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A month later, I believe that the transmission of SARS-CoV-2 via aerosols matters much more than has been officially acknowledged to date.

This confirms the results of a study from late May (not peer-reviewed) in which Covid-19 patients were found to release SARS-CoV-2 simply by exhaling — without coughing or even talking. The authors of that study said the finding implied that airborne transmission “plays a major role” in spreading the virus.

Accepting these conclusions wouldn’t much change what is currently being recommended as best behavior. The strongest protection against SARS-CoV-2, whether the virus is mostly contained in droplets or in aerosols, essentially remains the same: Keep your distance and wear masks.

Rather, the recent findings are an important reminder to also be vigilant about opening windows and improving airflow indoors. And they are further evidence that the quality of masks and their fit matter, too.

here is no neat and no meaningful cutoff point — at 5 microns or any other size — between droplets and aerosols: All are tiny specks of liquid, their size ranging along a spectrum that goes from very small to really microscopic.

Yes, droplets tend to fly through the air like mini cannonballs and they fall to the ground rather quickly, while aerosols can float around for many hours.

The practical implications are plain:Social distancing really is important. It keeps us out of the most concentrated parts of other people’s respiratory plumes. So stay away from one another by one or two meters at least — though farther is safer.

“The smaller the exhaled droplets, the more important the short-range airborne route.”

Can you walk into an empty room and contract the virus if an infected person, now gone, was there before you? Perhaps, but probably only if the room is small and stuffy.

Can the virus waft up and down buildings via air ducts or pipes? Maybe, though that hasn’t been established.

another, recent, preprint (not peer reviewed) about the Diamond Princess concluded that “aerosol inhalation was likely the dominant contributor to Covid-19 transmission” among the ship’s passengers.

It might seem logical, or make intuitive sense, that larger droplets would contain more virus than do smaller aerosols — but they don’t.

The Lancet Respiratory Medicine that analyzed the aerosols produced by the coughs and exhaled breaths of patients with various respiratory infections found “a predominance of pathogens in small particles” (under 5 microns). “There is no evidence,” the study also concluded, “that some pathogens are carried only in large droplets.”

I believe that, taken together, much of the evidence gathered to date suggests that close-range transmission by aerosols is significant — possibly very significant, and certainly more significant than direct droplet spray.

But basic physics also says that a 5-micron droplet takes about a half-hour to drop to the floor from the mouth of an adult of average height — and during that time, the droplet can travel many meters on an air current. Droplets expelled in coughs or sneezes also travel much farther than one meter.

Wear a mask. Masks help block aerosols released by the wearer. Scientific evidence is also building that masks protect the wearer from breathing in aerosols around them.

When it comes to masks, size does matter.

My lab has been testing cloth masks on a mannequin, sucking in air through its mouth at a realistic rate. We found that even a bandanna loosely tied over its mouth and nose blocked half or more of aerosols larger than 2 microns from entering the mannequin.

Ventilation counts. Open windows and doors. Adjust dampers in air-conditioning and heating systems. Upgrade the filters in those systems. Add portable air cleaners, or install germicidal ultraviolet technologies to remove or kill virus particles in the air.

Avoid crowds. The more people around you, the more likely someone among them will be infected. Especially avoid crowds indoors, where aerosols can accumulate.

We also found that especially with very small aerosols — smaller than 1 micron — it is more effective to use a softer fabric (which is easier to fit tightly over the face) than a stiffer fabric (which, even if it is a better filter, tends to sit more awkwardly, creating gaps).

One study from 2013 found that surgical masks reduced exposure to flu viruses by between 10 percent and 98 percent (depending on the mask’s design).A recent paper found that surgical masks can completely block seasonal coronaviruses from getting into the air.To my knowledge, no similar study has been conducted for SARS-CoV-2 yet, but these findings might apply to this virus as well since it is similar to seasonal coronaviruses in size and structure.

What about the outbreak on the Diamond Princess cruise ship off Japan early this year? Some 712 of the 3,711 people on board became infected.

Consider the case of a restaurant in Guangzhou, southern China, at the beginning of the year, in which one diner infected with SARS-CoV-2 at one table spread the virus to a total of nine people seated at their table and two other tables.Yuguo Li, a professor of engineering at the University of Hong Kong, and colleagues analyzed video footage from the restaurant and in a preprint (not peer reviewed) published in April found no evidence of close contact between the diners.Droplets can’t account for transmission in this case, at least not among the people at the tables other than the infected person’s: The droplets would have fallen to the floor before reaching those tables.But the three tables were in a poorly ventilated section of the restaurant, and an air conditioning unit pushed air across them. Notably, too, no staff member and none of the other diners in the restaurant — including at two tables just beyond the air conditioner’s airstream — became infected.

Ventilation systems in schools, nursing homes, residences and businesses may need to minimize recirculating air and add powerful new filters.

in an open letter to the W.H.O., 239 scientists in 32 countries have outlined the evidence showing that smaller particles can infect people, and are calling for the agency to revise its recommendations

Whether carried aloft by large droplets that zoom through the air after a sneeze, or by much smaller exhaled droplets that may glide the length of a room, these experts said, the coronavirus is borne through air and can infect people when inhaled

But the infection prevention and control committee in particular, experts said, is bound by a rigid and overly medicalized view of scientific evidence, is slow and risk-averse in updating its guidance and allows a few conservative voices to shout down dissent.

“If we started revisiting airflow, we would have to be prepared to change a lot of what we do,” she said. “I think it’s a good idea, a very good idea, but it will cause an enormous shudder through the infection control society.”

In early April, a group of 36 experts on air quality and aerosols urged the W.H.O. to consider the growing evidence on airborne transmission of the coronavirus. The agency responded promptly, calling Lidia Morawska, the group’s leader and a longtime W.H.O. consultant, to arrange a meeting.

But the discussion was dominated by a few experts who are staunch supporters of handwashing and felt it must be emphasized over aerosols, according to some participants, and the committee’s advice remained unchanged.

Dr. Morawska and others pointed to several incidents that indicate airborne transmission of the virus, particularly in poorly ventilated and crowded indoor spaces. They said the W.H.O. was making an artificial distinction between tiny aerosols and larger droplets, even though infected people produce both.

We’ve known since 1946 that coughing and talking generate aerosols,

Scientists have not been able to grow the coronavirus from aerosols in the lab.

