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Now, based on the U.S. response since February, Lipsitch believes that we’re still likely to see the virus spread to the point of becoming endemic.

That would mean it is with us indefinitely, and the current pandemic would end when we reach levels of “herd immunity,” traditionally defined as the threshold at which enough people in a group have immune protection so the virus can no longer cause huge spikes in disease.

Anthony Fauci, the head of the National Institute of Allergy and Infectious Diseases, said that, because of a “general anti-science, anti-authority, anti-vaccine feeling,” the U.S. is “unlikely” to achieve herd immunity even after a vaccine is available.

The case-fatality rate for COVID-19 is now very roughly 1 percent overall. In the absolute simplest, linear model, if 70 percent of the world were to get infected, that would mean more than 54 million deaths.

Without a better plan, this threshold—the percentage of people who have been infected that would constitute herd immunity—seems to have become central to our fates.

Some mathematicians believe that it’s much lower than initially imagined. At least, it could be, if we choose the right future.

Gomes explains, “There doesn’t need to be a lot of variation in a population for epidemics to slow down quite drastically.”

in dynamic systems, the outcomes are more like those in chess: The next play is influenced by the previous one. Differences in outcome can grow exponentially, reinforcing one another until the situation becomes, through a series of individually predictable moves, radically different from other possible scenarios. You have some chance of being able to predict the first move in a game of chess, but good luck predicting the last.

“selective depletion” of people who are more susceptible—can quickly decelerate a virus’s spread. When Gomes uses this sort of pattern to model the coronavirus’s spread, the compounding effects of heterogeneity seem to show that the onslaught of cases and deaths seen in initial spikes around the world are unlikely to happen a second time.

Based on data from several countries in Europe, she said, her results show a herd-immunity threshold much lower than that of other models.“We just keep running the models, and it keeps coming back at less than 20 percent,” Gomes said. “It’s very striking.”

If that proves correct, it would be life-altering news. It wouldn’t mean that the virus is gone. But by Gomes’s estimates, if roughly one out of every five people in a given population is immune to the virus, that seems to be enough to slow its spread to a level where each infectious person is infecting an average of less than one other person

That’s the classic definition of herd immunity. It would mean, for instance, that at 25 percent antibody prevalence, New York City could continue its careful reopening without fear of another major surge in cases.

Lipsitch also believes that heterogeneity is important to factor into any model. It was one reason he updated his prediction, not long after we spoke in February, of what the herd-immunity threshold would be. Instead of 40 to 70 percent, he lowered it to 20 to 60 percent. When we spoke last week, he said he still stands by that, but he is skeptical that the number lands close to the 20 percent end of the range. “I think it’s unlikely,” he said, but added, “This virus is proving there can be orders-of-magnitude differences in attack rates, depending on political and societal decisions, which I don’t know how to forecast.”

he believes that the best we can do is continually update models based on what is happening in the real world. She can’t say why the threshold in her models is consistently at or below 20 percent, but it is. “If heterogeneity isn’t the cause,” she said, “then I’d like for someone to explain what is.”

Biological variations in susceptibility could come down to factors as simple as who has more nose hair, or who talks the loudest and most explosively, and Langwig shares the belief that these factors can create heterogeneity in susceptibility and transmission. Those effects can compound to dramatically change the math behind predictions for the future.

What’s important to her, rather, is that people are not misled by the idea of herd immunity. In the context of vaccination, herd-immunity thresholds are relatively fixed and predictable. In the context of an ongoing pandemic, thinking of this threshold as some static concept can be dangerously misleading.

She worries that many people conflate academic projections about reaching herd immunity with a “let it run wild” fatalism. “My view is that trying to take that route would lead to mass death and devastation,” she says.

Left totally unchecked, Bansal says, the percentage of infected people could go even higher than 70 percent.

“Within certain populations that lack heterogeneity, like within a nursing home or school, you may even see the herd-immunity threshold be above 70 percent,” Bansal says. If a population average led people in those settings to get complacent, there could be needless death.

Bansal believes that heterogeneity of behavior is the key determinant of our futures. “That magic number that we’re describing as a herd-immunity threshold very much depends on how individuals behave,” Bansal says, since R0 clearly changes with behaviors. On average, the R0 of the coronavirus currently seems to be between 2 and 3, according to Lipsitch.

Social distancing and other reactive measures changed the R0 value, and they will continue to do so. The virus has certain immutable properties, but there is nothing immutable about how many infections it causes in the real world.

The threshold can change based on how a virus spreads. The spread keeps on changing based on how we react to it at every stage, and the effects compound. Small preventive measures have big downstream effects

In other words, the herd in question determines its immunity. There is no mystery in how to drop the R0 to below 1 and reach an effective herd immunity: masks, social distancing, hand-washing, and everything everyone is tired of hearing about. It is already being done.

“I think it no longer seems impossible that Switzerland or Germany could remain near where they are in terms of cases, meaning not very much larger outbreaks, until there’s a vaccine,” he said. They seem to have the will and systems in place to keep their economies closed enough to maintain their current equilibrium.

Other wealthy countries could hypothetically create societies that are effectively immune to further surges, where the effective herd-immunity threshold is low.

We have the wealth in this country to care for people, and to set the herd-immunity threshold where we choose. Parts of the world are illuminating a third way forward, something in between total lockdown and simply resuming the old ways of life. It happens through individual choices and collective actions, reimagining new ways of living, and having the state support and leadership to make those ways possible

as much attention as we give to the virus, and to drugs and our immune systems, the variable in the system is us. There will only be as much chaos as we allow.

But I do think it’s going to be a while before this virus becomes completely normal. And I’ve never been convinced that this current generation of elderly people will ever get to a place where it is completely normal. If you’re 65 or 75 or even older — it’s really hard to teach an immune system new tricks if you’re that age

And so while we may see excess mortality in the elderly decline somewhat, I don’t think we’ll see it ever disappear for this generation who was already old when the pandemic hit. Many will never develop that robust, long-term immunological memory we would want to see — and which happens naturally to someone who’s been exposed hundreds of times since they were a little baby.

There’s a similar story with measles. There is no routine later-life sequelae, like shingles, for measles. But what we do see is that, in measles outbreaks today, there are some people who were vaccinated who get it anyway. Maybe 5 to 15 percent of cases are not immunologically naïve people, but vaccinated people.

Is it really the case that, as babies, we are fighting off those viruses hundreds of times?The short answer is yeah. We start seeing viruses when we’re 2 months old, when we’re a month old. And a lot of these viruses we’ve seen literally tens, if not hundreds of times for some people by the time we’re adults. People tend to think that immunity is binary — you’re either immune or you’re not. That couldn’t be farther from the truth. It’s a gradient, and your protection gets stronger the more times you see a virus.

We used to think we just had this spectacular immune response when we first encountered the virus at, say, age 6, and that the immune response lasted until we were 70. But actually what we were seeing was the effect of an immune system being retrained every time it came into contact with the virus after the initial infection — at 6, and 7, and 8, and so on. Every time your friend got chickenpox, or your neighbor, you got a massive boost. You were re-upping your immune response and diversifying your immunological tools — potentially multiple times a year, a kind of natural booster.

But now, in America, kids get chickenpox vaccines. So you don’t have kids in America getting chickenpox today, and never will. But that means that older Americans, who did get it as kids, are not being exposed again — certainly not multiple times each year. And it turns out that, in the absence of routine re-exposures, that first exposure alone isn’t nearly as good at driving lifelong immunity and warding off shingles until your immune system begins to fall apart in old age — it can last until you’re in your 30s, for example but not until your 70s.

