York, came to the brink of chaos.

Only when tens of thousands of antibody tests are done will we know how many silent carriers there may be in the United States. The C.D.C. has suggested it might be 25 percent of those who test positive. Researchers in Iceland said it might be double that.

China is also revising its own estimates. In February, a major study concluded that only 1 percent of cases in Wuhan were asymptomatic. New research says perhaps 60 percent were.

The virus may also be mutating to cause fewer symptoms. In the movies, viruses become more deadly. In reality, they usually become less so, because asymptomatic strains reach more hosts. Even the 1918 Spanish flu virus eventually faded into the seasonal H1N1 flu.

The lockdowns will end, but haltingly.

it is likely a safe bet that at least 300 million of us are still vulnerable.

Until a vaccine or another protective measure emerges, there is no scenario, epidemiologists agreed, in which it is safe for that many people to suddenly come out of hiding. If Americans pour back out in force, all will appear quiet for perhaps three weeks.

The gains to date were achieved only by shutting down the country, a situation that cannot continue indefinitely. The White House’s “phased” plan for reopening will surely raise the death toll no matter how carefully it is executed.

Every epidemiological model envisions something like the dance

On the models, the curves of rising and falling deaths resemble a row of shark teeth.

Surges are inevitable, the models predict, even when stadiums, churches, theaters, bars and restaurants remain closed, all travelers from abroad are quarantined for 14 days, and domestic travel is tightly restricted to prevent high-intensity areas from reinfecting low-intensity ones.

In his wildly popular March 19 article in Medium, “Coronavirus: The Hammer and the Dance,” Tomas Pueyo correctly predicted the national lockdown, which he called the hammer, and said it would lead to a new phase, which he called the dance, in which essential parts of the economy could reopen, including some schools and some factories with skeleton crews.

Even the “Opening Up America Again” guidelines Mr. Trump issued on Thursday have three levels of social distancing, and recommend that vulnerable Americans stay hidden. The plan endorses testing, isolation and contact tracing — but does not specify how these measures will be paid for, or how long it will take to put them in place.

On Friday, none of that stopped the president from contradicting his own message by sending out tweets encouraging protesters in Michigan, Minnesota and Virginia to fight their states’ shutdowns.

China did not allow Wuhan, Nanjing or other cities to reopen until intensive surveillance found zero new cases for 14 straight days, the virus’s incubation period.

Compared with China or Italy, the United States is still a playground.Americans can take domestic flights, drive where they want, and roam streets and parks. Despite restrictions, everyone seems to know someone discreetly arranging play dates for children, holding backyard barbecues or meeting people on dating apps.

Even with rigorous measures, Asian countries have had trouble keeping the virus under control

But if too many people get infected at once, new lockdowns will become inevitable. To avoid that, widespread testing will be imperative.

Reopening requires declining cases for 14 days, the tracing of 90 percent of contacts, an end to health care worker infections, recuperation places for mild cases and many other hard-to-reach goals.

Immunity will become a societal advantage.

Imagine an America divided into two classes: those who have recovered from infection with the coronavirus and presumably have some immunity to it; and those who are still vulnerable.

“It will be a frightening schism,” Dr. David Nabarro, a World Health Organization special envoy on Covid-19, predicted. “Those with antibodies will be able to travel and work, and the rest will be discriminated against.”

Soon the government will have to invent a way to certify who is truly immune. A test for IgG antibodies, which are produced once immunity is established, would make sense

Dr. Fauci has said the White House was discussing certificates like those proposed in Germany. China uses cellphone QR codes linked to the owner’s personal details so others cannot borrow them.

As Americans stuck in lockdown see their immune neighbors resuming their lives and perhaps even taking the jobs they lost, it is not hard to imagine the enormous temptation to join them through self-infection

My daughter, who is a Harvard economist, keeps telling me her age group needs to have Covid-19 parties to develop immunity and keep the economy going,”

It would be a gamble for American youth, too. The obese and immunocompromised are clearly at risk, but even slim, healthy young Americans have died of Covid-19.

The virus can be kept in check, but only with expanded resources.

Resolve to Save Lives, a public health advocacy group run by Dr. Thomas R. Frieden, the former director of the C.D.C., has published detailed and strict criteria for when the economy can reopen and when it must be closed.

once a national baseline of hundreds of thousands of daily tests is established across the nation, any viral spread can be spotted when the percentage of positive results rises.

To keep the virus in check, several experts insisted, the country also must start isolating all the ill — including mild cases.