Most of the samples in those experiments have come from hospital rooms with good air flow that would dilute viral levels.

In most buildings, she said, “the air-exchange rate is usually much lower, allowing virus to accumulate in the air and pose a greater risk.”

The W.H.O. also is relying on a dated definition of airborne transmission, Dr. Marr said. The agency believes an airborne pathogen, like the measles virus, has to be highly infectious and to travel long distances.

“We have this notion that airborne transmission means droplets hanging in the air capable of infecting you many hours later, drifting down streets, through letter boxes and finding their way into homes everywhere,”

Experts all agree that the coronavirus does not behave that way.

Dr. Marr and others said the coronavirus seemed to be most infectious when people were in prolonged contact at close range, especially indoors, and even more so in superspreader events — exactly what scientists would expect from aerosol transmission.

The agency lagged behind most of its member nations in endorsing face coverings for the public. While other organizations, including the C.D.C., have long since acknowledged the importance of transmission by people without symptoms, the W.H.O. still maintains that asymptomatic transmission is rare.

Many experts said the W.H.O. should embrace what some called a “precautionary principle” and others called “needs and values” — the idea that even without definitive evidence, the agency should assume the worst of the virus, apply common sense and recommend the best protection possible.

“There is no incontrovertible proof that SARS-CoV-2 travels or is transmitted significantly by aerosols, but there is absolutely no evidence that it’s not,

So at the moment we have to make a decision in the face of uncertainty, and my goodness, it’s going to be a disastrous decision if we get it wrong,” she said. “So why not just mask up for a few weeks, just in case?”

he agency also must consider the needs of all its member nations, including those with limited resources, and make sure its recommendations are tempered by “availability, feasibility, compliance, resource implications,” she said.

if the W.H.O. were to push for rigorous control measures in the absence of proof, hospitals in low- and middle-income countries may be forced to divert scarce resources from other crucial programs.

That’s the balance that an organization like the W.H.O. has to achieve,” he said. “It’s the easiest thing in the world to say, ‘We’ve got to follow the precautionary principle,’ and ignore the opportunity costs of that.”

In interviews, other scientists criticized this view as paternalistic. “‘We’re not going to say what we really think, because we think you can’t deal with it?’ I don’t think that’s right,”

Even cloth masks, if worn by everyone, can significantly reduce transmission, and the W.H.O. should say so clearly, he added.

The W.H.O. tends to describe “an absence of evidence as evidence of absence,” Dr. Aldis added. In April, for example, the W.H.O. said, “There is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection.”

The statement was intended to indicate uncertainty, but the phrasing stoked unease among the public and earned rebukes from several experts and journalists. The W.H.O. later walked back its comments.

In a less public instance, the W.H.O. said there was “no evidence to suggest” that people with H.I.V. were at increased risk from the coronavirus. After Joseph Amon, the director of global health at Drexel University in Philadelphia who has sat on many agency committees, pointed out that the phrasing was misleading, the W.H.O. changed it to say the level of risk was “unknown.”

But W.H.O. staff and some members said the critics did not give its committees enough credit.“Those that may have been frustrated may not be cognizant of how W.H.O. expert committees work, and they work slowly and deliberately,”

Scientists have detected the virus in places that can be reached only by air, such as ductwork; asymptomatic transmission is occurring, meaning people are spreading this without coughing or sneezing large droplets; and basic aerosol physics shows that people shed an entire continuum of particles when they cough, sneeze or talk, including large particles that settle out quickly and smaller ones that stay afloat for hours.

Why is airborne transmission so important? One reason: super-spreader events. Covid-19 does not spread from one person to the next equally.

one recent paper found that 10 percent of cases led to about 80 percent of the spread.

Such super-spreader events appear to be happening exclusively indoors, where airborne transmission is more likely.

when people sing, they emit as many aerosol particles as they do when they’re coughing. The practice also happened from 6:30 to 9 p.m., when most buildings turn off their ventilation systems.

it also requires that we minimize exposure to airborne pathogens, especially indoors.

To do that, we need to do two things. First, maintain physical distancing. Six feet is good, but 10 feet is better

Second, we must deploy healthy building strategies, such as refreshing stale indoor air.

We do this by opening windows in our homes and cars and by increasing the outdoor air ventilation rate in buildings with HVAC systems

We also have to make sure places such as bathrooms and rooms with infected patients have enough exhaust, and are negatively pressurized relative to common areas, so any airborne virus is confined to limited areas.

Florence Nightingale said, "Cleanliness and fresh air from open windows, with unremitting attention to the patient are the only defence a true nurse either asks or needs."

Just looking at these next few statistics will tell you everything you need to know about where we need to target our intervention efforts

One in 3 deaths nationally are workers or residents of senior homes; nine of the top 10 clusters in the United States occur in meatpacking plants and prisons; the death rate in hot spots is 10-fold higher in areas with lower incomes; and communities of color have nearly five times the odds of infection.

if healthy individuals start stockpiling surgical masks and high-grade N95 masks, they can also make it harder for health workers to get the resources they need to help on the front lines.

But studies of influenza pandemics have shown that when high-grade N95 masks are not available, surgical masks do protect people a bit more than not wearing masks at all. And when masks are combined with hand hygiene, they help reduce the transmission of infections.

When researchers conducted systematic review of a variety of interventions used during the SARS outbreak in 2003, they found that washing hands more than 10 times daily was 55 percent effective in stopping virus transmission, while wearing a mask was actually more effective — at about 68 percent

Wearing gloves offered about the same amount of protection as frequent hand-washing, and combining all measures — hand-washing, masks, gloves and a protective gown — increased the intervention effectiveness to 91 percent.

Classified data from the Chinese government that was reported in the South China Morning Post indicated that up to a third of all people who tested positive for the coronavirus could have been silent carriers.

Widespread testing on the Diamond Princess showed that half of the positive cases on board the cruise ship had no symptoms.

While wearing a mask may not necessarily prevent healthy people from getting sick, and it certainly doesn’t replace important measures such as hand-washing or social distancing, it may be better than nothing,

“It’s still hard to tell what percentage of people are truly asymptomatic because many go on to develop symptoms a few days later,

“What we do know is that individuals can shed virus about 48 hours before they develop symptoms and masking can prevent transmission from those individuals.”