With Covid, when it infects you, it can land in your upper respiratory tract and it just start replicating right there. Immediately, it’s present and replicating in your lungs and in your nose. And that alone elicits enough of an immune response to cause us to feel really crappy and even cause us to feel disease.

But we could have just set the narrative better at the beginning: Look, you might get sick again, but your risk of landing in the hospital is going to be really low, and if you get a booster, you might still get sick again, but your risk of landing in the hospital is going to be even lower. That’s something I think humans can deal with, and I think the public could have understood it.

But it’s why we don’t see the severe disease as much, with a second exposure or an exposure after vaccination: For most people, it’s not getting into the heart and the liver and stuff nearly as easily.

But it doesn’t have to. It’s still causing symptomatic disease. And maybe mucosal vaccines could stop this, but without them we’re likely to continue seeing infections and even symptomatic infections.

through most of 2020 and into 2021, though. Back then, I think the conventional wisdom was that a single exposure — through infection or vaccination — would be the end of the pandemic for you. If this is basic virology and immunology, how did we get that so wrong?

The short answer is that epidemiologists are not immunologists and immunologists are not virologists and virologists are not epidemiologists. And, in general, physicians don’t know anything about the details.

But this failure had some pretty concrete impacts. When reinfections first began popping up, people were surprised, they were scared, and then, to some degree, they lost trust in vaccines. And the people they were turning to for guidance — not only did they not warn us about that, they were slow to acknowledge it, as well.

It had dramatic impacts and ripple effects that will last for years to limit our ability to get populations properly vaccinated.

the worst thing we can do during a pandemic is set inappropriately high expectations. These vaccines are incredible, they’ve had an enormously positive impact on mortality, but they were never going to end the pandemic.

And now, there’s a huge number of people questioning, do these vaccines even do anything?

For babies born today, though, I really think they’re not going to view Covid as any different than other viruses. By the time they are 20, it will be like any other virus to them. Because their immune systems will have grown up with it.

Instead, we set society up for failure, since people feel like the government failed everyone, that biology failed us, and that this was a crazy virus that has broken all the rules of our immune system, when it’s just doing what we’ve always known it would do.

How do you wish we had messaged things differently? What would it have meant to communicate early and clearly that Covid was a textbook virus, as you say?I think the biggest thing would have been just to say, we understand the enemy.

To say that this is a textbook virus, it doesn’t mean that it’s not killing people. Objectively, it’s still killing more people than any other infectious disease

What it means is that we could’ve taken action based on what we knew, rather than waiting around to prove everything and publish papers in Nature and Science talking about things we already knew.

We could have prepared for November and December of 2020 and then for November and December of 2021. But everyone kept saying, we don’t know if it’s going to come back. We knew it was going to come back and it makes me want to cry to think about it. We did nothing and hundreds of thousands of people died. We didn’t prepare nursing homes because we all got to the summer of 2020 and we said, cross our fingers.

We knew how tests worked. We knew about serial testing and why it was important for a public health approach. We knew that vaccines could have really good impacts once they were around. And if you were looking through the correct lens, we even knew that they weren’t going to stop transmission.

We didn’t have to live in a world where we were flying blind. We could have lived in a world where we’re knowledgeable. But instead, we chose almost across the board to will ourselves into this state of fear and anxiety.

And that really started in the earliest days. Almost the first experience I had was a lot like that movie with Jennifer Lawrence —Don’t Look Up.

none of this was complicated. You just had to ask a simple question: what would happen if you took away all immunity from an adult? Well, once you control for no immunity, adults are going to get very, very sick.

Of course, by and large, babies didn’t get very sick from this disease.Babies are immunologically naïve, but they are also resilient. A virus can tear up a baby, but a baby can repair its tissue so fast. Adults don’t have that. It’s just like a baby getting a cut. They’ll heal really quick

An adult getting a cut — you go by age, and every decade of age that you are, it’s going to take exponentially longer for that wound to heal. Eventually get to 80 or 90 and the wound can’t even heal. In the immunology world, this is called “tolerance.”

why are all these organ systems getting damaged when other viruses don’t seem to do that? It’s natural to think, it’s Covid — this is a weird disease. But it’s much more a story about immunity and how it develops than about the virus or the disease. None of our organ systems had any immune defenses around to help them out. And I think that the majority of post-acute sequelae and multi-organ complications and long Covid — they are not the result of the virus being a crazy different virus, but are a result of this virus replicating in an environment where there were such absent or exceedingly low defenses.

Is it the same whenever we encounter a virus for the first time?Think about travelers. Travelers get way more sick from a local disease than people who grew up with that virus. If you get malaria as a traveler, you’re much more likely to get really sick. You don’t see everyone in Nicaragua taking chloroquine every day. But you definitely see travelers taking it, because malaria can be deadly for adults.

What about, not severity, but post-acute complications — do we have long malaria? Do we have liver complications from dengue?

The really hard part of answering that question is there’s just not enough data on the frequency of long-term effects, because nothing like this has ever happened at such scale. It’s like everyone in Europe and North America suddenly traveled to a country where malaria was endemic.

Or think about H.I.V. It essentially kills your immune system, and once the immune barriers are down, other viruses that used to infect humans would get into tissues that we didn’t like them to get into. If there wasn’t such a clear signal of a loss of CD-4 T cells to explain it, people might still be scratching their heads and going, man, I wonder why all these patients are getting fungal infections. Well, there’s a virus there that’s depleting their immune system.

Covid is absolutely waking the world up to this — to the fact that there are really weird long-term sequelae to viruses when they infect organ systems that would normally be protected. And I think we’re going to find that more and more cancers are being attributed to viral infections.

It wasn’t that long ago that we first learned that most cases of cervical cancer were caused by H.P.V. — I think the 1980s. And now we have a vaccine for H.P.V. and rates of cervical cancer have fallen by two-thirds.

what about incidence? We’ve talked at a few points about how important it is to think about all of these questions in terms of the scale. What is the right scale for thinking about future long Covid, for instance, or other post-acute sequelae?

I think the absolute risk, per infection, is going down and down and down. That’s just true.

he U.K.’s Office of National Statistics, which shows a much lower risk of developing long Covid now, from reinfection, than from an initial infection earlier in the pandemic.

the worst is definitely behind us, which is a good thing, especially for people who worry that the problems will keep building and a lot of people — or even everyone — will get long Covid symptoms. I don’t think there’s a world where we’re looking at the babies of today dealing with long Covid at any meaningful scale.

a lot of the fear right now comes from the worst cases, and there’s a lot of worst cases. Even one of the people that I know well, I know in their mind they’re worried that they’ll never recover, but I think objectively they are recovering slowly. It might not be an eight month course. It might be a year and a half. But they will get better. Most of us will.

But for those hoping that 25 percent represents a true ceiling for pandemic spread in a given community, well, it almost certainly does not, considering that recent serological surveys have shown that perhaps 93 percent of the population of Iquitos, Peru, has contracted the disease; as have more than half of those living in Indian slums; and as many as 68 percent in particular neighborhoods of New York City

overshoot of that scale would seem unlikely if the “true” threshold were as low as 20 or 25 percent.