“If I was forced to select only one intervention, it would be the rapid isolation of all cases,”

In China, anyone testing positive, no matter how mild their symptoms, was required to immediately enter an infirmary-style hospital — often set up in a gymnasium or community center outfitted with oxygen tanks and CT scanners.

There, they recuperated under the eyes of nurses. That reduced the risk to families, and being with other victims relieved some patients’ fears.

Still, experts were divided on the idea of such wards

Ultimately, suppressing a virus requires testing all the contacts of every known case. But the United States is far short of that goal.

In China’s Sichuan Province, for example, each known case had an average of 45 contacts.

The C.D.C. has about 600 contact tracers and, until recently, state and local health departments employed about 1,600, mostly for tracing syphilis and tuberculosis cases.

China hired and trained 9,000 in Wuhan alone. Dr. Frieden recently estimated that the United States will need at least 300,000.

There will not be a vaccine soon.

any effort to make a vaccine will take at least a year to 18 months.

the record is four years, for the mumps vaccine.

for unclear reasons, some previous vaccine candidates against coronaviruses like SARS have triggered “antibody-dependent enhancement,” which makes recipients more susceptible to infection, rather than less. In the past, vaccines against H.I.V. and dengue have unexpectedly done the same.

A new vaccine is usually first tested in fewer than 100 young, healthy volunteers. If it appears safe and produces antibodies, thousands more volunteers — in this case, probably front-line workers at the highest risk — will get either it or a placebo in what is called a Phase 3 trial.

It is possible to speed up that process with “challenge trials.” Scientists vaccinate small numbers of volunteers, wait until they develop antibodies, and then “challenge” them with a deliberate infection to see if the vaccine protects them.

Normally, it is ethically unthinkable to challenge subjects with a disease with no cure, such as Covid-19.

“Fewer get harmed if you do a challenge trial in a few people than if you do a Phase 3 trial in thousands,” said Dr. Lipsitch, who recently published a paper advocating challenge trials in the Journal of Infectious Diseases. Almost immediately, he said, he heard from volunteers.

The hidden danger of challenge trials, vaccinologists explained, is that they recruit too few volunteers to show whether a vaccine creates enhancement, since it may be a rare but dangerous problem.

if a vaccine is invented, the United States could need 300 million doses — or 600 million if two shots are required. And just as many syringes.

“People have to start thinking big,” Dr. Douglas said. “With that volume, you’ve got to start cranking it out pretty soon.”

Treatments are likely to arrive first.

The modern alternative is monoclonal antibodies. These treatment regimens, which recently came very close to conquering the Ebola epidemic in eastern Congo, are the most likely short-term game changer, experts said.

as with vaccines, growing and purifying monoclonal antibodies takes time. In theory, with enough production, they could be used not just to save lives but to protect front-line workers.

Having a daily preventive pill would be an even better solution, because pills can be synthesized in factories far faster than vaccines or antibodies can be grown and purified.

Goodbye, ‘America First.’

A public health crisis of this magnitude requires international cooperation on a scale not seen in decades. Yet Mr. Trump is moving to defund the W.H.O., the only organization capable of coordinating such a response.

And he spent most of this year antagonizing China, which now has the world’s most powerful functioning economy and may become the dominant supplier of drugs and vaccines. China has used the pandemic to extend its global influence, and says it has sent medical gear and equipment to nearly 120 countries.

This is not a world in which “America First” is a viable strategy, several experts noted.

“If President Trump cares about stepping up the public health efforts here, he should look for avenues to collaborate with China and stop the insults,”

If we alienate the Chinese with our rhetoric, I think it will come back to bite us,” he said.“What if they come up with the first vaccine? They have a choice about who they sell it to. Are we top of the list? Why would we be?”

Once the pandemic has passed, the national recovery may be swift. The economy rebounded after both world wars, Dr. Mulder noted.

In one of the most provocative analyses in his follow-up article, “Coronavirus: Out of Many, One,” Mr. Pueyo analyzed Medicare and census data on age and obesity in states that recently resisted shutdowns and counties that voted Republican in 2016.

He calculated that those voters could be 30 percent more likely to die of the virus.

In the periods after both wars, Dr. Mulder noted, society and incomes became more equal. Funds created for veterans’ and widows’ pensions led to social safety nets, measures like the G.I. Bill and V.A. home loans were adopted, unions grew stronger, and tax benefits for the wealthy withered.

If a vaccine saves lives, many Americans may become less suspicious of conventional medicine and more accepting of science in general — including climate change

“Whether the Delta variant is already at that plateau, or whether there’s going to be further increases before it gets to that plateau, I can’t say. But I do think that plateau exists.”