Wearing a mask can also reduce the likelihood that people will touch their face

In many Asian countries, everyone is encouraged to wear masks, and the approach is about crowd psychology and protection. If everyone wears a mask, individuals protect each other, reducing overall community transmission

The sick automatically have one on and are also more likely to adhere to keeping their mask on because the stigma of wearing one is removed.

Masks are also an important signal that it’s not business as usual during a pandemic

They may also serve as an act of solidarity, showing that all citizens are on board with the precautionary measures needed to bring infections under control.

“If everyone in the community wears a mask, it could decrease transmission,” Dr. Fishman said. “But unfortunately I think that we don’t have enough masks to make that effective policy in the U.S.”

"The epidemiologists say if it's 'close contact' then it's not airborne. That's baloney," he says.

Of course, the world is struggling with a shortage of the most protective medical masks and gear.

What's more, one study of hospital rooms of patients with COVID-19 found that "swabs taken from the air exhaust outlets tested positive, suggesting that small virus-laden droplets may be displaced by airflows and deposited on equipment such as vents." Another study in Wuhan hospitals f

"The U.S. CDC has it exactly right,"

When epidemiologists are working in the field, trying to understand an outbreak of an unknown pathogen, it's not possible for them to know exactly what's going on as a pathogen is spread from person to person, Milton says. "Epidemiologists cannot tell the difference between droplet transmission and short-range aerosol transmission."

For the average person not working in a hospital, Milton says the recommendation to stay 6 feet away from others sounds reasonable.

People shouldn't cram into cars with the windows rolled up, he says, and officials need to keep crowding down in mass transit vehicles like trains and buses.

With coronavirus cases continuing to climb and hospitals facing the prospect of having to decide how to allocate limited staff and resources, the Department of Health and Human Services is reminding states and health care providers that civil rights laws still apply in a pandemic.

States are preparing for a situation when there's not enough care to go around by issuing "crisis of care" standards.

But disability groups are worried that those standards will allow rationing decisions that exclude the elderly or people with disabilities.

On Saturday, the HHS Office for Civil Rights put out guidance saying states, hospitals and doctors cannot put people with disabilities or older people at the back of the line for care.

Severino said his office has opened or is about to open investigations of complaints in multiple states. He did not say which states could be the focus of investigation, but in the last several days, disability groups in four states — Alabama, Kansas, Tennessee and Washington — have filed complaints.

In Kansas and Tennessee, disability groups and people with disabilities say state guidelines would allow doctors to deny care to some people with traumatic brain injuries or people who use home ventilators to help them breathe.

The ventilator issue is coming up in New York, which may soon be the first place where there are not enough ventilators to meet the demand of patients. Gov. Andrew Cuomo said the state will need double its current amount in about three weeks.

Severino said Saturday that his office was concerned about complaints of possible ventilator reallocation, an issue that had been raised in New York and Kansas.

The PREP Act provides immunity to tort liability claims for manufacturers or drug companies that are asked to scale up quick responses to a disaster such as a nuclear attack or a pandemic.

Severino said his office would investigate civil rights violations and it would be up to another office at HHS, the general counsel's office, to make waivers under the PREP Act.

Some disability advocates have worried whether that exception could be used to trump civil rights laws that protect people with disabilities from treatment decisions.

He was 98 years old.

The Reverend Joseph Lowery, co-founder of the Southern Christian Leadership Conference, died Friday, according to a statement by the Joseph & Evelyn Lowery Institute for Justice and Human Rights.

The statement said Lowery died peacefully at home Friday night, surrounded by his daughters.

Known affectionately as the "Dean" of the Civil Rights Movement, Lowery was a part of pivotal moments in the nation's history

At an appearance on the national mall in 2013, at the age of 91, he led the crowd in the chant "Fired Up? Ready to go?" The event marked 50 years since the 1963 March on Washington, which Lowery attended as a contemporary of the Rev. Martin Luther King, Jr. At that 50th anniversary appearance, he warned that hard-fought gains were under attack.

Joseph Echols Lowery was born in Huntsville, Alabama in 1921. He was the son of a teacher and a shopkeeper. The young Lowery experienced firsthand the brutalities of the Jim Crow South and would spend his life fighting for racial justice.

One of the first protests he organized was as a young Methodist minister in Mobile, Alabama in the early 1950s. It was aimed at desegregating city buses.

From there, Lowery helped coordinate the 1955 Montgomery bus boycott, the non-violent movement that desegregated the city's public transportation and led to the formation of the Southern Christian Leadership Conference.

Four decades later, at a gathering of civil rights foot soldiers in Montgomery, Lowery reflected on that accomplishment, noting that the number of black elected officials in the country had gone from less than 300 in 1965 to nearly 10,000 by 2005.

"It changed the face of the nation," said Lowery.

In May, the Centers for Disease Control and Prevention updated its guidelines to clarify that while COVID-19 spreads easily among speakers and sneezers in close encounters, touching a surface “isn’t thought to be the main way the virus spreads.” Other scientists have reached a more forceful conclusion. “Surface transmission of COVID-19 is not justified at all by the science,”

n the past few months, scientists have converged on a theory of how this disease travels: via air. The disease typically spreads among people through large droplets expelled in sneezes and coughs, or through smaller aerosolized droplets, as from conversations, during which saliva spray can linger in the air.

Surface transmission—from touching doorknobs, mail, food-delivery packages, and subways poles—seems quite rare.

All those studies that made COVID-19 seem likely to live for days on metal and paper bags were based on unrealistically strong concentrations of the virus. As he explained to me, as many as 100 people would need to sneeze on the same area of a table to mimic some of their experimental conditions. The studies “stacked the deck to get a result that bears no resemblance to the real world," Goldman said.

an obsession with contaminated surfaces distracts from more effective ways to combat COVID-19. “People have prevention fatigue,” Goldman told me. “They’re exhausted by all the information we’re throwing at them. We have to communicate priorities clearly; otherwise, they’ll be overloaded.”

Hygiene theater can take limited resources away from more important goals. Goldman shared with me an email he had received from a New Jersey teacher after his Lancet article came out. She said her local schools had considered shutting one day each week for “deep cleaning.” At a time when returning to school will require herculean efforts from teachers and extraordinary ingenuity from administrators to keep kids safely distanced, setting aside entire days to clean surfaces would be a pitiful waste of time and scarce local tax revenue.

