Why Are Some People So Much More Infectious Than Others?

As the coronavirus tears through the country, scientists are asking: Are some people more infectious than others? Are there superspreaders, people who seem to just spew out virus, making them especially likely to infect others?

It seems that the answer is yes. There do seem to be superspreaders, a loosely defined term for people who infect a disproportionate number of others, whether as a consequence of genetics, social habits or simply being in the wrong place at the wrong time.

But those virus carriers at the heart of what are being called superspreading events can drive and have driven epidemics, researchers say, making it crucial to figure out ways to identify spreading events or to prevent situations, like crowded rooms, where superspreading can occur.

Just as important are those at the other end of the spectrum — people who are infected but unlikely to spread the infection.

Distinguishing between those who are more infectious and those less infectious could make an enormous difference in the ease and speed with which an outbreak is contained, said Jon Zelner, an epidemiologist at the University of Michigan. If the infected person is a superspreader, contact tracing is especially important. But if the infected person is the opposite of a superspreader, someone who for whatever reason does not transmit the virus, contact tracing can be a wasted effort.

“The tricky part is that we don’t necessarily know who those people are,” Dr. Zelner said.

Two factors are at play, said Martina Morris, emeritus professor of statistics and sociology at the University of Washington.

“There has to be a link between people in order to transmit an infection,” she said. But, she added, a link “is necessary but not sufficient.” The second factor is how infectious a person is. “We almost never have independent data on those two things,” Dr. Morris said.

She pointed out that it can be easy to misattribute multiple infections to an individual — possibly exposing the person to public attack — when the spread has nothing to do with the person’s infectiousness.

“If you are the first person in a crowded room to get infected and if this is an easily spread disease, you will look like a superspreader,” she said. “Anyone in that room could have had the same impact. You were just the first in line.”

Yet there do seem to be situations in which a few individuals spark large outbreaks. With Covid-19, it is not yet known whether those highly infectious people include individuals with silent infections who do not realize they are sick, said Dr. Thomas Frieden, former director of the Centers for Disease Control and Prevention and chief executive at Resolve to Save Lives, an initiative of Vital Strategies. More likely, he adds, superspreading events may involve people with symptoms that linger but who are not sick enough to stay home.

Or they could involve infected people who shed an unusual amount of virus — a poorly studied factor that might be due to variations in the amount of virus in the aerosol droplets from a patient’s cough or the amount of infectious virus in feces, for example.

No matter what the cause, public health measures, like avoiding crowds, and what Dr. Frieden calls cough hygiene, can prevent a superspreading event, he said.

Medical history is replete with stories of superspreading in outbreaks of parasitic disease, tuberculosis, measles and other illness.

There is Mary Mallon, a cook better known as Typhoid Mary, who spread typhoid fever to more than 50 people in the early years of the twentieth century. She herself was not ill but was asymptomatic — silently infected with typhoid.

Superspreading also played important roles in outbreaks of two other coronaviruses, SARS and MERS.

“The MERS-CoV outbreak in South Korea was driven primarily by three infected individuals, and approximately 75 percent of cases can be traced back to three superspreaders who have each infected a disproportionately high number of contacts,” wrote George F. Gao, an immunologist and virologist at the Chinese Centers for Disease Control and Prevention in Beijing, in a recent paper.

The outbreak in South Korea began in 2015 when a 68-year-old man became infected with MERS during travel to the Middle East. He returned to South Korea where he directly infected 29 people, two of whom infected 106 people. The total number of cases in South Korea at that time was 166 — that superspreading event accounted for most of the outbreak.

In 2003 during the SARS outbreak, the first patient in Hong Kong appears to have infected at least 125 others. Other superspreading events involved 180 people in a housing complex in Hong Kong and another 22 people on a jet from Hong Kong to Beijing.

In the Ebola outbreak in Africa between 2014 and 2016, 61 percent of infections were traced to just 3 percent of infected people.

Superspreading also appears to have driven outbreaks of the new coronavirus.

One event occurred at the end of February when 175 Biogen executives gathered for a conference at the Boston Marriott Long Wharf Hotel. At least one was infected with the coronavirus. Two weeks later, seventy five percent of the 108 Massachusetts residents infected with the virus were associated with Biogen. The infections rippled out from there, to other states and other Massachusetts residents.

“Why at that conference?” asked Dr. Eric Topol, director of the Scripps research translational institute in San Diego. “At the time there were so many conferences — it was before social distancing. Something was going on there.”

Then there was the March 12 birthday party in Westport, Conn. About 50 people attended. Half ended up infected. The cluster of cases expanded so fast health officials gave up contact tracing.

At a funeral on Feb. 29 in Albany, Ga., someone unwittingly spread the virus among the 200 mourners. At Illinois’s current hot spot, the Cook County jail, at least 400 are known to be infected.

At the other end of the bell curve of infectiousness are infected people who do not seem to infect others. During the MERS outbreak in South Korea, 89 percent of patients did not appear to transmit the disease.

In the Covid-19 pandemic, there is a striking example from the far end of uninfectious — a couple in Illinois.

On Jan. 23 the wife, who had returned from a visit to Wuhan — became the first laboratory-confirmed case of Covid-19 in the state. On Jan. 30, her husband was infected. It was the first known person-to person transmission in the United States.

Both husband and wife became gravely ill and were hospitalized. Both recovered.

State public health officials traced their contacts — 372 people, including 195 health care workers. Not a single one became infected.

Dr. Jennifer Layden, chief medical officer for the Chicago Department of Public Health, said the remarkable lack of spread probably arose from several factors. Where were the couple in the course of their infection when they came into contact with those other people? Were they sneezing or coughing? How close were the contacts? Were the people they interacted with simply less susceptible to infections?

As grimly alluring as it is to look for viral superspreaders, there are pitfalls.

There is a good chance that a cluster of infections would be attributed to a superspreader when, instead, public health officials missed some transmissions by other people, Dr. Zelner said. And there are social consequences to superspreader stories.

“The nature of our society right now is that we are very much interested in the catastrophic,” said Samuel K. Roberts, a medical historian at Columbia. “The best way to do that is to have something that looks like a zombie story. It’s a powerful narrative.”

The general public doesn’t need to know if an outbreak was traced to one person, he said.

“What’s more important is, How do we protect ourselves?” he said. “Finding patient zero is not going to help. It only stokes fear of the other.”

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