Antibodies, which can prevent the virus from entering our cells, are engineered to latch onto specific molecules on the surface of the virus, snapping into place like puzzle pieces. But genetic mutations in the virus can change the shape of those binding sites.

“If you change that shape, you can make it impossible for an antibody to do its job,”

But as more people acquire antibodies against the virus, mutations that allow the virus to slip past these antibodies will become even more advantageous.

The good news is that there are many different kinds of antibodies, and a variant with a few new mutations is unlikely to escape them all, experts said.

its sleeve to counteract the evolution of the virus,” Dr. Pepper said. “Knowing that there is this complex level of diversity in the immune system allows me to sleep better at night.”

“It’s a lot harder to evade T cell responses than antibody responses,”

And then there are B cells, which generate our army of antibodies. Even after we clear the infection, the body keeps churning out B cells for a while, deliberately introducing small genetic mutations. The result is an enormously diverse collection of B cells producing an array of antibodies, some of which might be a good match for the next variant that comes along.

Whether the virus will become more virulent — that is, whether it will cause more serious disease — is the hardest to predict,

Unlike transmissibility or immune evasion, virulence has no inherent evolutionary advantage.

Some scientists predict that the virus will ultimately be much like the flu, which can still cause serious illness and death, especially during seasonal surges.

“The virus has no interest in killing us,” Dr. Metcalf said. “Virulence only matters for the virus if it works for transmission.”

It is too early to say whether SARS-CoV-2 will change in virulence over the long-term. There could certainly be trade-offs between virulence and transmission; variants that make people too sick too quickly may not spread very far.

Then again, this virus spreads before people become severely ill. As long as that remains true, the virus could become more virulent without sacrificing transmissibility.

Moreover, the same thing that makes the virus more infectious — faster replication or tighter binding to our cells — could also make it more virulent.

Although many possible paths remain open to us, what is certain is that SARS-CoV-2 will not stop evolving — and that the arms race between the virus and us is just beginning.

We lost the first few rounds, by allowing the virus to spread unchecked, but we still have powerful weapons to bring to the fight. The most notable are highly effective vaccines, developed at record speed. “I think there is hope in the fact that the SARS-CoV-2 vaccines at this point are more effective than flu vaccines have probably ever been,”

“I have great faith that we can sort any detrimental evolutionary trajectories out by improving our current or next generation vaccines,”

be you have a re-infection, but it’s relatively mild, which also boosts your immunity,”

rising vaccination rates may already be suppressing new mutations.And the evolution rate could also slow down as the virus becomes better adapted to humans.

“There’s low-hanging fruit,” Dr. Lauring said. “So there are certain ways it can evolve and make big improvements, but after a while there aren’t areas to improve — it’s figured out all the easy ways to improve.”

Eventually, as viral evolution slows down and our immune systems catch up, we will reach an uneasy equilibrium with the virus, scientists predict. We will never extinguish it, but it will smolder rather than rage.

So far, studies suggest that our antibody, T cell and B cell responses are all working as expected when it comes to SARS-CoV-2. “This virus is mostly playing by immunological rules we understand,”

Others are more optimistic. “My guess is that one day this is going to be another cause of the common cold,”

There are four other coronaviruses that have become endemic in human populations. We are exposed to them early and often, and all four mostly cause run-of-the-mill colds.

much of the world remains unvaccinated, and this virus has already proved capable of surprising us. “We should be somewhat cautious and humble about trying to predict what it is capable of doing in the future,”

While we can’t guard against every eventuality, we can tip the odds in our favor by expanding viral surveillance, speeding up global vaccine distribution and tamping down transmission until more people can be vaccinated

The future, he said, “depends much, much more on what humans do than on what the virus does.”

The pandemic may be global but, as Yonatan Grad, an immunologist at Harvard University, told me, “it is made up of hyperlocal epidemics that are differentially impacting communities.” If neighboring cities, states, or countries are at very different points in their outbreak trajectory, it could create difficult questions about when and how to reopen. The places that have best succeeded in stopping COVID-19 will be the ones most vulnerable to infections in the future. Singapore, for example, succeeded in containing the virus early on, only to see a huge surge of cases in March and April. “At some point,” Grad said, “we’re going to need to think about How do we all get to the same place?”