As long as people wear masks and don’t lick one another, New York’s subway-germ panic seems irrational. In Japan, ridership has returned to normal, and outbreaks traced to its famously crowded public transit system have been so scarce that the Japanese virologist Hitoshi Oshitani concluded, in an email to The Atlantic, that “transmission on the train is not common.”

By funneling our anxieties into empty cleaning rituals, we lose focus on the more common modes of COVID-19 transmission and the most crucial policies to stop this plague. “My point is not to relax, but rather to focus on what matters and what works,” Goldman said. “Masks, social distancing, and moving activities outdoors. That’s it. That’s how we protect ourselves. That’s how we beat this thing.”

“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.”

Try to avoid exchanging money or credit cards with the cashier. Use a credit-card reader when possible.

all Americans to wear cloth face masks when out in public, which includes when in the grocery store.

wash your hands with soap and water before going out and when coming home, and use hand sanitizer when out.

If you use gloves, choose disposable ones and throw them before getting into your car or as soon as you get home

Try not to use your phone when in the store. If you do, clean it when you get home.

it’s a good idea to bring your own, mainly to wipe the grocery cart

Wipes can also be used for other high-touch areas in the store like freezer handles or tongs used in self-serve bins.

To be extra cautious, you could heat food in the oven or microwave, though this hasn’t been specifically studied so it’s unclear if there’s a particular length of time needed.

People over 65 and those who have medical conditions that put them at greater risk of hospitalization and serious illness should avoid going to the grocery store, if possible. Try to order groceries online or have a family member or friend deliver them while taking precautions.

Though there have been no documented cases of transmission of the novel coronavirus through food packaging, a recent NEJM study found that the virus can live on cardboard for up to 24 hours and on hard surfaces such as plastic and stainless steel for two to three days.

use self-checkout when possible and use hand sanitizer when you’re done.

experts say wiping down cereal boxes and other packages isn’t necessary. “Use the wipes when you need them,” says Dr. Chapman. If you’re home you can easily wash your hands. “That’s going to reduce your risk as much if not more than trying to wipe everything down,” he says.

instead of being preoccupied with wiping down packaging and containers, focus on washing your hands. “It’s much better to treat your hands, wash your hands, rather than dealing with all the surfaces,”

Your mouth is a gateway to both your respiratory system (your lungs) and your digestive system (stomach). Respiratory viruses like the novel coronavirus are believed to enter the body and reproduce through the respiratory tract, not the digestive tract.

Experts say it is possible that if the virus rubs off from any object to the inside of your mouth, it could infect you if it goes into your respiratory system. But there doesn’t appear to be any risk of infection via your digestive trac

doctors say getting the virus through ingestion of contaminated food seems unlikely. Gregory Poland, director of the Mayo Clinic’s Vaccine Research Group in Rochester, Minn., speculates that the gastric acid in the stomach would kill it. “My own speculation is that the GI route would be very low likelihood compared to known and efficient methods of infection,”

the studies were done in a laboratory with high doses of the virus, so it’s unknown if in real life the virus can be transmitted that way. Most likely if someone were to sneeze or cough on a cardboard container, the virus would degrade more quickly due to environmental factors, such as sunlight.

If you touch virus particles on raw food and then touch your nose, eyes or mouth, that is a potential source of transmission. But experts note that is very unlikely. To be vigilant, thoroughly wash your hands with soap and water, and don’t eat your food with your hands.

Experts urge people not to wash fruits and vegetables with anything but water. The chemicals on wipes and chlorine solutions especially can be dangerous—don’t ingest those.

There’s no evidence that the virus can be transmitted through clothing, but it hasn’t been specifically studied

The good news is it can be killed by doing laundry. So if you were in a grocery store where people near you were coughing, it’s a good idea to remove your clothes when you get home. Don’t shake clothing. Place it in your laundry hamper.

The CDC recommends laundering contaminated clothes in the warmest appropriate water setting and drying them thoroughly.

The CDC says there are no known cases of the novel coronavirus being spread through the fecal-oral route, which is a common route of transmission for stomach viruses like the norovirus.

But a recent study that hasn’t been peer reviewed yet found the virus in the stool of some patients. This route of transmission remains unknown, and experts say it’s unlikely to be contributing significantly to the pandemic.

Almost all of these accidental mutations are inconsequential: The virus still looks and functions just as its parent before it did. Over time, though, sets of mutations can layer on top of one another and accumulate, and the virus begins to function differently. Some of these differences confer an advantage of one sort or another—for example, increased transmissibility.

He has been at the forefront of identifying and tracking the variant, but says huge questions remain unanswered. “There’s still considerable uncertainty as to the long-term consequences” of B.1.1.7, Pybus told me. “We don’t even know whether this lineage truly originated in the U.K., with so many countries not doing this surveillance.”

The group’s reopening recommendations include widespread testing, contact tracing and isolation of people who are infected or exposed.

One important factor in transmission is that seemingly benign activities like speaking and breathing produce respiratory bits of varying sizes that can disperse along air currents and potentially infect people nearby.

Health agencies have so far identified respiratory-droplet contact as the major mode of Covid-19 transmission. These large fluid droplets can transfer virus from one person to another if they land on the eyes, nose or mouth. But they tend to fall to the ground or on other surfaces pretty quickly.

Proper ventilation—such as forcing air toward the ceiling and pumping it outside, or bringing fresh air into a room—dilutes the amount of virus in a space, lowering the risk of infection.

The so-called attack rate—the percentage of people who were infected in a specific place or time

that is only a rule of thumb, he cautioned. It could take much less time with a sneeze in the face or other intimate contact where a lot of respiratory droplets are emitted, he said.

When singing, people can emit many large and small respiratory particles. Singers also breathe deeply, increasing the chance they will inhale infectious particles.

Similar transmission dynamics could be at play in other settings where heavy breathing and loud talking are common over extended periods, like gyms, musical or theater performances, conferences, weddings and birthday parties.

An estimated 10% of people with Covid-19 are responsible for about 80% of transmissions, according to a study published recently in Wellcome Open Research. Some people with the virus may have a higher viral load, or produce more droplets when they breathe or speak, or be in a confined space with many people and bad ventilation when they’re at their most infectious point in their illness

additional protocols to interrupt spread, like social distancing in workspaces and providing N95 respirators or other personal protective equipment, might be necessary as well, she said.

overall, “the risk of a given infected person transmitting to people is pretty low,” said Scott Dowell, a deputy director overseeing the Bill & Melinda Gates Foundation’s Covid-19 response. “For every superspreading event you have a lot of times when nobody gets infected.”

