Most People With Coronavirus Won’t Spread It. Why Do a Few Infect Many? - The New York Times

At a May 30 birthday party in Texas, one man reportedly infected 18 friends and family with the coronavirus.

Reading reports like these, you might think of the virus as a wildfire, instantly setting off epidemics wherever it goes. But other reports tell another story altogether.

In Italy, for example, scientists looked at stored samples of wastewater for the earliest trace of the virus. Last week they reported that the virus was in Turin and Milan as early as Dec. 18. But two months would pass before northern Italy’s hospitals began filling with victims of Covid-19. So those December viruses seem to have petered out.

As strange as it may seem, these reports don’t contradict each other. Most infected people don’t pass on the coronavirus to someone else. But a small number pass it on to many others in so-called superspreading events.

“You can think about throwing a match at kindling,” said Ben Althouse, principal research scientist at the Institute for Disease Modeling in Bellevue, Wash. “You throw one match, it may not light the kindling. You throw another match, it may not light the kindling. But then one match hits in the right spot, and all of a sudden the fire goes up.”

Understanding why some matches start fires while many do not will be crucial to curbing the pandemic, scientists say. “Otherwise, you’re in the position where you’re always one step behind the virus,” said Adam Kucharski, an epidemiologist at the London School of Hygiene and Tropical Medicine.

When the virus first emerged in China, epidemiologists scrambled to understand how it spread from person to person. One of their first tasks was to estimate the average number of people each sick person infected, or what epidemiologists call the reproductive number.

The new coronavirus turned out to have a reproductive number somewhere between two and three. It’s impossible to pin down an exact figure, since people’s behavior can make it easier or harder for the virus to spread. By going into lockdown, for instance, Massachusetts drove its reproductive number down from 2.2 at the beginning of March to 1 by the end of the month; it’s now at .74.

This averaged figure can also be misleading because it masks the variability of spread from one person to the next. If nine out of 10 people don’t pass on a virus at all, while the 10th passes it to 20 people, the average would still be two.

In some diseases, such as influenza and smallpox, a large fraction of infected people pass on the pathogen to a few more. These diseases tend to grow steadily and slowly. “Flu can really plod along,” said Kristin Nelson, an associate professor at Emory University.

But other diseases, like measles and SARS, are prone to sudden flares, with only a few infected people spreading the disease.

Epidemiologists capture the difference between the flare-ups and the plodding with something known as the dispersion parameter. It is a measure of how much variation there is from person to person in transmitting a pathogen.

But James Lloyd-Smith, a U.C.L.A. disease ecologist who developed the dispersion parameter 15 years ago, cautioned that just because scientists can measure it doesn’t mean they understand why some diseases have more superspreading than others. “We just understand the bits of it,” he said.

When Covid-19 broke out, Dr. Kucharski and his colleagues tried to calculate that number by comparing cases in different countries.

If Covid-19 was like the flu, you’d expect the outbreaks in different places to be mostly the same size. But Dr. Kucharski and his colleagues found a wide variation. The best way to explain this pattern, they found, was that 10 percent of infected people were responsible for 80 percent of new infections. Which meant that most people passed on the virus to few, if any, others.

Dr. Kucharski and his colleagues published their study in April as a preprint, a report that has not been reviewed by other scientists and published in a scientific journal. Other epidemiologists have calculated the dispersion parameter with other methods, ending up with similar estimates.

In Georgia, for example, Dr. Nelson and her colleagues analyzed over 9,500 Covid-19 cases from March to May. They created a model for the spread of the virus through five counties and estimated how many people each person infected.

In a preprint published last week, the researchers found many superspreading events. Just 2 percent of people were responsible for 20 percent of transmissions.

Now researchers are trying to figure out why so few people spread the virus to so many. They’re trying to answer three questions: Who are the superspreaders? When does superspreading take place? And where?

As for the first question, doctors have observed that viruses can multiply to bigger numbers inside some people than others. It’s possible that some people become virus chimneys, blasting out clouds of pathogens with each breath.