While the team found similar antibody levels in women vaccinated with both vaccines, Alter said they found higher levels of IgA antibodies in pregnant women who received the Moderna vaccine. She said this particular type of antibody may be transferred more efficiently to babies, for a longer period of time.

mRNA vaccines illicit antibodies in pregnant women that can be transferred to their babies, though this is the largest study on vaccines in pregnant women, to date. Pregnant and lactating women were not included in the initial clinical trials of the vaccines.

regnant people with Covid-19 are at increased risk for severe illness and may be at increased risk for adverse outcomes, like preterm birth. The CDC says it hopes to study vaccine safety in around 13,000 pregnant people for each of the three authorized coronavirus vaccines.

Several of my colleagues and students and I have statistically analyzed thousands of seasonal coronavirus cases in the United States and used a mathematical model to infer that immunity over a year or so is likely for the two seasonal coronaviruses most closely related to SARS-CoV-2 — an indication perhaps of how immunity to SARS-CoV-2 itself might also behave.

Mild illness, in other words, might not always build up protection. Similarly, it will be important to study the immune responses of people with asymptomatic cases of SARS-CoV-2 infection to determine whether symptoms, and their severity, predict whether a person becomes immune.

the issue might be resolved by comparing the viral genome sequence from the first and the second periods of infection.

it is reasonable to assume that only a minority of the world’s population is immune to SARS-CoV-2, even in hard-hit areas. How could this tentative picture evolve as better data come in? Early hints suggest that it could change in either direction.

One recent study (not yet peer-reviewed) suggests that rather than, say, 10 times the number of detected cases, the United States may really have more like 100, or even 1,000, times the official number

if this one is correct, then herd immunity to SARS-CoV-2 could be building faster than the commonly reported figures suggest.

another recent study (also not yet peer-reviewed) suggests that not every case of infection may be contributing to herd immunity. Of 175 Chinese patients with mild symptoms of Covid-19, 70 percent developed strong antibody responses, but about 25 percent developed a low response and about 5 percent developed no detectable response at all

An alternative possibility, which many scientists think is more likely, is that these patients had a false negative test in the middle of an ongoing infection, or that the infection had temporarily subsided and then re-emerged

The balance between these uncertainties will become clearer when more serologic surveys, or blood tests for antibodies, are conducted on large numbers of people. Such studies are beginning and should show results soon. Of course, much will depend on how sensitive and specific the various tests are: how well they spot SARS-CoV-2 antibodies when those are present and if they can avoid spurious signals from antibodies to related viruses.

Based on the volunteer experiments with seasonal coronaviruses and the antibody-persistence studies for SARS and MERS, one might expect a strong immune response to SARS-CoV-2 to protect completely against reinfection and a weaker one to protect against severe infection and so still slow the virus’s spread.

But designing valid epidemiologic studies to figure all of this out is not easy — many scientists, including several teams of which I’m a part — are working on the issue right now.

getting a handle on this fast is extremely important: not only to estimate the extent of herd immunity, but also to figure out whether some people can re-enter society safely, without becoming infected again or serving as a vector, and spreading the virus to others. Central to this effort will be figuring out how long protection lasts.

Experimental and statistical evidence suggests that infection with one coronavirus can offer some degree of immunity against distinct but related coronaviruses.

then there is the question of immune enhancement: Through a variety of mechanisms, immunity to a coronavirus can in some instances exacerbate an infection rather than prevent or mitigate it.

administering a vaccine against dengue fever, a flavivirus infection, can sometimes make the disease worse.

concern that they might be at play is one of the obstacles that have slowed the development of experimental vaccines against SARS and MERS.

The good news is that research on SARS and MERS has begun to clarify how enhancement works, suggesting ways around it, and an extraordinary range of efforts is underway to find a vaccine for Covid-19, using multiple approaches.

with Germany now carrying out about 50,000 coronavirus tests per day. Britain’s chief medical officer Chris Whitty on Tuesday said the United Kingdom had “a lot to learn” from Germany, which had “got ahead” in mass testing.

European governments are mulling the introduction of what have been dubbed “immunity passports” to let individuals who have had the virus return to a more normal life.

German scientists caution that this stage of the response is still a long way off. There remain crucial questions over how long someone’s immunity might last and at what level, and there are concerns about tests producing false positives and insufficient capacity for widespread testing

there needs to be more validation of the test and there needs to be science on how long immunity lasts.”

“This is one of many tools to address coronavirus, but it may be a very important one,”

antibody testing studies could provide an indication of how many people have been infected and thus help governments assess the risk of easing restrictions.