The attack rate for Covid-19 in households ranges between 4.6% and 19.3%, according to several studies. It was higher for spouses, at 27.8%, than for other household members, at 17.3%, in one study in China.

The 37-year-old stay-at-home mother was hospitalized with a stroke on April 18 that her doctors attributed to Covid-19, and was still coughing when she went home two days later.

She pushed to get home quickly, she said, because her 4-year-old son has autism and needed her. She kept her distance from family members, covered her mouth when coughing and washed her hands frequently. No one else in the apartment has fallen ill, she said. “Nobody went near me when I was sick,” she said.

Being outside is generally safer, experts say, because viral particles dilute more quickly. But small and large droplets pose a risk even outdoors, when people are in close, prolonged contac

No one knows for sure how much virus it takes for someone to become infected, but recent studies offer some clues

In one small study published recently in the journal Nature, researchers were unable to culture live coronavirus if a patient’s throat swab or milliliter of sputum contained less than one million copies of viral RNA.

“Based on our experiment, I would assume that something above that number would be required for infectivity,” said Clemens Wendtner, one of the study’s lead authors

He and his colleagues found samples from contagious patients with virus levels up to 1,000 times that, which could help explain why the virus is so infectious in the right conditions: It may take much lower levels of virus than what’s found in a sick patient to infect someone else.

Current CDC workplace guidelines don’t talk about distribution of aerosols, or small particles, in a room, said Lisa Brosseau, a respiratory-protection consultan

Another factor is prolonged exposure. That’s generally defined as 15 minutes or more of unprotected contact with someone less than 6 feet away

Some scientists say while aerosol transmission does occur, it doesn’t explain most infections. In addition, the virus doesn’t appear to spread widely through the air.

“If this were transmitted mainly like measles or tuberculosis, where infectious virus lingered in the airspace for a long time, or spread across large airspaces or through air-handling systems, I think you would be seeing a lot more people infected,” said the CDC’s Dr. Brooks.

High-touch surfaces like doorknobs are a risk, but the virus degrades quickly so other surfaces like cardboard boxes are less worrisome,

years of painstakingly worded treaties, high-level visits and cutting-edge disease surveillance—all meant to encourage good-faith cooperation—have only bitten around the edges of the problem.

“It can’t demand entry into a country because they think something bad is happening.”

Nearly 200 countries were counting on an agency whose budget—roughly $2.4 billion in 2020—is less than a sixth of the Maryland Department of Health’s. Its donors, largely Western governments, earmark most of that money for causes other than pandemic preparedness.

In 2018 and 2019, about 8% of the WHO’s budget went to activities related to pandemic preparedness

the agency’s bureaucratic structure, diplomatic protocol and funding were no match for a pandemic as widespread and fast-moving as Covid-19.

To write its recommendations, the WHO solicits outside experts, which can be a slow process.

It took those experts more than four months to agree that widespread mask-wearing helps, and that people who are talking, shouting or singing can expel the virus through tiny particles that linger in the air. In that time, about half a million people died.

As months rolled on, it became clear that governments were reluctant to allow the U.N. to scold, shame or investigate them.

In particular, The Wall Street Journal found:

* China appears to have violated international law requiring governments to swiftly inform the WHO and keep it in the loop about an alarming infectious-disease cluster

—there are no clear consequences for violations

* The WHO lost a critical week waiting for an advisory panel to recommend a global public-health emergency, because some of its members were overly hopeful that the new disease wasn’t easily transmissible from one person to another.

* The institution overestimated how prepared some wealthy countries were, while focusing on developing countries, where much of its ordinary assistance is directed

Public-health leaders say the WHO plays a critical role in global health, leading responses to epidemics and setting health policies and standards for the world. It coordinates a multinational effort every year to pick the exact strains that go into the seasonal flu vaccine, and has provided public guidance and advice on Covid-19 when many governments were silent.

The world’s public-health agency was born weak, created in 1948 over U.S. and U.K. reluctance. For decades, it was legally barred from responding to diseases that it learned about from the news. Countries were required to report outbreaks of only four diseases to the WHO: yellow fever, plague, cholera and smallpox, which was eradicated in 1980.

Nearly three times that amount was budgeted for eradicating polio, a top priority for the WHO’s two largest contributors: the U.S. and the Bill & Melinda Gates Foundation.

SARS convinced governments to retool the WHO. The next year, delegates arrived in the Geneva palace where the League of Nations once met to resolve a centuries-old paradox: Countries don’t report outbreaks, because they fear—correctly—their neighbors will respond by blocking travel and trade.

“Everybody pushed back. No sovereign country wants to have this.”

China wanted an exemption from immediately reporting SARS outbreaks. The U.S. argued it couldn’t compel its 50 states to cooperate with the treaty. Iran blocked American proposals to make the WHO focus on bioterrorism. Cuba had an hourslong list of objections.

Around 3:15 a.m. on the last day, exhausted delegates ran out of time. The treaty they approved, called the International Health Regulations, imagined that each country would quickly and honestly report, then contain, any alarming outbreaks

In return, the treaty discouraged restrictions on travel and trade. There would be no consequences for reporting an outbreak—yet no way to punish a country for hiding one.

The treaty’s key chokepoint: Before declaring a “public health emergency of international concern,” or PHEIC, the WHO’s director-general would consult a multinational emergency committee and give the country in question a chance to argue against such a declaration.

Delegates agreed this could give some future virus a head start but decided it was more important to discourage the WHO from making any unilateral announcements that could hurt their economies.

On Jan. 22, a committee of 15 scientists haggled for hours over Chinese data and a handful of cases in other countries. Clearly, the virus was spreading between people in China, though there was no evidence of that in other countries. The question now: Was it mainly spreading from very sick people in hospitals and homes—or more widely?

On Jan. 3, representatives of China’s National Health Commission arrived at the WHO office in Beijing. The NHC acknowledged a cluster of pneumonia cases, but didn’t confirm that the new pathogen was a coronavirus, a fact Chinese officials already knew.

That same day, the NHC issued an internal notice ordering laboratories to hand over or destroy testing samples and forbade anyone from publishing unauthorized research on the virus.

