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Superspreader Events Offer a Clue on Curbing Coronavirus

The Wall Street Journal. logo The Wall Street Journal. 5/21/2020 Bojan Pancevski
FILE - In this Tuesday, Feb. 25, 2020, file photo, Bourbon Street is a sea of humanity on Mardi Gras day in New Orleans. In the city here, the old saying “Let the good times roll” has given way to a new municipal maxim: “Wash your hands.” A month ago the city was awash in people and steeped in its annual tradition of communal joy — the Mardi Gras season. But now New Orleans has joined those places shutting down bars, eliminating restaurant dining and banning crowds. (AP Photo/Rusty Costanza, File) © Associated Press FILE - In this Tuesday, Feb. 25, 2020, file photo, Bourbon Street is a sea of humanity on Mardi Gras day in New Orleans. In the city here, the old saying “Let the good times roll” has given way to a new municipal maxim: “Wash your hands.” A month ago the city was awash in people and steeped in its annual tradition of communal joy — the Mardi Gras season. But now New Orleans has joined those places shutting down bars, eliminating restaurant dining and banning crowds. (AP Photo/Rusty Costanza, File)

Some scientists looking for ways to prevent a return to exponential growth in coronavirus infections after lockdowns are lifted are zeroing in on a new approach: focus on avoiding superspreading events.

The theory is that banning mass public events where hundreds of attendees can infect themselves in the space of a few hours, along with other measures such as wearing face masks, might slow the pace of the new coronavirus’s progression to a manageable level even as shops and factories reopen.

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Researchers believe that the explosive growth of coronavirus infections that overwhelmed hospitals in some countries was primarily driven by such events earlier this year—horse races in Britain, carnival festivities in the U.S. and Germany or a soccer match in Italy.

The study of superspreading events could help scientists better understand how the virus can propagate in crowded conditions—in offices, schools, churches, gyms and public transportation—and guide governments in regulating such public occasions as weddings, trade conferences and sports games.

There is little doubt about the mechanisms involved in superspreading events. A study published by the National Academy of Sciences in the U.S. last week found that one minute of loud speech was enough to produce thousands of droplets that remain airborne for about 12 minutes, potentially able to infect anyone in the area. Similar studies have shown that virus-laden aerosols, particles smaller than droplets, can levitate for hours after being released in indoors spaces.

A more surprising finding is that mass infections tend to be more serious than those contracted in other circumstances, perhaps because of sustained exposure to a larger amount of virus.

“Most cases globally, and especially most deaths, happened after superspreading events,” said Hendrik Streeck, a virologist with the University Hospital Bonn, Germany, who published the world-wide first study of a novel coronavirus superspreading event.

His research into the outbreak in the western county of Heinsberg, which in March became a center of the epidemic in Germany, established that the infection spread across the region like wildfire after around 400 people took part in a traditional carnival party. They drank, sang, kissed and danced for several hours in a large hall on Feb. 15.

The people who attended not only got infected and then spread the virus across the county, but also showed stronger symptoms and a comparatively severe illness, Dr. Streeck says—possibly because they received a higher load of the virus from close and prolonged exposure. Weeks later, thousands were infected across the region and dozens died.

Superspreading events exist in many infectious diseases, but with Covid-19 they are especially dangerous because the virus has a longer period of incubation in which patients show no symptoms but can infect others. Sars and MERS, two other deadly coronaviruses that produced smaller global outbreaks in recent years, were also driven by superspreading events, research has shown.

The Mardi Gras festivities in Louisiana, a choir practice in Skagit County, Washington and a meeting of executives of the Biogen drug company near Boston are among the one-off events scientists think helped give the pandemic a fateful boost.

U.S. meatpacking plants, where hundreds have become infected, have also emerged as superspreading sites: counties with or near meatpacking plants have been found to have nearly twice as many Covid-19 cases as the national average, according to an analysis by the Environmental Working Group, a nonprofit organization.