Knowing the level of infection among the population and particular risk groups could also let the government determine whether it can be “a little bit more relaxed” during a potential second wave of infections

In Germany, studies already underway or planned over the coming months involve a sampling of roughly 100,000 people

One by the University of Bonn will study 1,000 people from the hard-hit town of Heinsberg. Another in Munich will repeat tests on the same sample group regularly to monitor the spread.

Merkel, who has been tight-lipped on Germany’s strategy for lifting restrictions, says she doesn’t want to raise the population’s hopes prematurely. “We would be a bad government if we weren’t thinking about an exit strategy,”

it might take two years for the population to achieve herd immunity — meaning that so many people have been infected that the number of those still susceptible is no longer enough to sustain further spread of the virus.

the number of people infected could be as much as 10 times greater than now thought.

“If you want to assess the real danger and the real mortality, you have to know the number of people who have had the disease,”

knowing who is immune could be “extremely effective,” he said, for instance by helping identify which front-line workers are immune and do not need protective gear.

Testing accuracy also remains a concern. Some tests are overly sensitive, reacting to other types of coronaviruses and producing false positives. Errors could be particularly problematic when testing those who have not shown symptoms

Another concern is that tests are most effective several weeks after infection. So the timing of wide-scale studies is important,

“The question is, if someone tests for the antibody, what does that mean? Immunity? Part immunity? For how long?” he said. “If someone asked me, ‘What can I do with the results?’ I wouldn’t be able to answer.”

Kevin McCullough, a columnist for the conservative website Town Hall, wrote a column that ran last week with the headline “Antibody Testing: Proves We’ve Been Had!”

“Those higher numbers of case fatality rates, they’re still true,” Viboud said. “It’s your probability of dying if you’re clinically sick with it, which is something that people may care about, too.”

Moreover, the fatality rate of a virus, however it is defined, is not an innate feature of the pathogen. It depends on many variables, including the age and health of the population and access to health care.

The most controversial such study came out of Santa Clara County, Calif., the heart of Silicon Valley. It claimed that the true number of infections in the county in early April may have been 50 to 85 times the official coronavirus case counts. Extrapolating from that, it gave an estimated infection fatality rate of between 0.12 and 0.2 percent.

That provided what seemed like reliable ammunition to those who believe the danger of covid-19 has been overstated. They said the numbers show this disease is not much different from seasonal flu.

A commonly cited statistic about seasonal flu is that it has a fatality rate of 0.1 percent, That, however, is a case fatality rate. The infection fatality rate for flu is perhaps only half that, Viboud said. Shaman estimated that it’s about one-quarter the case fatality rate.

Shaman, the epidemiologist, said he hopes the higher estimates for infections and commensurately lower fatality rates are correct. Everyone needs more testing and better data, he said.

But even those high figures mean that within the whole population of the county, only 3% have been infected and have antibodies to the virus. A study in the Netherlands of 7,000 blood donors also found that just 3% had antibodies.

“We are working with a number of countries carrying out these serology studies,” she added. The WHO-supported studies would use robust methods and the tests would be validated for accuracy.

The hope will be that people who have had Covid-19 will be able to resume their lives. But Van Kerkhove last week said that even if tests showed a person had antibodies, it did not prove that they were immune.

“There are a lot of countries that are suggesting using rapid diagnostic serological tests to be able to capture what they think will be a measure of immunity,” she said. “Right now, we have no evidence that the use of a serological test can show that an individual has immunity or is protected from reinfection.”

The coronavirus vaccines, in contrast, are injected deep into the muscles and stimulate the immune system to produce antibodies. This appears to be enough protection to keep the vaccinated person from getting ill.Some of those antibodies will circulate in the blood to the nasal mucosa and stand guard there, but it’s not clear how much of the antibody pool can be mobilized, or how quickly. If the answer is not much, then viruses could bloom in the nose — and be sneezed or breathed out to infect others.

That leaves open the possibility that some vaccinated people get infected without developing symptoms, and could then silently transmit the virus — especially if they come in close contact with others or stop wearing masks.

The coronavirus vaccines have proved to be powerful shields against severe illness, but that is no guarantee of their efficacy in the nose. The lungs — the site of severe symptoms — are much more accessible to the circulating antibodies than the nose or throat, making them easier to safeguard

Pfizer will test a subset of its trial participants for antibodies against a viral protein called N. Because the vaccines have nothing to do with this protein, N antibodies would reveal whether the volunteers had become infected with the virus after immunization, said Jerica Pitts, a spokeswoman for the company.

Only people who have virus teeming in their nose and throat would be expected to transmit the virus, and the lack of symptoms in the immunized people who became infected suggests that the vaccine may have kept the virus levels in check.