China’s failure to notify the WHO of the cluster of illnesses is a violation of the International Health Regulations

China also flouted the IHR by not disclosing all key information it had to the WHO

The WHO said it’s up to member states to decide whether a country has complied with international health law, and that the coming review will address those issues.

While Chinese scientists had sequenced the genome and posted it publicly, the government was less forthcoming about how patients might be catching the virus.

WHO scientists pored over data they did get, and consulted with experts from national health agencies, including the CDC, which has 33 staff detailed to the WHO.

Then a 61-year-old woman was hospitalized in Thailand on Jan. 13.

The next day, Dr. van Kerkhove told reporters: “It’s certainly possible that there is limited human-to-human transmission.” MERS and SARS, both coronaviruses, were transmissible among people in close quarters. Epidemiological investigations were under way, she said.

Over the next few years, emergency committees struggled over how to determine whether an outbreak was a PHEIC. It took months to declare emergencies for two deadly Ebola epidemics

The committee met over two days, but was split. They mostly agreed on one point: The information from China “was a little too imprecise to very clearly state that it was time” to recommend an emergency declaration,

On Jan. 28, Dr. Tedros and the WHO team arrived for their meeting with Mr. Xi

Leaning across three wooden coffee tables, Dr. Tedros pressed for cooperation. In the absence of information, countries might react out of fear and restrict travel to China, he repeated several times throughout the trip. Mr. Xi agreed to allow a WHO-led international team of experts to visit. It took until mid-February to make arrangements and get the team there.

China also agreed to provide more data, and Dr. Tedros departed, leaving Dr. Briand behind with a list of mysteries to solve. How contagious was the virus? How much were children or pregnant women at risk? How were cases linked? This was vital information needed to assess the global risk, Dr. Briand said

Back in Geneva, Dr. Tedros reconvened the emergency committee. By now it was clear there was human-to-human transmission in other countries. When it met on Jan. 30, the committee got the information the WHO had been seeking. This time the committee recommended and Dr. Tedros declared a global public-health emergency.

President Trump and New York Gov. Andrew Cuomo both assured constituents their health systems would perform well. The U.K.’s chief medical officer described the WHO’s advice as largely directed at poor and middle-income countries. As for keeping borders open, by then many governments had already closed them to visitors from China.

The WHO shifted focus to the developing world, where it believed Covid-19 would exact the heaviest toll. To its surprise, cases shot up just across the border, in northern Italy.

Lessons learned

If there were one thing the WHO might have done differently, it would be to offer wealthier countries the type of assistance with public-health interventions that the WHO provides the developing world

the WHO’s warning system of declaring a global public-health emergency needs to change. Some want to see a warning system more like a traffic light—with color-coded alarms for outbreaks, based on how worried the public should be

Emergency committees need clearer criteria for declaring a global public-health emergency and should publicly explain their thinking

The WHO should have more powers to intervene in countries to head off a health crisis

the WHO’s health emergencies unit should report to the director-general and not member states, and its budget should be protected so it doesn’t have to compete with other programs for money.

Implementing many of those ideas would require herding diplomats back for another monthslong slog of treaty revisions. If and when such talks begin, new governments will likely be in place, and political priorities will float elsewher

“Unfortunately, I’m very cynical about this,” he said. “We are living through cycles of panic and neglect. We’ve been through all of this before.”

What is clear, they said, is that people should consider minimizing time indoors with people outside their families. Schools, nursing homes and businesses should consider adding powerful new air filters and ultraviolet lights that can kill airborne viruses.

For the coronavirus, the definition has been more complicated. Experts agree that the virus does not travel long distances or remain viable outdoors. But evidence suggests it can traverse the length of a room and, in one set of experimental conditions, remain viable for perhaps three hours.

Aerosols are droplets, droplets are aerosols — they do not differ except in size. Scientists sometimes refer to droplets less than five microns in diameter as aerosols.

From the start of the pandemic, the W.H.O. and other public health organizations have focused on the virus’s ability to spread through large droplets that are expelled when a symptomatic person coughs or sneezes.

These droplets are heavy, relatively speaking, and fall quickly to the floor or onto a surface that others might touch. This is why public health agencies have recommended maintaining a distance of at least six feet from others

But some experts have said for months that infected people also are releasing aerosols when they cough and sneeze. More important, they expel aerosols even when they breathe, talk or sing

Scientists know now that people can spread the virus even in the absence of symptoms — without coughing or sneezing — and aerosols might explain that phenomenon.

Because aerosols are smaller, they contain much less virus than droplets do. But because they are lighter, they can linger in the air for hours, especially in the absence of fresh air. In a crowded indoor space, a single infected person can release enough aerosolized virus over time to infect many people

“We should be placing as much emphasis on masks and ventilation as we do with hand washing,” Dr. Marr said. “As far as we can tell, this is equally important, if not more important.”

As for how long is safe, that is frustratingly tough to answer. A lot depends on whether the room is too crowded to allow for a safe distance from others and whether there is fresh air circulating through the room.

What does airborne transmission mean for reopening schools and colleges?This is a matter of intense debate. Many schools are poorly ventilated and are too poorly funded to invest in new filtration systems. “There is a huge vulnerability to infection transmission via aerosols in schools,”

When indoors, one simple thing people can do is to “open their windows and doors whenever possible,” Dr. Marr said. You can also upgrade the filters in your home air-conditioning systems, or adjust the settings to use more outdoor air rather than recirculated air.

If none of those things are possible, try to minimize the time you spend in an indoor space, especially without a mask. The longer you spend inside, the greater the dose of virus you might inhale.

Of all the measures South Africa put in place, Dawood said the most important against the flu was school closures. Studies have shown that children exhibit the highest rates of infection and illness due to influenza. “I think that was the one that interrupted flu’s whole chain,”

experts have offered numerous explanations for why it is more infectious than the seasonal flu. Foremost is the extent to which it spreads when those who carry it are asymptomatic. Flu is almost always transmitted through symptoms such as sneezing and coughing.

People also began taking extra health precaution

Many people also carry “background immunities” for influenza, Dawood said. Repeated infections, as well as worldwide annual vaccination campaigns, have given large portions of the global population at least some influenza antibodies, although new strains appear nearly every year.

With coronavirus cases climbing steeply at the beginning of the country’s flu season, many people rushed to clinics to get flu shots, hoping to avoid at least one co-morbidity.