But, of course, that threshold may not be the same in all places, across all populations, and is surely affected, to some degree, by the social behavior taken to protect against the spread of the disease.

we probably err when we conceive of group immunity in simplistically binary terms. While herd immunity is a technical term referring to a particular threshold at which point the disease can no longer spread, some amount of community protection against that spread begins almost as soon as the first people are exposed, with each case reducing the number of unexposed and vulnerable potential cases in the community by one

you would not expect a disease to spread in a purely exponential way until the point of herd immunity, at which time the spread would suddenly stop. Instead, you would expect that growth to slow as more people in the community were exposed to the disease, with most of them emerging relatively quickly with some immune response. Add to that the effects of even modest, commonplace protections — intuitive social distancing, some amount of mask-wearing — and you could expect to get an infection curve that tapers off well shy of 60 percent exposure.

Looking at the data, we see that transmissions in many severely impacted states began to slow down in July, despite limited interventions. This is especially notable in states like Arizona, Florida, and Texas. While we believe that changes in human behavior and changes in policy (such as mask mandates and closing of bars/nightclubs) certainly contributed to the decrease in transmission, it seems unlikely that these were the primary drivers behind the decrease. We believe that many regions obtained a certain degree of temporary herd immunity after reaching 10-35 percent prevalence under the current conditions. We call this 10-35 percent threshold the effective herd immunity threshold.

Indeed, that is more or less what was recently found by Youyang Gu, to date the best modeler of pandemic spread in the U.S

he cautioned again that he did not mean to imply that the natural herd-immunity level was as low as 10 percent, or even 35 percent. Instead, he suggested it was a plateau determined in part by better collective understanding of the disease and what precautions to take

Gu estimates national prevalence as just below 20 percent (i.e., right in the middle of his range of effective herd immunity), it still counts, I think, as encouraging — even if people in hard-hit communities won’t truly breathe a sigh of relief until vaccines arrive.

If you can get real protection starting at 35 percent, it means that even a mediocre vaccine, administered much more haphazardly to a population with some meaningful share of vaccination skeptics, could still achieve community protection pretty quickly. And that is really significant — making both the total lack of national coordination on rollout and the likely “vaccine wars” much less consequential.

At least 20 percent of the public, and perhaps 50 percent, had some preexisting, cross-protective T-cell response to SARS-CoV-2, according to one much-discussed recent paper. An earlier paper had put the figure at between 40 and 60 percent. And a third had found an even higher prevalence: 81 percent.

The T-cell story is similarly encouraging in its big-picture implications without being necessarily paradigm-changing

These numbers suggest their own heterogeneity — that different populations, with different demographics, would likely exhibit different levels of cross-reactive T-cell immune response

The most optimistic interpretation of the data was given to me by Francois Balloux, a somewhat contrarian disease geneticist and the director of the University College of London’s Genetics Institute

According to him, a cross-reactive T-cell response wouldn’t prevent infection, but would probably mean a faster immune response, a shorter period of infection, and a “massively” reduced risk of severe illness — meaning, he guessed, that somewhere between a third and three-quarters of the population carried into the epidemic significant protection against its scariest outcomes

the distribution of this T-cell response could explain at least some, and perhaps quite a lot, of COVID-19’s age skew when it comes to disease severity and mortality, since the young are the most exposed to other coronaviruses, and the protection tapers as you get older and spend less time in environments, like schools, where these viruses spread so promiscuously.

Balloux told me he believed it was also possible that the heterogeneous distribution of T-cell protection also explains some amount of the apparent decline in disease severity over time within countries on different pandemic timelines — a phenomenon that is more conventionally attributed to infection spreading more among the young, better treatment, and more effective protection of the most vulnerable (especially the old).

Going back to Youyang Gu’s analysis, what he calls the “implied infection fatality rate” — essentially an estimated ratio based on his modeling of untested cases — has fallen for the country as a whole from about one percent in March to about 0.8 percent in mid-April, 0.6 percent in May, and down to about 0.25 percent today.

even as we have seemed to reach a second peak of coronavirus deaths, the rate of death from COVID-19 infection has continued to decline — total deaths have gone up, but much less than the number of cases

In other words, at the population level, the lethality of the disease in America has fallen by about three-quarters since its peak. This is, despite everything that is genuinely horrible about the pandemic and the American response to it, rather fantastic.

there may be some possible “mortality displacement,” whereby the most severe cases show up first, in the most susceptible people, leaving behind a relatively protected population whose experience overall would be more mild, and that T-cell response may play a significant role in determining that susceptibility.

That, again, is Balloux’s interpretation — the most expansive assessment of the T-cell data offered to me

The most conservative assessment came from Sarah Fortune, the chair of Harvard’s Department of Immunology

Fortune cautioned not to assume that cross-protection was playing a significant role in determining severity of illness in a given patient. Those with such a T-cell response, she told me, would likely see a faster onset of robust response, yes, but that may or may not yield a shorter period of infection and viral shedding

Most of the scientists, doctors, epidemiologists, and immunologists I spoke to fell between those two poles, suggesting the T-cell cross-immunity findings were significant without necessarily being determinative — that they may help explain some of the shape of pandemic spread through particular populations, but only some of the dynamics of that spread.

he told me he believed, in the absence of that data, that T-cell cross-immunity from exposure to previous coronaviruses “might explain different disease severity in different people,” and “could certainly be part of the explanation for the age skew, especially for why the very young fare so well.”

the headline finding was quite clear and explicitly stated: that preexisting T-cell response came primarily via the variety of T-cells called CD4 T-cells, and that this dynamic was consistent with the hypothesis that the mechanism was inherited from previous exposure to a few different “common cold” coronaviruses

“This potential preexisting cross-reactive T-cell immunity to SARS-CoV-2 has broad implications,” the authors wrote, “as it could explain aspects of differential COVID-19 clinical outcomes, influence epidemiological models of herd immunity, or affect the performance of COVID-19 candidate vaccines.”

“This is at present highly speculative,” they cautioned.

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.

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 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

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 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.

In severe cases, however, the clash between the virus and the immune system rages much longer. Other parts of the body, including those not directly affected by the virus, become collateral damage, prompting serious and potentially life-threatening symptoms

On Friday, the president received an experimental antibody cocktail developed by drug maker Regeneron. The next day he began a course of the antiviral remdesivir. Experts say such treatments might be best administered early in infection, to rein in the virus before it runs amok.

A typical immune response launches its defense in two phases. First, a cadre of fast-acting fighters rushes to the site of infection and attempts to corral the invader. This so-called innate response buys the rest of the immune system time to mount a second, more tailored attack, called the adaptive response, which kicks in about a week later, around the time the first wave begins to wane.

Eventually, a second wave of immune cells and molecules arrives, more targeted than their early counterparts and able to home in on the coronavirus and the cells it infects.

If the innate immune system makes early progress against the virus, the infection may be mild. But if the body’s defenses flag, the coronavirus may continue replicating, ratcheting up the viral load. Faced with a growing threat, innate immune cells will continue to call for help, fueling a vicious cycle of recruitment and destruction. Prolonged, excessive inflammation can cause life-threatening damage to vital organs like the heart, kidneys and lungs.

On Sunday, President Trump’s doctors reported that he had also received a course of dexamethasone, a steroid that broadly blunts the immune response by curbing the activity of several cytokines. Dexamethasone has been shown to reduce death rates in hospitalized Covid-19 patients who are ill enough to require ventilation or supplemental oxygen. But it is far less likely to help and may even harm patients at an earlier stage of infection, or those who have milder disease. Experts say that administering dexamethasone inappropriately, or too soon, could undermine a helpful immune response, allowing the virus to ravage the body.