Vaccinated people who have a high viral load but don’t have symptoms “would actually be, in some ways, even worse spreaders because they may be under a false sense of security,” Dr. Maldonado said.

While the vaccine had promising results in the lab, it's not known if it will protect people in the real world. The US Food and Drug Administration has cleared the company to begin Phase 2 trials, which typically involve several hundred of people, and Moderna plans to start large-scale clinical trials, known as Phase 3 trials, in July, which typically involve tens of thousands of people.

"We've demonstrated that these antibodies, this immune response, can actually block the virus," Zaks said. "I think this is a very important first step in our journey towards having a vaccine."

"It shows that not only did the antibody bind to the virus, but it prevented the virus from infecting the cells," said Dr. Paul Offit, a member of the NIH panel that's setting a framework for vaccine studies in the US.

In the Moderna study, three participants developed fever and other flu-like symptoms when they received the vaccine at a dose of 250 micrograms. Moderna anticipates the Phase 3 study on dosage will be between 25 and 100 micrograms.

So far, the Moderna study subjects who were vaccinated even at 25 and 100 micrograms achieved antibody levels similar to or even higher than people who naturally became infected with coronavirus.

“That amount of memory would likely prevent the vast majority of people from getting hospitalized disease, severe disease, for many years,” said Shane Crotty, a virologist at the La Jolla Institute of Immunology who co-led the new study.

A study published last week also found that people who have recovered from Covid-19 have powerful and protective killer immune cells even when antibodies are not detectable.

These studies “are all by and large painting the same picture, which is that once you get past those first few critical weeks, the rest of the response looks pretty conventional,” said Deepta Bhattacharya, an immunologist at the University of Arizona

A small number of infected people in the new study did not have long-lasting immunity after recovery, perhaps because of differences in the amounts of coronavirus they were exposed to.

Although antibodies in the blood are needed to block the virus and forestall a second infection — a condition known as sterilizing immunity — immune cells that “remember” the virus more often are responsible for preventing serious illness.

More often, people become infected a second time with a particular pathogen, and the immune system recognizes the invader and quickly extinguishes the infection. The coronavirus in particular is slow to do harm, giving the immune system plenty of time to kick into gear.

“It may be terminated fast enough that not only are you not experiencing any symptoms but you are not infectious,” Dr. Sette said.

Dr. Sette and his colleagues recruited 185 men and women, aged 19 to 81, who had recovered from Covid-19. The majority had mild symptoms not requiring hospitalization

The team tracked four components of the immune system: antibodies, B cells that make more antibodies as needed; and two types of T cells that kill other infected cells.

He and his colleagues found that antibodies were durable, with modest declines at six to eight months after infection, although there was a 200-fold difference in the levels among the participants.

The study is the first to chart the immune response to a virus in such granular detail, experts said. “For sure, we have no priors here,” Dr. Gommerman said. “We’re learning, I think for the first time, about some of the dynamics of these populations through time.”

Exactly how long immunity lasts is hard to predict, because scientists don’t yet know what levels of various immune cells are needed to protect from the virus. But studies so far have suggested that even small numbers of antibodies or T and B cells may be enough to shield those who have recovered.

My high level of vaccine antibodies probably explains my success more than my behavior. I make some effort to avoid Covid, but have been far from perfect. And I’ve been potentially exposed at least twice: Once last December, when someone at a small holiday gathering I’d attended developed symptoms the next day, and more recently, when I shared a large indoor space with two people who later tested positive. But according to my lab work, I’ve never had even a silent infection.

As for me, I have some data that can, to an extent, quantify and explain my own good luck in avoiding Covid so far. I’m participating in a study on immunity which allowed me to learn that my blood still carries loads of antibodies induced by my vaccine and December booster shot, and no signs of any prior infection. Not everyone’s antibodies wane at the same rate, and in some people, the antibodies don’t wane much at all. (At some point it should become routine to collect this information to help people decide whether to get additional booster shots.)

But one piece of advice almost no one is giving? Be lucky. Pediatrician Neil Stone says that there’s no “secret” for staying Covid-19 free because there’s just too much luck involved.

It’s possible I was protected by my high antibodies, or that some quirk of air flow meant I never breathed in enough virus to get sick. Or perhaps I benefited from a different form of luck. There’s another facet to immunity called the innate immune system, which acts as a first line of defense and sometimes knocks out a virus or other pathogen before it replicates enough to elicit the production of antibodies. Good innate immunity might help explain something many of us have experienced — not getting a cold or flu even when sleeping in the same bed with the sick person through the whole illness.