The continued implementation of personal protective measures may be so effective against flu transmission that entire strains of the virus that relied on incubation in the Southern Hemisphere’s winter before being passed back north might be eliminated for good

“If it works out like this, it would be a very beautiful positive side effect of covid, one of the few good things to come of it.”

The main threat to that possibility is reintroduction from the Northern Hemisphere, where many countries, and parts of the United States in particular, have not required mask-wearing and school closures.

The president and Mr. Biden were at lecterns more than six feet apart at the debate. At that distance, Mr. Biden might have had protection from larger respiratory droplets released by the president, but not from aerosols lingering in the air.

Indoors, aerosols may travel farther than six feet. In August, for example, scientists reported that they had successfully isolated live virus from aerosols collected at a distance of seven feet and 16 feet from an infected patient in a hospital.

Dr. Marr likened the aerosols released while talking or shouting to cigarette smoke. People closest to the smoker — like Mr. Biden and the moderator, Chris Wallace (who plans to take a test on Monday) — would have been exposed first.

But over time, the smoke in a poorly ventilated room will drift throughout the space and reach even those much farther away.

The C.D.C. has not fully acknowledged the risk of airborne transmission of the virus indoors, beyond indirect nods to the importance of ventilation in schools and businesses.On Sept. 18, the C.D.C. published a new version of its guidance on how the virus spreads that acknowledged the importance of aerosols, and said inhalation of the virus was the main way the virus was transmitted.

The version posted Sept. 18 also said that indoors, “there is growing evidence that droplets and airborne particles can remain suspended in the air and be breathed in by others, and travel distances beyond 6 feet (for example, during choir practice, in restaurants, or in fitness classes).”

The slowdown in cases, however, is already prompting action from Spain’s regions, which are responsible for the healthcare systems, Covid-19 vaccination drives and coronavirus restrictions in their territories.

According to the Cantabria health department, the Covid pass is no longer able to achieve the goal of cutting transmission in indoor areas now that the omicron variant has become dominant. This new strain is highly transmissible, and is able to infect both the vaccinated and unvaccinated.

For this same reason, Catalonia is also considering lifting the use of the Covid pass in the region. “We think that it has made an effect: it has raised awareness on the importance of vaccination and was also applied to make the public aware that we were in a pandemic situation,” said Carme Cabezas, the Catalan secretary of Public Health.

For the other Spanish regions with coronavirus restrictions in force, no changes have been announced, but many of the measures that are currently in place will expire soon if not extended.

And suppose it takes five days between a person’s getting infected and their infecting others. After 30 days, the more transmissible virus is now causing 26, or 64, times as many new cases as the less transmissible one.

Not every case will be mild, though, and even a small hospitalization rate on top of a huge case number will be a big number.

Now, as my colleague Ed Yong reports, Omicron could push a collapsing health-care system further into disaster. Hospitals are already dealing with the flu and other winter viruses. They’re already canceling elective surgeries.

If there are no changes to behavior or policy, this year’s winter wave would peak at about double the hospitalizations of last winter at its worst, and 20 percent more deaths, according to the most pessimistic of projections

The most optimistic projection sees a caseload similar to last winter’s, but hospitalizations and deaths at about half of where they were back then, assuming the vaccines keep up their very high protection against severe illness.

Vaccine protection against severe illness should be more durable than it is against infection, but may still take a hit

Very preliminary data from South Africa’s largest health insurer suggest that two doses of the Pfizer-BioNTech vaccine were 70 percent effective at preventing hospitalization from Omicron infections, down from 93 percent before.

If that holds, it’s a “huge decrease,” Meyers says, and one that matches the assumptions of her team’s grimmer—but not grimmest—projections. When they modeled scenarios where vaccine effectiveness against hospitalization dropped by about that much, they saw a difference of tens of thousands of deaths.

The available evidence on Omicron’s inherent severity is likely to be biased in ways that make it appear more promising. First of all, hospitalizations lag infections.

Second, the first people infected may skew young and are thus more likely to have mild cases regardless of the variant

third, some of the mildness attributed to the virus may result instead from existing immunity. In South Africa, where doctors are reporting relatively low hospitalizations compared with previous waves, many cases are probably reinfections

The South Africa health-insurer data suggest that Omicron might carry a 29 percent lower risk of hospitalization than the original virus, when adjusted for risk factors including age, sex, vaccination status, and documented prior infection—but many prior infections may be undocumented, which would make the reduction in risk seem bigger than it really is. (A recent analysis of early U.K. cases found “at most, limited changes in severity compared with Delta.”)

Either way, in the short run, we will have a massive number of Omicron cases on top of a massive number of Delta cases. Together they will infect huge numbers of people, vaccinated or not

The effect is dramatic in some countries. Modern Sweden has some of the most rapid social mobility rates estimated in the world. Yet surnames in modern Sweden show status persistence exactly in this 0.7-0.8 range. This result was completely unexpected. Understanding why that is the case is a key puzzle the book tackles.

The book mainly concentrates on measures such as education, occupational status, wealth and longevity as indicators of status. Another surprising puzzle that emerged is that with surnames, the persistence of status was the same for all these measures. We might expect wealth to persist in a different way, since it can be transmitted across generations in a different way than education. You do not need any talent to inherit wealth. This is another regularity the book tries to account for.

Another remarkable feature of the surname data is how seemingly impervious social mobility rates are to government interventions. In all societies, what seems to matter is just who your parents are. At the extreme, we see in modern Sweden an extensive system of public education and social support. Yet underlying mobility rates are no higher in modern Sweden than in pre-industrial Sweden or medieval England.

Interestingly, in China, the extreme social intervention represented by the Communist Revolution of 1949, which included executing large numbers of members of the old upper class, has not resulted in much of an increase in social mobility. Surnames of high status in the Imperial and Republican era continue to be overrepresented among modern elites, including Communist Party officials.

families that have high social competence, whatever the social system is, typically find their way to the top of the social ladder.

 It is clear that families are very powerful determinants of children’s outcomes. But what do parents transmit to their children? Is it mainly some type of culture? Or is it mainly genetics?

The data does not exist to provide any conclusive answer to this question. But even if this is cultural transmission, it looks in all respects just the same as biological inheritance. The book performs a series of tests to see if biological transmission can be ruled out as the important link, and the empirical patterns never rule this out.