Raymond Sanchez, an M.T.A. driver, disinfecting buses during the changing of shifts in East Harlem in New York in May.Credit... Brittainy Newman/The New York Times

Some people also have more opportunity to get sick, and to then make other people sick. A bus driver or a nursing home worker may sit at a hub in the social network, while most people are less likely to come into contact with others — especially in a lockdown.

The Coronavirus Outbreak

• Frequently Asked Questions and Advice

  Updated June 24, 2020

  • What’s the best material for a mask?

    Scientists around the country have tried to identify everyday materials that do a good job of filtering microscopic particles. In recent tests, HEPA furnace filters scored high, as did vacuum cleaner bags, fabric similar to flannel pajamas and those of 600-count pillowcases. Other materials tested included layered coffee filters and scarves and bandannas. These scored lower, but still captured a small percentage of particles.

  • Is it harder to exercise while wearing a mask?

    A commentary published this month on the website of the British Journal of Sports Medicine points out that covering your face during exercise “comes with issues of potential breathing restriction and discomfort” and requires “balancing benefits versus possible adverse events.” Masks do alter exercise, says Cedric X. Bryant, the president and chief science officer of the American Council on Exercise, a nonprofit organization that funds exercise research and certifies fitness professionals. “In my personal experience,” he says, “heart rates are higher at the same relative intensity when you wear a mask.” Some people also could experience lightheadedness during familiar workouts while masked, says Len Kravitz, a professor of exercise science at the University of New Mexico.

  • I’ve heard about a treatment called dexamethasone. Does it work?

    The steroid, dexamethasone, is the first treatment shown to reduce mortality in severely ill patients, according to scientists in Britain. The drug appears to reduce inflammation caused by the immune system, protecting the tissues. In the study, dexamethasone reduced deaths of patients on ventilators by one-third, and deaths of patients on oxygen by one-fifth.

  • What is pandemic paid leave?

    The coronavirus emergency relief package gives many American workers paid leave if they need to take time off because of the virus. It gives qualified workers two weeks of paid sick leave if they are ill, quarantined or seeking diagnosis or preventive care for coronavirus, or if they are caring for sick family members. It gives 12 weeks of paid leave to people caring for children whose schools are closed or whose child care provider is unavailable because of the coronavirus. It is the first time the United States has had widespread federally mandated paid leave, and includes people who don’t typically get such benefits, like part-time and gig economy workers. But the measure excludes at least half of private-sector workers, including those at the country’s largest employers, and gives small employers significant leeway to deny leave.

  • Does asymptomatic transmission of Covid-19 happen?

    So far, the evidence seems to show it does. A widely cited paper published in April suggests that people are most infectious about two days before the onset of coronavirus symptoms and estimated that 44 percent of new infections were a result of transmission from people who were not yet showing symptoms. Recently, a top expert at the World Health Organization stated that transmission of the coronavirus by people who did not have symptoms was “very rare,” but she later walked back that statement.

  • What’s the risk of catching coronavirus from a surface?

    Touching contaminated objects and then infecting ourselves with the germs is not typically how the virus spreads. But it can happen. A number of studies of flu, rhinovirus, coronavirus and other microbes have shown that respiratory illnesses, including the new coronavirus, can spread by touching contaminated surfaces, particularly in places like day care centers, offices and hospitals. But a long chain of events has to happen for the disease to spread that way. The best way to protect yourself from coronavirus — whether it’s surface transmission or close human contact — is still social distancing, washing your hands, not touching your face and wearing masks.

  • How does blood type influence coronavirus?

    A study by European scientists is the first to document a strong statistical link between genetic variations and Covid-19, the illness caused by the coronavirus. Having Type A blood was linked to a 50 percent increase in the likelihood that a patient would need to get oxygen or to go on a ventilator, according to the new study.

  • How many people have lost their jobs due to coronavirus in the U.S.?

