Illustration: Sarah Grillo/Axios

A small percentage of people — called superspreaders — may be responsible for a large number of COVID-19 infections, research is starting to indicate.

Why it matters: While there's no method to detect who these people are before they infect others, there are ways to control behaviors that cause superspreading events — a key issue as states start to reopen and debate what types of events are OK.

The latest: Three recent studies by the London School of Hygiene & Tropical Medicine, Tel Aviv University and the Institute for Disease Modeling in Washington, which have not yet been peer-reviewed, came to similar conclusions: Roughly 10% of COVID-19 cases appear to have caused around 80% of new infections.

It reflects the law of the "vital few," where a small number (between 5% and 20%) are responsible for the majority of cases, says Eric Topol, executive vice president of Scripps Research, who's also tweeted about it.

  • Indoor congregations — such as at churches, prisons, meatpacking plants and nursing homes — offer the highest risk, he says.
  • "These places, they deserve even tighter scrutiny for contact tracing. ... If you focused on tracing from these venues, these settings, that would be a wise use of resources," Topol adds.

Case in point: Allison James, a CDC epidemic intelligence service officer assigned to the Arkansas Department of Health, investigated a superspreader event in March at a church where 35 out of around 92 attendees fell ill, resulting in three deaths.

  • Similar clusters of COVID-19 cases have been reported in other places of worship, conferences, night clubs and parties.
  • As states begin reopening, if people choose to attend in-person services, James says she hopes they follow CDC safety guidelines.

"Superspreader events can sometimes give an outbreak a new trajectory, or allow it to sustain itself in a way that it wouldn't" otherwise, Amesh Adalja, senior scholar at the Johns Hopkins Center for Health Security, tells Axios.

What's happening: Why some people are superspreaders remains unknown. The person's genetics, immune system, how much virus they shed, and their behavior (such as how they speak, if they wash their hands often, if they socialize with large groups) likely play a role.

  • Topol says research is targeting this phenomenon because "if we knew the genomics or immunologic markers that these people have, maybe there's something we could do."
  • But Adalja says, "It's less about the biology than it is about the habits and interactions that individuals with the virus have, and what types of events they go to which puts them into contact with many different people in a way that spreads the virus."

Background: Different pathogens have different "reproductive numbers" (called R0 or R-naught), which is the average number of infections one person transmits. The R0 for the SARS-CoV-2 virus is thought to be between 2–3.

  • The problem is, these are averages and they don't reflect diseases where one person may infect 20 and others none.
  • The 1918 Spanish flu and the 2014–2016 Ebola outbreak both had an Ro of 2, but superspreaders played a greater role with Ebola. With contact tracing, the Ebola outbreak was contained to West Africa and only infected 30,000 people, whereas the Spanish flu was not contained and infected roughly 500 million people worldwide.
  • Another important factor is dispersion (called k), which indicates how much a disease clusters. The k for SARS-CoV-2 is unknown, with estimated rates ranging from o.1 to 10.

Flashback: Typhoid Mary, who was an Irish-born cook and asymptomatic carrier of typhoid, was considered a superspreader of the disease in New York in the early 20th century.

  • But Judith Leavitt, professor emerita in the department of medical history and bioethics at the University of Wisconsin, Madison, says Mary Mallon may have only been directly linked to about 50 cases over a 12-year period when she wasn't forcibly quarantined and may have been mislabeled due to fear and prejudice.
  • "She was made an example of for multiple reasons. She was Irish, representing an immigrant group that was then stigmatized and denigrated. She was a woman and not a family breadwinner," she adds.
  • Privacy is another important issue to discuss, Leavitt says, as contact tracing is super important for containment but must be done carefully.
  • Labeling people a superspreader can lead to stigmatization, as seen in Mary's case.

Besides contact tracing, some recommended steps to make events safer are reducing crowd density, wearing masks, changing seating layouts and installing good ventilation.

The bottom line: Understanding how superspreading works could help to fine-tune responses to the coronavirus pandemic — and curb it.

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