At 74 years old and about 240 pounds, Mr. Trump occupies a high-risk age group and verges on obesity, a condition that can exacerbate the severity of Covid-19. Men also tend to have a poorer disease prognosis.

“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.

How long might immunity to the coronavirus last? Years, maybe even decades, according to a new study

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.

Consider a calculation to determine the threshold for herd immunity: Vc=(1− 1/R0)/E. "Vc" is the proportion of people who must be vaccinated to protect the rest of the herd, "R0," pronounced R-naught, is an estimate of the number of secondary cases from the original infected person and "E" represents effectiveness of a given vaccine against transmission. And this, which resembles a brutal SAT math section entry, is the dumbed-down version.

This is not a fund-raiser with a fixed universal goal we all are striving to reach. The above equation evaluates the nation as a homogenized entity, but people live in communities

In other words, susceptible cows should be culled to lessen the risk of new infections

Though, of course, the fix -- culling -- is not an option for human disease, the benefit of an immune herd is self evident.

Fast forward to the 21st century world of vaccines. Pandemics and health care are decidedly more complex, which has led all to wonder: what is the magic number of people we need to vaccinate so we can all forget these disastrous last 14 months?close dialogOur free Provoke/Persuade newsletter compiles the week’s most thought-provoking pieces and delivers them straight to your inbox. Please enter aboveSign me upBy subscribing you agree to ourprivacy policy.Thanks for Subscribing!Continue ReadingBy subscribing you agree to ourprivacy policy.close dialog/* effects for .bx-campaign-1295603 *//* custom css .bx-campaign-1295603 *//* custom css from creative 52220 */.bxc.bx-custom.bx-campaign-1295603 .bx-row-image-logo img { height: 42px;}@media screen and (max-width:736px) { .bxc.bx-custom.bx-campaign-1295603 .bx-row-image-logo img { height: 35px;}}/*Validation border*/.bxc.bx-custom.bx-campaign-1295603 .bx-row-validation .bx-input { border-color: #B50000; /*Specify border color*/ border-width: 1px; box-shadow: none; background-color: transparent; color: #B50000; /*Specify text color*/}/* rendered styles .bx-campaign-1295603 */.bxc.bx-campaign-1295603.bx-active-step-1 .bx-creative:before {min-height: 220px;}.bxc.bx-campaign-1295603.bx-active-step-1 .bx-creative {border-color: #c1c1c1;border-style: solid;background-size: contain;background-color: white;border-width: 1px 0;border-radius: 0;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603.bx-active-step-1 .bx-creative:before {min-height: 200px;}}.bxc.bx-campaign-1295603.bx-active-step-1 .bx-creative> *:first-child {width: 780px;vertical-align: middle;padding: 10px;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603.bx-active-step-1 .bx-creative> *:first-child {width: 340px;padding: 20px;}}.bxc.bx-campaign-1295603.bx-active-step-1 .bx-close {stroke: rgb(193, 193, 193);stroke-width: 2px;width: 24px;height: 24px;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603.bx-active-step-1 .bx-close {width: 30px;height: 30px;padding: 0 0 10px 10px;}}.bxc.bx-campaign-1295603 .bx-group-1295603-y4M7jyO {width: 660px;text-align: left;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603 .bx-group-1295603-y4M7jyO {text-align: center;width: 315px;}}.bxc.bx-campaign-1295603 .bx-element-1295603-tVcUlRZ {padding: 0;width: auto;}.bxc.bx-campaign-1295603 .bx-element-1295603-tVcUlRZ> *:first-child {background-color: transparent;background-size: contain;}.bxc.bx-campaign-1295603 .bx-group-1295603-BpRQ7DR {width: 660px;text-align: left;padding: 25px 0 15px;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603 .bx-group-1295603-BpRQ7DR {width: 310px;padding: 15px 0 15px;text-align: center;}}.bxc.bx-campaign-1295603 .bx-element-1295603-oUX5Jvf {width: 100%;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603 .bx-element-1295603-oUX5Jvf {width: auto;}}.bxc.bx-campaign-1295603 .bx-element-1295603-oUX5Jvf> *:first-child {font-family: CNN Business,CNN,Helvetica Neue,Helvetica,Arial,Utkal,sans-serif;font-weight: 400;font-size: 24px;line-height: 1.1em;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603 .bx-element-1295603-oUX5Jvf> *:first-child {font-size: 16px;padding: 6px 0 0;line-height: 1.2;}}.bxc.bx-campaign-1295603 .bx-group-1295603-PZ8dLrW {width: 660px;padding: 0;min-width: 550px;text-align: left;}@media all and (max-width: 736px) {.bxc.bx-campaign-1295603 .bx-group-1295603-PZ8dLrW {min-width: auto;width: 310px;padding: 0;}}@media all and (min-wi

Lessening the threat of fetal loss therefore was straightforward: farmers should "retain" immune cattle -- those who had already had a spontaneous abortion -- and not waste "material, time, and energy ... on animals of doubtful value." Rather, they advised to butcher the non-immune cows and concentrate on the immune, "profitable" ones.

A famous mumps outbreak in adolescent boys from the Orthodox Jewish community is thought to have been exacerbated by the school practice of promoting close, sustained (15 hours a day) contact with a study partner ("chavrusa") including "animated" face-to-face discussion resulting in transmission despite the fact that most had been vaccinated years before.

Stated more simply, the herd likely is protected at a very different percent of vaccinated people in an Orthodox Jewish community in San Diego where people live near the school and walk to most activities compared to a gated community in a Minneapolis suburb where many prefer to keep to themselves.

We have received a master class in viral variants in recent months, witnessing day by day the alarming uptick in new cases as the B.1.1.7. variant has been introduced to new communities. But a single R-naught cannot fit all variants of Covid-19; a community with higher rates of B.1.1.7. and, therefore, a higher R-naught will require, among other things, a higher level of vaccination to designate the herd as sufficiently immune.

There is not one magic number to signal to the entire country that we have finally made it;

This is extremely important to keep in mind in the weeks and months ahead as we continue to vaccinate and wait and vaccinate and wait, chasing a number that is fundamentally misleading.

The heterogeneity of human behavior, geography and the virus itself explains the vagueness of the pronouncements of Dr. Anthony Fauci, the chief medical adviser to President Joe Biden, and other experts as they seek to evade specifying just how many more people need vaccination before we officially can claim victory.

As we have seen in the US during the 15-month arc of the pandemic, trust in science and scientists has been the key to progress. Masks work. Vaccines work. Certain medications work.

Two important factors seem to be playing a role in Brazil’s resurgence. The first is that, after a COVID-19 infection, the natural immunity that our bodies develop seems to vary in strength and permanence. Protection wanes after infection with most respiratory viruses, including coronaviruses

The solution, then, depends on vaccination. The immune response that the vaccines create is generally more robust than the immune response we get after being infected by a virus, and should buy a population more protected time than would a surge in exposure to the virus

it at least has a capacity to infect people who have already recovered from COVID-19, even if their defenses protect them against other versions of the virus.

The mutations that help the virus spread and evade immune responses have arisen independently in multiple places. Combined with waning immunity, these factors underscore the challenge before the world: Populations may still be vulnerable to disaster scenarios just when things seem to be getting better.

the virus’s capacity to cause such a deadly second surge in Brazil suggests a dangerous evolutionary potential.

