May 21, 2020

Scientists around the globe are tracking so-called "superspreading events" in which a single person infected with the new coronavirus triggers a larger outbreak—and they're beginning to learn why some infected patients are more likely to be superspreaders than others, Kai Kupferschmidt reports for Science Magazine.

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The importance of understanding how the virus spreads

Scientists tracking the new coronavirus' spread have generally focused on two numbers, Kupferschmidt reports. The first is called the reproduction number (R), which represents the average number of new infections caused by a single infected person. The second is called the dispersion factor (k), a number indicating the likelihood that a particular disease will spread in clusters.

Without any social distancing, the new coronavirus' R-number is around three, Kupferschmidt reports.  

But Jamie Lloyd-Smith, who has studied the spread of pathogens at the University of California-Los Angeles, said the vast majority of infected individuals don't spread the virus at all. "The consistent pattern is that the most common number is zero," he said. "Most people do not transmit."

This is why scientists also look at the dispersion factor (k); the lower the k number, the more likely it is the virus is spread by small group of people.

For example, in a 2005 paper published in Nature co-authored by Lloyd-Smith, SARS—which also is caused by a type of coronavirus—was determined to have a k of 0.16, indicating that superspreading played a significant role in the disease's spread. Meanwhile, MERS—which is caused by a different coronavirus—had about a 0.25 k number, while the 1918 Spanish Flu had a k number of around one, meaning superspreading wasn't much of a factor.

So far, estimates of the k number for SARS-CoV-2, which is the name of the new coronavirus, have varied, Kupferschmidt reports. One estimate from researchers at the University of Bern concluded the k number for the new coronavirus is higher than the k numbers for SARS and MERS.

However, a recent research article by Adam Kucharski of the London School of Hygiene & Tropical Medicine (LSHTM) estimated that k for Covid-19, the disease caused by the new coronavirus, is 0.1. "Probably about 10% of cases lead to 80% of the spread," Kucharski said.

Experts explain that these numbers are important because it allows policymakers to target social distancing policies at the types of gatherings where superspreading is likely to occur. "If you can predict what circumstances are giving rise to these events, the math shows you can really, very quickly curtail the ability of the disease to spread," Lloyd-Smith said.

Why some people are superspreaders and others aren't

As scientists learn more about how the new coronavirus spreads, they're also learning why some people are more likely to be superspreaders than others—and the environments most likely to cause superspreading events.

It turns out the two are closely related, Kupferschmidt reports. Research to date suggests that the new coronavirus mostly transmits through droplets, though it has also been found to occasionally spread through aerosols that suspend in the air, which can allow one person to infect many more. A 2019 study of healthy people found that some people exhale more droplets than others when they talk, some of which was explained by their speaking volume.

In addition, a study out of Japan found people are nearly 19 times as likely to become infected with Covid-19 indoors when compared with outdoors.

According to Kupferschmidt, these findings may explain a recent CDC case study in which a single individual who attended choir practice on March 10 triggered an outbreak that sickened 53 of the 61 choir members who gathered for practice. According to CDC, three of those individuals were hospitalized and two died. The initial patient had been experiencing cold-like symptoms and was later diagnosed with Covid-19..

Gwenan Knight from LSHTM said the existing data and reports of clusters indicate that enclosed spaces where people are shouting, singing, or breathing heavy from exercise may be riskier than others.

Knight said she noticed there have been reports of outbreaks at places where people typically shout or sing, like choir practices or Zumba classes, while places like Pilates classes have not been associated with outbreaks. "Maybe slow, gentle breathing is not a risk factor, but heavy, deep, or rapid breathing and shouting is," she said.

Other factors that determine whether a superspreading event could occur include the timing in which a person enters a high-risk setting and the infected individual, Kupferschmidt reports.

For instance, Kucharski noted that research is showing Covid-19 patients are most infectious for a brief period of time, which means entering a high-risk setting during that period could cause a superspreading event. "Two days later, that person could behave in the same way and you wouldn't see the same outcome," Kucharski said.

In addition, Kupferschmidt reports that some people may simply be more likely to spread the new coronavirus than others because of differences in how their body reacts to the virus (Kupferschmidt, Science Magazine, 5/19).

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