Why are some people so much more infectious than others? Scientists ponder coronavirus puzzle

Super-spreading events may involve people who shed an unusual amount of virus.
Super-spreading events may involve people who shed an unusual amount of virus.PHOTO: AFP

NEW YORK (NYTIMES) - As the coronavirus spreads around the world, scientists are asking: Are some people more infectious than others? Are there "super spreaders", people who seem to just spew out the virus, making them especially likely to infect others?

It seems that the answer is yes. There do seem to be super spreaders, 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 super-spreading 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 super spreading 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 Dr Jon Zelner, an epidemiologist at the University of Michigan. If the infected person is a super spreader, contact tracing is especially important. But if the infected person is the opposite of a super spreader, 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 Prof 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," she 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 super spreader," 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 realise they are sick, said Dr Thomas Frieden, former director of the Centres for Disease Control and Prevention and chief executive at Resolve to Save Lives, an initiative of Vital Strategies.

More likely, he added, super spreading 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 faeces, for example.

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

Medical history is replete with stories of super spreading 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 20th century. She herself was not ill but was asymptomatic - silently infected with typhoid.

Super spreading also played important roles in outbreaks of two other coronaviruses, which caused severe acute respiratory syndrome (Sars) and Middle East respiratory syndrome (Mers).

"The Mers-CoV outbreak in South Korea was driven primarily by three infected individuals, and approximately 75 per cent of cases can be traced back to three super spreaders who have each infected a disproportionately high number of contacts," wrote Dr George Gao, an immunologist and virologist at the Chinese Centres 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 super-spreading event accounted for most of the outbreak.

 
 
 

In 2003, during the Sars outbreak, the first patient in Hong Kong appeared to have infected at least 125 others. Other super-spreading 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 per cent of infections were traced to just 3 per cent of infected people.

Super spreading 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, 75 per cent 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."

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 per cent of patients did not appear to transmit the disease.

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

On Jan 23, the wife - who had returned from a visit to Wuhan, China - 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 hospitalised. Both recovered.

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