New, dangerous variants are all but inevitable when there are extremely high levels of transmission of the virus. As more people gain immunity, the selective pressure on the virus will favor the variants that can most effectively evade immune responses

the basic nature of evolutionary biology means that the virus should be expected to evolve in ways that circumvent defense mechanisms. Evidence that it is already doing so has been clear in the latest vaccine trial data.

the variant in Brazil, known as the P.1 (or B.1.1.248) lineage, has a potent combination of mutations. Not only does this variant seem to be more transmissible; its lineage carries mutations that help it escape the antibodies that we develop in response to older lineages of the coronavirus.

Wealthy countries have time to avoid a fate like Brazil’s through immediate, efficient vaccination.

In most places, however, this is not close to happening. And as of last week, only one of the world’s 29 poorest countries had vaccinated anyone at all. A study in the journal BMJ estimated that vaccines will not be available to more than a fifth of the world’s population until 2022.

The coronavirus’s constantly evolving nature is a stark reminder that the entire world is in this crisis together.

Vaccine distribution is more than just an issue of justice or morality

Ensuring that every human is vaccinated is in everyone’s interest, as global distribution of vaccines is the most effective way to drive down the virus’s capacity to replicate and evolve.

The key will be bringing down the global rates of transmission as quickly as possible—not getting any single country to 100 percent vaccination while dozens of countries roil.

“It is truly confounding that wealthier nations think that hoarding vaccines is the way to protect their citizens from a global pandemic that doesn’t respect borders,”

As the virus currently surges across Africa, some 2.5 million health-care workers are unvaccinated. “Clearly, the failure to address vaccine allocation based on health and epidemiological needs, rather than national interest, is now promising to have a dire impact on the world’s ability to achieve rapid, global control of COVID,”

Certain countries will approach herd immunity by vaccinating almost every citizen. Other countries could see mass casualties and catastrophic waves of reinfection—potentially with variants that evolved in response to the immunity conferred by the very vaccines to which these populations do not have access. In the process, these hot spots themselves will facilitate rapid evolution, giving rise to even more variants that could make the vaccinated populations susceptible to disease once again

The countries that hoard the vaccine without a plan to help others do so at their own peril.

he and other researchers estimated that population heterogeneity shaves the coronavirus herd immunity rate to 43 percent. Others say it might be lower.

Gabriela Gomes, a mathematician at the University of Strathclyde in Glasgow, has scrutinized European cities overwhelmed by the disease. In a paper she wrote with nine other researchers, which hasn’t yet been peer-reviewed, she arrived at a striking conclusion: Herd immunity could be lower than 20 percent.

many researchers have been reluctant to say whether they believe the worst in some of the hardest-hit cities has already passed. No one knows how long immunity lasts. The virus could mutate.

“I would love that as well. But the reality is that it’s wishful thinking. It’s confirmation bias. We can’t pick evidence we hope is true. We have to be very careful about this because it could blow up in your face very quickly.”

Officials weren’t willing to impose a lockdown. In impoverished Manaus, where many already live on the brink, the mayor said it would lead to social chaos and violence. So Pinheiro Alves spent his off hours trying to jury-rig ventilators.

“There isn’t a concrete explanation,” said Henrique dos Santos Pereira, a scientist at the Federal University of Amazonas. Maybe there’s an unseen biological immunity in the population. Or the city’s relative youth staved off the worst.

“The problem is that we don’t know how many people are susceptible,” dos Santos Pereira said. “In the beginning, we were thinking it was everywhere, but it doesn’t seem like the whole world is susceptible … It is causing us to reconsider the theory of herd immunity.”

Virgílio, the mayor, hopes the scientists are right. The medical system in Manaus has failed once. If a second wave does come, he has little doubt what would happen.“Our capacities would be overwhelmed.”

What’s more, even if it is determined that reinfection cannot occur shortly after recovery, it could still happen later if immunity is only seasonal

If reinfection is indeed possible, we need to know whether it will result in disease that is milder or more severe.

While antibodies to a previous infection generally reduce risk the second time around, for some viruses, such as dengue fever, they can lead to severe and even fatal disease.

3. The virus could continue living inside youWe don’t know that recovered patients actually clear the virus from their bodies. Many viruses can remain in reservoirs, parts of the body where they hang out quietly, and re-emerge to cause disease later in life

We now know that in some patients, detectable virus can be found in feces and even blood after apparent recovery. Does the coronavirus remain in the body, or are these just residual bits of virus?

2. Reinfection could be possibleThere are documented cases where people who appear to recover from the virus test positive again, which calls even short-term immunity into question.

5. Survivors could suffer long-term damage

We do know that previously healthy people are being left with potentially long-term lung and heart damage.

6. A ‘mild’ case is hardly mildAs more patients recount enduring painful coughing, disorientation and difficulties breathing, people are coming to understand that the 80 percent to 85 percent of cases considered mild are not necessarily “mild” in its usual sense

7. There’s no shortcut to immunityHerd immunity requires a high proportion of a population to be immune (the actual percentage varies for different infections), but we want to get there slowly or, ideally, through vaccines

Slowing down the spread of the coronavirus won’t just save lives in the coming few months; it also gives us time to study treatments, and to expand or reconfigure hospital services for Covid-19 patients.

We need to keep in mind that the science is moving fast right now. It is unprecedented to see such an intensive effort internationally being put into studying one disease.

While it is hard to be patient, the best way out of this will likely be much clearer to us in a month or two than it is now

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.”

That’s a large gamble based on very little scientific understanding.

Contrary to the popular understanding, Sweden has not done nothing. Social distancing is in place, people are to work from home where possible and to isolate when even mildly symptomatic. Steps have been taken to shield vulnerable citizens

WHO mortality data shows Italy, the United Kingdom, Spain and the United States all have higher death rates, despite having imposed significant mitigation and suppression protocols.

by traditional metrics of deaths/capita, cases/capita, or even crude case fatality rate, they ended up seemingly in a worse position than Sweden, where bars remain open and coffee culture is booming.

A strategy should be based on a broad empirically derived consensus which takes into account both what we know, and what we do not.

We don’t know how many people have a strong immune response to the virus.

We don’t know how long that immune response lasts.

We don’t know how many people have been infected and how those infections might manifest.

We don’t know how many individuals need immunity to produce effective herd immunity.

And we don’t know if immunity to the current strain would carry over to future strains.

Even more bracing: We are unlikely to find definitive answers to these questions in the near future.

One of the key perceived advantages of Sweden’s approach was the expected reduction in economic damage. But Sweden’s economy is projected to contract by 7 percent to 10 percent of GDP this year, only slightly better than its Nordic neighbors who shut down their societies and have far less morbidity and mortality

All of that said, it is unlikely that we’ll be able to say definitively whether or not Sweden’s choice was optimal any time soon. Even comparing key metrics like cases and deaths between countries is difficult because of methodological differences in collecting data

When we consider economic damages, long-term sequelae of infection and the differential impact across marginalized groups, direct comparisons becomes extremely challenging.

sometimes there are no good decisions; only less bad ones. Sweden may be on the road to herd immunity but even if they are in fact on the path, the road is neither swift nor smooth. And it is littered with friends and relatives.

that would be on par with what we experience with flu viruses, which scientists estimate hit us about every two to five years, less often in adulthood. It also matches up well with the documented cadence of the four other coronaviruses that seasonally trouble humans, and cause common colds.