Stress, diet, general health and even sunlight might all affect innate immunity. So could other factors. There’s so much we still don’t know about the immune system. And that’s one reason we talk about “luck.”

understanding how the luck works could help other people avoid Covid, whether for the first time or for the second or third time. Taking a closer look at what passed for luck helped researchers like Gary Taubes discover that public health had obesity all wrong, and the standard high carb/low fat diets were causing people to gain weight.

The government hopes to use the findings to unravel a riddle that will allow Germany to move securely into the next phase of the pandemic: Which of the far-reaching social and economic restrictions that have slowed the virus are most effective and which can be safely lifted?

Other countries like Iceland and South Korea have tested broadly for infections, or combined testing with digital tracking to undercut the spread of the virus. But even the best laid plans can go awry; Singapore attempted to reopen only to have the virus re-emerge.

President Trump is in a hurry to restart the economy in an election year, but experts warn that much wider testing is needed to open societies safely.

Both Britain and the United States, where some of the first tests were flawed, virtually forfeited the notion of widespread testing early in their outbreaks and have since had to ration tests in places as they scramble to catch up

Germany, which produces most of its own high-quality test kits, is already testing on a greater scale than most — 120,000 a day and growing in a nation of 83 million.

Merkel, a trained scientist, said this week that the aim was nothing less than tracing “every infection chain.”

Every 10 people infected with the virus now pass it to seven others — a sharp decline in the infection rate for a virus that has spread exponentially.

The generosity and solidarity on such striking display inside of Europe’s largest and richest economy have been missing in Germany’s response to poorer European nations in the south, which were hit hardest by the virus.

the chancellor’s mixture of calm reassurance and clear-eyed realism — as well as her ability to understand the science and explain it to citizens — has been widely praised and encouraged Germans to follow social distancing rules. Her approval ratings are now higher than 80 percent.

That broad confidence in government has given Germany a tremendous advantage. It is much of the reason a knock on the door by a police officer and strangers dressed like aliens asking for blood can engender good will rather than alarm

“We are leading the thinking of what to do next.”

Its most ambitious project, aiming to test a nationwide random sample of 15,000 people across the country, is scheduled to begin next month.

“In the free world, Germany is the first country looking into the future,”

Nationally, the Robert Koch Institute, the government’s central scientific institution in the field of biomedicine, is testing 5,000 samples from blood banks across the country every two weeks and 2,000 people in four hot spots who are farther along in the cycle of the disease.

In Gangelt, a small town of about 12,000 in northwest Germany, tests of a first group of 500 residents found that 14 percent had antibodies to the virus. Another 2 percent tested positive for the coronavirus, raising hopes that about 15 percent of the local population may already have some degree of immunity.

“The process toward reaching herd immunity has begun,”

t may hold valuable insights for places that lag behind as the pandemic runs its course.

The mortality rate in the town, for example, turned out to be 0.37 percent, much lower than the national rate of 2.9 percent which is calculated based only on detected infections.

“We are at a crossroads,” said Mr. Hoelscher, the professor. “Are we going the route of loosening more and increasing immunity in the summer to slow the spread of this in the winter and gain more freedom to live public life? Or are we going to try to minimize transmissions until we have a vaccine?

“This is a question for politicians, not for scientists,” he added. “But politicians need the data to make an informed risk assessment.”

“I thought to myself if we’re going into lockdown, we need to start working on an exit strategy now,”

The next day, he said he wrote a short pitch to the Bavarian government. Six hours later, he had the green light. It took another three weeks until the test kits had arrived, a new lab was opened and teams of medics started fanning out across the city.

Six days after they first rung his doorbell, a doctor and two medical students came back to Mr. Germann’s apartment, household number 420 out of 3,000.They put on disposable protection suits, gloves and goggles and one of them sat down on a plastic stool they had brought along to take a small vial of his blood. Then they removed and bagged their suits, disinfected the stool and any surface they had touched and left. It took all of 10 minutes.

The treatments — a monoclonal antibody developed by Eli Lilly and a cocktail of two monoclonal antibodies developed by Regeneron — won emergency use authorization, or an E.U.A., from the Food and Drug Administration last month for outpatients with “mild to moderate” disease

“One of the challenges is the E.U.A. criteria really are so broad, it could be half of the people with Covid could qualify, but there is clearly not enough,”

Even some top officials at the F.D.A. — both career employees and political appointees — have privately expressed concern in recent months that people with connections to the White House appeared to be getting access to the antibody treatments,

In fact, the antibody treatments are so scarce that officials in Utah have developed a ranking system to determine who is most likely to benefit from the drugs, while Colorado is using a lottery system.