    The unemployment rate fell to 13.3 percent in May, the Labor Department said on June 5, an unexpected improvement in the nation’s job market as hiring rebounded faster than economists expected. Economists had forecast the unemployment rate to increase to as much as 20 percent, after it hit 14.7 percent in April, which was the highest since the government began keeping official statistics after World War II. But the unemployment rate dipped instead, with employers adding 2.5 million jobs, after more than 20 million jobs were lost in April.

  • What are the symptoms of coronavirus?

    Common symptoms include fever, a dry cough, fatigue and difficulty breathing or shortness of breath. Some of these symptoms overlap with those of the flu, making detection difficult, but runny noses and stuffy sinuses are less common. The C.D.C. has also added chills, muscle pain, sore throat, headache and a new loss of the sense of taste or smell as symptoms to look out for. Most people fall ill five to seven days after exposure, but symptoms may appear in as few as two days or as many as 14 days.

  • How can I protect myself while flying?

    If air travel is unavoidable, there are some steps you can take to protect yourself. Most important: Wash your hands often, and stop touching your face. If possible, choose a window seat. A study from Emory University found that during flu season, the safest place to sit on a plane is by a window, as people sitting in window seats had less contact with potentially sick people. Disinfect hard surfaces. When you get to your seat and your hands are clean, use disinfecting wipes to clean the hard surfaces at your seat like the head and arm rest, the seatbelt buckle, the remote, screen, seat back pocket and the tray table. If the seat is hard and nonporous or leather or pleather, you can wipe that down, too. (Using wipes on upholstered seats could lead to a wet seat and spreading of germs rather than killing them.)

  • What should I do if I feel sick?

    If you’ve been exposed to the coronavirus or think you have, and have a fever or symptoms like a cough or difficulty breathing, call a doctor. They should give you advice on whether you should be tested, how to get tested, and how to seek medical treatment without potentially infecting or exposing others.

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Dr. Nelson suspects the biological differences between people are less significant. “I think the circumstances are a lot more important,” she said. Dr. Lloyd-Smith agreed. “I think it’s more centered on the events.”

A lot of transmission seems to happen in a narrow window of time starting a couple days after infection, even before symptoms emerge. If people aren’t around a lot of people during that window, they can’t pass it along.

And certain places seem to lend themselves to superspreading. A busy bar, for example, is full of people talking loudly. Any one of them could spew out viruses without ever coughing. And without good ventilation, the viruses can linger in the air for hours.

A study from Japan this month found clusters of coronavirus cases in health care facilities, nursing homes, day care centers, restaurants, bars, workplaces, and musical events such as live concerts and karaoke parties.

This pattern of superspreading could explain the puzzling lag in Italy between the arrival of the virus and the rise of the epidemic. And geneticists have found a similar lag in other countries: The first viruses to crop up in a given region don’t give rise to the epidemics that come weeks later.

Many countries and states have fought outbreaks with lockdowns, which have managed to draw down Covid-19’s reproductive number. But as governments move toward reopening, they shouldn’t get complacent and forget the virus’s potential for superspreading.

“You can really go from thinking you’ve got things under control to having an out-of-control outbreak in a matter of a week,” Dr. Lloyd-Smith said.

Singapore’s health authorities earned praise early on for holding down the epidemic by carefully tracing cases of Covid-19. But they didn’t appreciate that huge dormitories where migrant workers lived were prime spots for superspreading events. Now they are wrestling with a resurgence of the virus.

On the other hand, knowing that Covid-19 is a superspreading pandemic could be a good thing. “It bodes well for control,” Dr. Nelson said.

Since most transmission happens only in a small number of similar situations, it may be possible to come up with smart strategies to stop them from happening. It may be possible to avoid crippling, across-the-board lockdowns by targeting the superspreading events.

“By curbing the activities in quite a small proportion of our life, we could actually reduce most of the risk,” said Dr. Kucharski.

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