For now, every infection, and every subsequent reinfection, remains a toss of the dice. “Really, it’s a gamble,” says Ziyad Al-Aly, a clinical epidemiologist and long-COVID researcher at Washington University in St. Louis. Vaccination and infection-induced immunity may load the dice against landing on severe disease, but that danger will never go away completely, and scientists don’t yet know what happens to people who contract “mild” COVID over and over again

Or maybe not. This virus seems capable of tangling into just about every tissue in the body, affecting organs such as the heart, brain, liver, kidneys, and gut; it has already claimed the lives of millions, while saddling countless others with symptoms that can linger for months or years.

considering our current baseline, “less dangerous” could still be terrible—and it’s not clear exactly where we’re headed. When it comes to reinfection, we “just don’t know enough,”

Perhaps, as several experts have posited since the pandemic’s early days, SARS-CoV-2 will just become the fifth cold-causing coronavirus.

A third or fourth bout might be more muted still; the burden of individual diseases may be headed toward an asymptote of mildness that holds for many years

Future versions of SARS-CoV-2 could continue to shape-shift out of existing antibodies’ reach, as coronaviruses often do. But the body is flush with other fighters that are much tougher to bamboozle—among them, B cells and T cells that can quash a growing infection before it spirals out of control

Those protections tend to build iteratively, as people see pathogens or vaccines more often. People vaccinated three times over, for instance, seem especially well equipped to duke it out with all sorts of SARS-CoV-2 variants, including Omicron and its offshoots.

promising patterns: Second infections and post-vaccination infections “are significantly less severe,” she told me, sometimes to the point where people don’t notice them at all

Bodies, wised up to the virus’s quirks, can now react more quickly, clobbering it with sharper and speedier strikes.

“There are still very good reasons” to keep exposures few and far between, Landon, of the University of Chicago, told me. Putting off reinfection creates fewer opportunities for harm: The dice are less likely to land on severe disease (or chronic illness) when they’re rolled less often overall. It also buys us time to enhance our understanding of the virus, and improve our tools to fight it.

Immunity, though, is neither binary nor permanent. Even if SARS-CoV-2’s assaults are blunted over time, there are no guarantees about the degree to which that happens, or how long it lasts.

A slew of factors could end up weighting the dice toward severe disease—among them, a person’s genetics, age, underlying medical conditions, health-care access, and frequency or magnitude of exposure to the virus.

for everyone else, no amount of viral dampening can totally eliminate the chance, however small it may be, of getting very sick.

Long COVID, too, might remain a possibility with every discrete bout of illness. Or maybe the effects of a slow-but-steady trickle of minor, fast-resolving infections would sum together, and bring about the condition.

Every time the body’s defenses are engaged, it “takes a lot of energy, and causes tissue damage,” Thomas told me. Should that become a near-constant barrage, “that’s probably not great for you.”

Bodies are resilient, especially when they’re offered time to rest, and she doubts that reinfection with a typically ephemeral virus such as SARS-CoV-2 would cause mounting damage. “The cumulative effect is more likely to be protective than detrimental,” she said, because of the immunity that’s laid down each time.

people who have caught the virus twice or thrice may be more likely to become long-haulers than those who have had it just once.

Some other microbes, when they reinvade us, can fire up the immune system in unhelpful ways, driving bad bouts of inflammation that burn through the body, or duping certain defensive molecules into aiding, rather than blocking, the virus’s siege. Researchers don’t think SARS-CoV-2 will do the same. But this pathogen is “much more formidable than even someone working on coronaviruses would have expected,

Seasonal encounters with pathogens other than SARS-CoV-2 don’t often worry us—but perhaps that’s because we’re still working to understand their toll. “Have we been underestimating long-term consequences from other repeat infections?” Thomas said. “The answer is probably, almost certainly, yes.”

the rhythm of reinfection isn’t just about the durability of immunity or the pace of viral evolution. It’s also about our actions and policies, and whether they allow the pathogen to transmit and evolve. Strategies to avoid infection—to make it as infrequent as possible, for as many people as possible—remain options, in the form of vaccination, masking, ventilation, paid sick leave, and more.

Gordon and Swartz are both hopeful that the slow accumulation of immunity will also slash people’s chances of developing long COVID.

The outlooks of the experts I spoke with spanned the range from optimism to pessimism, though all agreed that uncertainty loomed. Until we know more, none were keen to gamble with the virus—or with their own health. Any reinfection will likely still pose a threat, “even if it’s not the worst-case scenario,” Abdool Karim told me. “I wouldn’t want to put myself in that position.”

For at-home fitness essentials, Browning recommends getting a set of yellow, green and red resistance bands (the colors correspond with varying levels of resistance). "These can be used for back, bicep, triceps, shoulders and leg work," Browning said.

And don't forget about the joy of dancing

While that program has a subscription-based app, you can also find free workouts on Alexia Clark's Instagram and IGTV.

If you haven't tried mediation, now might be a good time to start. A recent review involving 20 randomized, controlled trials including more than 1,600 people suggested that meditation may help keep our immune system functioning optimally.

"Life is messy, and although meditation isn't a cure all it can help us to remember to breathe and that we'll never be able to clean it all up," Gluck said.

Research dating back over 25 years has revealed that psychological stress increases susceptibility to illness (PDF).

To calm our anxiety during this stressful time, first acknowledge that it is okay to feel stressed, anxious and afraid. "It is okay to feel panicked ... look for ways to ground yourself in a safe and healthy way that does not cause harm to others," Forti said.

"Holding on to rigid patterns of thinking exacerbates stress and anxiety," Forti said. "Flexibility is required during this time of uncertainty and rapid change."

In my home, that means working with several interruptions, and allowing my girls to have some access to TikTok on my iPhone, along with some extra cookies.

Lastly, get your z's. Not doing so can negatively affect your immune system, according to the National Sleep Foundation.

According to the NSF, taking two naps that are no longer than 30 minutes each — one in the morning and one in the afternoon — has been shown to help decrease stress and offset the negative effects that sleep deprivation has on the immune system. If that's not realistic, a 20-minute catnap during a lunch break or before dinner can help too

To contain such a pathogen, nations must develop a test and use it to identify infected people, isolate them, and trace those they’ve had contact with. That is what South Korea, Singapore, and Hong Kong did to tremendous effect. It is what the United States did not.

That a biomedical powerhouse like the U.S. should so thoroughly fail to create a very simple diagnostic test was, quite literally, unimaginable. “I’m not aware of any simulations that I or others have run where we [considered] a failure of testing,”

The testing fiasco was the original sin of America’s pandemic failure, the single flaw that undermined every other countermeasure. If the country could have accurately tracked the spread of the virus, hospitals could have executed their pandemic plans, girding themselves by allocating treatment rooms, ordering extra supplies, tagging in personnel, or assigning specific facilities to deal with COVID-19 cases.

None of that happened. Instead, a health-care system that already runs close to full capacity, and that was already challenged by a severe flu season, was suddenly faced with a virus that had been left to spread, untracked, through communities around the country.

With little room to surge during a crisis, America’s health-care system operates on the assumption that unaffected states can help beleaguered ones in an emergency.

That ethic works for localized disasters such as hurricanes or wildfires, but not for a pandemic that is now in all 50 states. Cooperation has given way to competition

Partly, that’s because the White House is a ghost town of scientific expertise. A pandemic-preparedness office that was part of the National Security Council was dissolved in 2018. On January 28, Luciana Borio, who was part of that team, urged the government to “act now to prevent an American epidemic,” and specifically to work with the private sector to develop fast, easy diagnostic tests. But with the office shuttered, those warnings were published in The Wall Street Journal, rather than spoken into the president’s ear.