The infusions must be administered in outpatient settings, but infusion centers, which also care for immune-suppressed cancer patients, are loath to treat people who have an infectious disease.

Dr. Wynia in Colorado expressed concern that the therapies might not be distributed equitably across racial and ethnic lines, with hard-hit minority communities not getting their fair share.

Both Mr. Trump and Mr. Christie, a longtime friend of his and former New Jersey governor, got the antibodies before they were approved by the F.D.A. Dr. Caplan, the medical ethicist, said he had no problem with Mr. Trump, 74, getting the therapy — he is, after all, the president, “a special person unto him- or herself.”

“Exactly the same, his doctor sent me here; he talked me into it,” Mr. Giuliani said of Mr. Trump’s physician, adding, “The minute I took the cocktail yesterday, I felt 100 percent better. It works very quickly, wow.”

Once state and local health agencies determine which hospitals or medical facilities should get the drugs, they are shipped out by a third-party distributor. Then it is up to health care providers to figure out what to do with them.

“The notion that we are going to be able to treat a significant percentage of the people who qualify for the drug with the drug — it’s not going to happen,”

Scientists are only beginning to guess why the vaccine’s profile is improving with the passing months.

Levels of antibodies skyrocket in the first few weeks after immunization, but then rapidly wane. The J.&J. vaccine may produce antibodies that decline more slowly than those produced by the other vaccines, some research suggests. Or those antibodies may become more sophisticated over time, through a biological phenomenon called affinity maturation.

Perhaps, some researchers suggest, the vaccine offered a more robust defense against the Omicron variant, responsible for the huge increase in infections over the past few months. And studies have shown that the vaccine trains other parts of the immune system at least as well as the other two vaccines.

“It is a shame that we don’t have more direct study of outcomes among people who received J.&J.,” she said. That is in part because fewer people got the vaccine than the mRNA vaccines, she said, but also “because we’re relying on other countries generating data.”

Still, the data so far suggest that two doses of the J.&J. vaccine had an effectiveness of about 75 percent against hospitalization with the Omicron variant, comparable to the protection from the Pfizer-BioNTech vaccine. The researchers presented the findings last month at the Conference on Retroviruses and Opportunistic Infections in Denver.

Although the trial looked only at people who got two doses of the Johnson & Johnson vaccine, it suggests that the vaccine may make an excellent booster for people who initially got two doses of an mRNA vaccine, experts said.

When his hospital asked for volunteers for a study of an emerging Covid-19 therapy, Dr. Planer was among the first to sign up. His blood carries an especially valuable quantity of antibodies, Dr. Donato said.

While there is no evidence that convalescent-plasma treatments can help with Covid-19, the technique has been used to fight other viruses, including Ebola, influenza and severe acute respiratory syndrome, or SARS.

Enthusiasm for the potential treatment grew after a paper published in the Journal of the American Medical Association on March 27 suggested that a small study of five critically ill patients in China had shown promising results.

The National Covid-19 Convalescent Plasma Project is a related effort that began several weeks ago as a clearinghouse for information and a way to match willing plasma donors with hospitals and doctors authorized to perform infusions.

In addition to being healthy and showing no signs of infection after testing positive for the virus, potential donors must satisfy all other requirements for giving blood.

The issue of some vaccines being less effective after a single dose rather than two is well known, as well as the fact that protection is not immediate. While the first dose can take several weeks to promote an effective antibody response, the second dose can trigger different responses, supercharging the protection. Pfizer itself says a single dose of its vaccine is about 52% effective.

Questions over the effectiveness of the vaccine were raised amid reports that thousands of Israelis were still becoming sick after receiving the vaccine, although the public health services head, Sharon Alroy-Preis, said that in most cases this was because the individuals had not built up sufficient antibodies after being inoculated before being exposed to the virus.

Scientists don't understand why the variant has spread so explosively in Brazil, and the variant carries a particularly dangerous set of mutations.

One study estimated that the population should have reached herd immunity and the virus shouldn't be able to spread easily in the community. So why would the city see an even bigger surge 10 months later? Could P.1 be evading the antibodies made against the previous version of the virus, making reinfections easier?

"In fact, it was really quite a dramatic drop-off in sensitivity. We saw that in half of the serum, the antibodies were significantly less effective against the new variant [from South Africa]." So far, scientists haven't tested out P.1 in similar neutralization experiments, but P.1 has two mutations that scientists have already shown reduce antibody binding.