A recent book, "The Triple Package"  [by Amy Chua and Jed Rubenfeld], argues the extreme opposite of biology in explaining social status, with the claim that successful cultural groups in the U.S. have three key features leading to success, one being impulse control. But what is remarkable is how disparate the culturally successful groups they identify are – Jews, Chinese, Indians, Mormons, Iranians, Lebanese, Nigerians, Cubans. And it is demonstrable that most of the successful groups identified here were elites selected from the parent populations as a combined result of politics at home and immigration policy in the U.S.

All this information on social mobility within societies unfortunately does not offer much insight into why societies as a whole succeed or fail in economic terms.

The surname data suggests that you will not be able to do much to increase social mobility through social policies of any type.  We already live in societies of massive social intervention in terms of the provision of education and health care. Yet we have not been able to raise social mobility rates above those of the pre-industrial era.

But if we're learning that we can predict the majority of people's outcomes at conception, that should lead us to reexamine our assumption that whatever income distribution comes out in society is fine. Because if it's the case that a lot of this is determined before someone enters the game, it weakens the case for letting the market determine the distribution. You'd be much more likely to favor a society with much less inequality.

And that's where Sweden's system does provide advantages over the U.S.'s. They haven't changed mobility rates, but they've changed the consequences, strongly, of ending up at various points in the distribution. It's a much better place for people who end up at the bottom of the distribution.

One piece of news that most people will find encouraging in these various studies is that eventually all elites become average in their characteristics, even if that takes 300 years

Accounts that emphasize cultural transmission all have a hard time explaining why successful groups, and successful families in general, all experience regression to the mean. There is nothing to stop a cultural trait being inherited unchanged

Only biological inheritance has an inbuilt mechanism to explain observed regression to the mean. It also has predictions about when this regression to the mean will not be observed (complete endogamy). It further implies that the rate of regression to the mean will be the same at the top of the status distribution as at the bottom. So the biological pathway has two advantages over the cultural. It produces a mechanism to explain the regression of all elites and underclasses that we observe, and it has testable implications about the speed and character of that mobility.

The final surprise in conducting this study was to find a seeming simple physics that underlies social mobility.

When we observe an elite group in 1800 in England, we can predict to seven generations in the future what their relative social position will be, despite the arrival of the Industrial Revolution and the rise of the modern welfare state in the interval.

Two very simple equations are sufficient to describe a major feature of the social world, and a feature that you would think impossible to model in any such manner.

Researchers believe the incubation period before symptoms is roughly five days on average. In studying the pattern of illness, epidemiologists have made the dismaying discovery that people start shedding the virus — potentially making others sick — in advance of symptoms.

The bulldozer nature of coronavirus means widespread severe illnesses and deaths from covid-19 can happen with terrifying speed.

a large percentage of the world’s population, potentially billions of people, could become infected within the next couple of years. Frantic efforts to develop a safe and effective vaccine are likely to take a year or more.

covid-19 may be many times as lethal for an infected person as seasonal flu.

the virus has a gift for stealth transmission. It seeds itself in communities far and wide,

The pandemic appears to be largely driven by direct, human-to-human transmission. That is why public health officials have told people to engage in social distancing

R0, pronounced “R naught.” That is the average number of new infections generated by each infected person.

he R0 is not an intrinsic feature of the virus. It can be lowered through containment, mitigation and ultimately “herd immunity,”

In the early days in China, before the government imposed extreme travel restrictions in Wuhan and nearby areas, and before everyone realized exactly how bad the epidemic might be, the R0 was 2.38, according to a study published in the journal Science. That is a highly contagious disease.

But on Jan. 23, China imposed extreme travel restrictions and soon put hundreds of millions of people into some form of lockdown as authorities aggressively limited social contact. The R0 plummeted below 1, and the epidemic has been throttled in China, at least for now.

its ability to spread depends also on the vulnerability of the human population, including the density of the community.

Without a vaccine or a drug to stop infections, the best hope is to break the chain of transmission one infection at a time

“Social distancing is building speed bumps so that we can slow the spread of the virus. We have to respect the speed bumps,”

the efficacy of social distancing “is the million-dollar question right now.”

She compared the current public measures to what happened during the 1918 influenza pandemic that killed an estimated 675,000 people in the United States, and in which some cities were more careful than others about enforcing social distancing.

“The USA is currently in a natural experiment of sorts, which each state implementing their own version of social distancing,” she said. “We will be able to compare the efficacy of these various public health policies, but not until more time has passed.”

Not only must people limit their direct contact, they need to limit the amount that their paths overlap, because the virus can linger on surfaces.

The virus degrades outside a host because of exposure to moisture and sunlight, or from drying out

in pristine laboratory conditions, some SARS-CoV-2 particles can remain potentially viable on metal or plastic for up to three days.

Absent hard data, limiting contact with shared surfaces, such as door handles or checkout machines, and frequent hand-washing is highly advisable.

people have some innate, mechanical defenses against viruses just like they do against pollen and dust, Taubenberger noted. Cells in the respiratory tract have tiny hairlike projections, called cilia, that move mucus toward the throat in a manner that helps clear invasive particles. This is not our body’s first viral rodeo.

“The window of opportunity is still there, but the window of opportunity is narrowing,” WHO Director General Tedros Adhanom Ghebreyesus said on Friday. “We need to act quickly before it closes completely.”

At the beginning of any disease outbreak, public health experts painstakingly trace the contacts of every person who becomes sick. The experts build a family tree of possible illness, with branches that include anyone who might have shaken hands with, or been sneezed on by an infected person

with confirmed infections approaching 80,000 people, contact tracing on a case-by-case basis could soon be impractical.

“What we find is that this virus is going to be very difficult to contain,” said Jeffrey Shaman, an infectious disease researcher at Columbia University and co-author of the study posted Monday. “Personally, I don’t think we can do it.”

The word ‘pandemic’ invokes fear, but it describes how widespread an outbreak may be, not its deadliness.

“I think we should assume that this virus is very soon going to be spreading in communities here, if it isn’t already, and despite aggressive actions, we should be putting more efforts to mitigate impacts,”

The virus would be easier to contain if people who are contagious were obviously so, as was the case with SARS, which started an outbreak that burned itself out in 2003. But the new virus appears to spread among people who in some cases are not noticeably sick.

among the more than 600 passengers from the Diamond Princess cruise ship who have tested positive, about half had no obvious symptoms.