In April, Blaine County in Idaho became one of America’s coronavirus hot spots when hundreds of people tested positive following an apres-ski party. Smaller events like weddings, parties and funerals have also served to turbocharge contagion. In one case, an infected individual visited a funeral and a birthday party within three days in February, spreading the virus to 16 people, three of whom died.

“It is now pretty clear that large groups of people close together are good opportunities to spread the virus,’’ said Marc Lipsitch, professor of epidemiology at the Harvard T.H. Chan School of Public Health.

The experience of several European countries seems to confirm the special role played by superspreading events. Over the past four weeks, Germany, Austria, Denmark, Norway and other countries that have exited early from lockdowns have removed most restrictions on public life except those targeting mass gatherings. So far, new infections have remained low and constant. Sweden, which never had a mandatory lockdown, managed to control and then reduce the spread by relying on only one restrictive measure: prohibiting gatherings of over 50 people.

One remaining question mark regards schools. While no country where schools have reopened has so far reported a sharp increase in infections, some scientists fear schools could act as accelerators for the pandemic.

Sars, another coronavirus that originated in China and is genetically near-identical to Covid-19, briefly spread world-wide in 2003 after a guest at the Metropole Hotel in Hong Kong infected international visitors who then spread the disease across continents, according to Professor Michael Small, a lecturer in applied mathematics at the University of Western Australia.

Prof. Small, who holds the Chair in Complex Engineering Systems at CSIRO, the country’s national science agency, studied both coronavirus outbreaks and says the lesson is that authorities must curb all gatherings of more than 100 people.

“This could well be the end of the open-plan office,” he said. “You can see it clearly from the data in many, many places: superspreading events cause bursts of infection that fuel exponential growth, but that can very quickly be reduced to linear growth if you limit the mass gathering of people.”

His modeling shows that lockdowns could be replaced by targeted measures with a much smaller economic impact, such as banning mass events, asking a significant number of white-collar workers to work from home and encouraging widespread use of smartphone contact-tracing apps.

What about crowded subways and commuter trains? Prof. Small is confident that the use of subways during rush-hour is certain to turn into a super-spreading event.

When London authorities reduced the number of subway trains in March—causing greater crowding than usual—they created superspreading conditions, said Prof. Michael Levitt, a Stanford lecturer and Nobel Prize laureate. He advocates the use of face masks and regular testing of bus drivers, shopkeepers and delivery couriers. Bars should also be regulated, he said, because loud music there forces patrons to speak louder.

In Britain, which has one of the worst Covid-19 death rates in the world, authorities allowed for a series of mass events to take place in March, including large-scale concerts, soccer games and horse races. George Batchelor, director of Edge Health, a data analytics firm that works with Britain’s health-care provider, thinks those gatherings prompted a significant increase in hospitalizations and mortality related to Covid-19 in the respective regions. He studied two soccer matches and a horse race—all of which took place outdoors, preceded and followed by the mass use of public transport and visits to bars and pubs.

“It would seem very unwise to allow for any such events any time soon,” Mr. Batchelor said.

Some of the lessons from the research are already being applied. In Germany, choral singing has been banned from religious services and Bundesliga soccer games are taking place without spectators, while churches in Britain are considering a ticket system to avoid crowding.

Austria will allow cultural events, such as concerts with up to 1,000 visitors, under strict security measures starting from August, while clubs and nightlife venues will remain closed, a government spokesman said, after outbreaks in such establishments in South Korea. A study published this week found that banning mass gatherings had the biggest contribution to bringing the epidemic under control in Germany.

Superspreading events could even reignite the epidemic when the situation appears under control, said Prof. Cristopher Moore, a physicist with the Santa Fe Institute.

Dr. Streeck, the German virologist, agrees. While most experts expect a deadly second wave of coronavirus infections in the fall, he thinks a sharper focus on preventing superspreading events and vigilant monitoring could help avoid such a scenario.

“We are all conducting experiments in our countries—no one knows how to do this right,” he said.

Write to Bojan Pancevski at 


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