Rudderless, blindsided, lethargic, and uncoordinated, America has mishandled the COVID-19 crisis to a substantially worse degree than what every health expert I’ve spoken with had feared. “Much worse,”

“Beyond any expectations we had,” said Lauren Sauer, who works on disaster preparedness at Johns Hopkins Medicine. “As an American, I’m horrified,” said Seth Berkley, who heads Gavi, the Vaccine Alliance. “The U.S. may end up with the worst outbreak in the industrialized world.”

it will be difficult—but not impossible—for the United States to catch up. To an extent, the near-term future is set because COVID-19 is a slow and long illness. People who were infected several days ago will only start showing symptoms now, even if they isolated themselves in the meantime. Some of those people will enter intensive-care units in early April

A “massive logistics and supply-chain operation [is] now needed across the country,” says Thomas Inglesby of Johns Hopkins Bloomberg School of Public Health. That can’t be managed by small and inexperienced teams scattered throughout the White House. The solution, he says, is to tag in the Defense Logistics Agency—a 26,000-person group that prepares the U.S. military for overseas operations and that has assisted in past public-health crises, including the 2014 Ebola outbreak.

The first and most important is to rapidly produce masks, gloves, and other personal protective equipment

it would also come at a terrible cost: SARS-CoV-2 is more transmissible and fatal than the flu, and it would likely leave behind many millions of corpses and a trail of devastated health systems.

This agency can also coordinate the second pressing need: a massive rollout of COVID-19 tests.

These measures will take time, during which the pandemic will either accelerate beyond the capacity of the health system or slow to containable levels. Its course—and the nation’s fate—now depends on the third need, which is social distancing.

There are now only two groups of Americans. Group A includes everyone involved in the medical response, whether that’s treating patients, running tests, or manufacturing supplies. Group B includes everyone else, and their job is to buy Group A more time. Group B must now “flatten the curve” by physically isolating themselves from other people to cut off chains of transmission.

Given the slow fuse of COVID-19, to forestall the future collapse of the health-care system, these seemingly drastic steps must be taken immediately, before they feel proportionate, and they must continue for several weeks.

Persuading a country to voluntarily stay at home is not easy, and without clear guidelines from the White House, mayors, governors, and business owners have been forced to take their own steps.

when the good of all hinges on the sacrifices of many, clear coordination matters—the fourth urgent need

Pundits and business leaders have used similar rhetoric, arguing that high-risk people, such as the elderly, could be protected while lower-risk people are allowed to go back to work. Such thinking is seductive, but flawed. It overestimates our ability to assess a person’s risk, and to somehow wall off the ‘high-risk’ people from the rest of society. It underestimates how badly the virus can hit ‘low-risk’ groups, and how thoroughly hospitals will be overwhelmed if even just younger demographics are falling sick.

A recent analysis from the University of Pennsylvania estimated that even if social-distancing measures can reduce infection rates by 95 percent, 960,000 Americans will still need intensive care.

There are only about 180,000 ventilators in the U.S. and, more pertinently, only enough respiratory therapists and critical-care staff to safely look after 100,000 ventilated patients. Abandoning social distancing would be foolish. Abandoning it now, when tests and protective equipment are still scarce, would be catastrophic.

If Trump stays the course, if Americans adhere to social distancing, if testing can be rolled out, and if enough masks can be produced, there is a chance that the country can still avert the worst predictions about COVID-19, and at least temporarily bring the pandemic under control. No one knows how long that will take, but it won’t be quick. “It could be anywhere from four to six weeks to up to three months,” Fauci said, “but I don’t have great confidence in that range.”

there are three possible endgames: one that’s very unlikely, one that’s very dangerous, and one that’s very long.

The first is that every nation manages to simultaneously bring the virus to heel, as with the original SARS in 2003. Given how widespread the coronavirus pandemic is, and how badly many countries are faring, the odds of worldwide synchronous control seem vanishingly small.

The second is that the virus does what past flu pandemics have done: It burns through the world and leaves behind enough immune survivors that it eventually struggles to find viable hosts. This “herd immunity” scenario would be quick, and thus tempting

The U.S. has fewer hospital beds per capita than Italy. A study released by a team at Imperial College London concluded that if the pandemic is left unchecked, those beds will all be full by late April. By the end of June, for every available critical-care bed, there will be roughly 15 COVID-19 patients in need of one.  By the end of the summer, the pandemic will have directly killed 2.2 million Americans,

The third scenario is that the world plays a protracted game of whack-a-mole with the virus, stamping out outbreaks here and there until a vaccine can be produced. This is the best option, but also the longest and most complicated.

there are no existing vaccines for coronaviruses—until now, these viruses seemed to cause diseases that were mild or rare—so researchers must start from scratch.

The first steps have been impressively quick. Last Monday, a possible vaccine created by Moderna and the National Institutes of Health went into early clinical testing. That marks a 63-day gap between scientists sequencing the virus’s genes for the first time and doctors injecting a vaccine candidate into a person’s arm. “It’s overwhelmingly the world record,” Fauci said.

The initial trial will simply tell researchers if the vaccine seems safe, and if it can actually mobilize the immune system. Researchers will then need to check that it actually prevents infection from SARS-CoV-2. They’ll need to do animal tests and large-scale trials to ensure that the vaccine doesn’t cause severe side effects. They’ll need to work out what dose is required, how many shots people need, if the vaccine works in elderly people, and if it requires other chemicals to boost its effectiveness.

No matter which strategy is faster, Berkley and others estimate that it will take 12 to 18 months to develop a proven vaccine, and then longer still to make it, ship it, and inject it into people’s arms.

as the status quo returns, so too will the virus. This doesn’t mean that society must be on continuous lockdown until 2022. But “we need to be prepared to do multiple periods of social distancing,” says Stephen Kissler of Harvard.

First: seasonality. Coronaviruses tend to be winter infections that wane or disappear in the summer. That may also be true for SARS-CoV-2, but seasonal variations might not sufficiently slow the virus when it has so many immunologically naive hosts to infect.

Second: duration of immunity. When people are infected by the milder human coronaviruses that cause cold-like symptoms, they remain immune for less than a year. By contrast, the few who were infected by the original SARS virus, which was far more severe, stayed immune for much longer.

scientists will need to develop accurate serological tests, which look for the antibodies that confer immunity. They’ll also need to confirm that such antibodies actually stop people from catching or spreading the virus. If so, immune citizens can return to work, care for the vulnerable, and anchor the economy during bouts of social distancing.

Aspects of America’s identity may need rethinking after COVID-19. Many of the country’s values have seemed to work against it during the pandemic. Its individualism, exceptionalism, and tendency to equate doing whatever you want with an act of resistance meant that when it came time to save lives and stay indoors, some people flocked to bars and clubs.

“We can keep schools and businesses open as much as possible, closing them quickly when suppression fails, then opening them back up again once the infected are identified and isolated. Instead of playing defense, we could play more offense.”

The vaccine may need to be updated as the virus changes, and people may need to get revaccinated on a regular basis, as they currently do for the flu. Models suggest that the virus might simmer around the world, triggering epidemics every few years or so. “But my hope and expectation is that the severity would decline, and there would be less societal upheaval,”

After infections begin ebbing, a secondary pandemic of mental-health problems will follow.

But “there is also the potential for a much better world after we get through this trauma,”

Testing kits can be widely distributed to catch the virus’s return as quickly as possible. There’s no reason that the U.S. should let SARS-CoV-2 catch it unawares again, and thus no reason that social-distancing measures need to be deployed as broadly and heavy-handedly as they now must be.

Pandemics can also catalyze social change. People, businesses, and institutions have been remarkably quick to adopt or call for practices that they might once have dragged their heels on, including working from home, conference-calling to accommodate people with disabilities, proper sick leave, and flexible child-care arrangements.

Perhaps the nation will learn that preparedness isn’t just about masks, vaccines, and tests, but also about fair labor policies and a stable and equal health-care system. Perhaps it will appreciate that health-care workers and public-health specialists compose America’s social immune system, and that this system has been suppressed.

Attitudes to health may also change for the better. The rise of HIV and AIDS “completely changed sexual behavior among young people who were coming into sexual maturity at the height of the epidemic,”

Years of isolationist rhetoric had consequences too.

“People believed the rhetoric that containment would work,” says Wendy Parmet, who studies law and public health at Northeastern University. “We keep them out, and we’ll be okay. When you have a body politic that buys into these ideas of isolationism and ethnonationalism, you’re especially vulnerable when a pandemic hits.”

Pandemics are democratizing experiences. People whose privilege and power would normally shield them from a crisis are facing quarantines, testing positive, and losing loved ones. Senators are falling sick. The consequences of defunding public-health agencies, losing expertise, and stretching hospitals are no longer manifesting as angry opinion pieces, but as faltering lungs.

After COVID-19, attention may shift to public health. Expect to see a spike in funding for virology and vaccinology, a surge in students applying to public-health programs, and more domestic production of medical supplies.

The lessons that America draws from this experience are hard to predict, especially at a time when online algorithms and partisan broadcasters only serve news that aligns with their audience’s preconceptions.

“The transitions after World War II or 9/11 were not about a bunch of new ideas,” he says. “The ideas are out there, but the debates will be more acute over the next few months because of the fluidity of the moment and willingness of the American public to accept big, massive changes.”

One could easily conceive of a world in which most of the nation believes that America defeated COVID-19. Despite his many lapses, Trump’s approval rating has surged. Imagine that he succeeds in diverting blame for the crisis to China, casting it as the villain and America as the resilient hero.

One could also envisage a future in which America learns a different lesson. A communal spirit, ironically born through social distancing, causes people to turn outward, to neighbors both foreign and domestic. The election of November 2020 becomes a repudiation of “America first” politics. The nation pivots, as it did after World War II, from isolationism to international cooperation

The U.S. leads a new global partnership focused on solving challenges like pandemics and climate change.

In 2030, SARS-CoV-3 emerges from nowhere, and is brought to heel within a month.

On the Global Health Security Index, a report card that grades every country on its pandemic preparedness, the United States has a score of 83.5—the world’s highest. Rich, strong, developed, America is supposed to be the readiest of nations. That illusion has been shattered. Despite months of advance warning as the virus spread in other countries, when America was finally tested by COVID-19, it failed.

“The messaging has been really confusing, and I think that was really unfortunate,” says Petra Klepac, who is also an infectious-disease modeler at the London School of Hygiene and Tropical Medicine. “It’s been a case of how not to communicate during an outbreak,”

critics of the U.K. strategy argue that swift, decisive action matters more than future hypotheticals do. The country’s current caseload puts it only a few weeks behind Italy, where more than 24,000 cases have so overburdened hospitals that doctors must now make awful decisions about whom to treat.

First, we don’t know how long immunity against the new coronavirus, SARS-CoV-2, lasts. When people are infected with OC43 and HKU1—two other coronaviruses that regularly circulate among humans and cause common colds—they stay immune for less than a year. By contrast, immunity against the first SARS virus (from 2003) holds for much longer. No one knows whether SARS-CoV-2 will hew to either of these extremes, and according to one recent study, its behavior could mean anything from annual outbreaks to a decades-long quiet spell.

making a decent long-term strategy is hard when there are still two big unknowns that substantially affect how the pandemic will progress

a more granular analysis across Chinese provinces showed that the virus can still easily spread in humid areas, and a third modeling study concluded that “SARS-CoV-2 can proliferate at any time of year.” The bottom line: There’s a very wide range of possible futures.

South Korea, by contrast, seems to have brought COVID-19 to heel through a combination of social-distancing measures and extensive testing. Hong Kong, Singapore, and Taiwan have been similarly successful.

In a similar letter, more than 500 behavioral scientists called on the government to disclose the evidence behind its contention that the public will experience “behavioral fatigue” if restrictions are put in place too early.

Without strong guidance, British institutions and citizens have begun making their own decisions, going well over what the government recommends. Universities haven’t been told to close, but many have, sending students home, moving classes and exams online, and postponing graduations. Many care homes will not be admitting visitors. Soccer leagues have been suspended. The Queen has canceled public engagements. The Scottish government is planning to close schools and expand testing.

“We really need people to engage and to sustain individual control measures, like social distancing, for months at a time,” Klepac adds. “We’re in this for the long term and we need everyone to do their part. It is a very big ask.

One of the first questions experts asked me was whether my family was vaccinated. Yes, I said: My husband and I are vaccinated and boosted, and our kids are vaccinated but not yet boosted. This is a relevant question because, if you’re exposed to the virus that causes Covid-19, “your immune system kicks into action a lot faster if you’re vaccinated versus not vaccinated,”

First, the swift immune reaction slows the rate of viral reproduction and spread. “This is what the vaccines are there for — to educate your immune system so that it gets a jump on the invaders before they are able to replicate out of control,”

Because the virus doesn’t replicate as quickly in vaccinated people, they may be less likely to test positive for Covid-19 after coronavirus exposure, because their immune system “keeps the viral load below the level of detection,”

It’s possible, then, that my husband and son did catch Covid-19, but their vaccinated immune systems fended off the infection so well that they never had enough viral proteins in their nose or throat to test positive. And their continual negative tests probably meant that they were never that contagious

If my husband and son never tested positive, why did they feel sick? Even if a vaccinated person doesn’t have much virus in their body, they can still have powerful Covid symptoms

That’s because many illness symptoms — fever, malaise, runny nose, fatigue — are actually caused by the immune system’s response to the virus, rather than the virus itself

And as for why I felt fine, Dr. Morrison said that perhaps my immune system fought off the incoming virus so quickly that I didn’t even have a chance to feel sick. “It sounds to me like you were definitely exposed,”

maybe I had high levels of vaccine antibodies or immune cells called T cells that were able to kill the invading virus before it had a chance to alert the parts of my immune system that would incite symptoms.

All this said, nobody really knows what happened to me, my son or my husband. When it comes to understanding how Covid-19 affects the body, “there are so many open questions,”

people can have different experiences for many different reasons. For instance, Dr. Andino said, it’s possible that the virus was replicating in parts of my husband’s or my son’s body that the tests didn’t reach

Research suggests that the coronavirus can replicate in the pancreas, heart, brain, kidneys and other organs, although vaccination may reduce the chance that the virus spreads outside the respiratory system.

Dr. Andino said that he and his colleagues have been conducting studies in which they follow and repeatedly test entire households after one person in the home tests positive for Covid-19. “What we see is exactly what you described — that some people in the household don’t test positive,”

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.