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When scientists think of the perfect setting for coronavirus transmission, they describe an indoor space where people from various households gather. The people might be six feet apart, but they are still sharing the same space and air with strangers for longer than they do in most public settings. It’s a place where people linger, take their masks down, and speak and laugh with one another.

This describes what one expert termed “superspreader destinations.” Dine-in restaurants happen to fit the bill perfectly.

There are a host of reasons why banning indoor dining is a politically and morally difficult choice, with the restaurant industry and its workers already facing financial ruin after months of drastically reduced business.

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But on the question of COVID-19 safety, science is clear: Going out to eat is associated with increased risk of coronavirus transmission.

“People talk a lot about superspreader events, but there are also superspreader destinations — types of places that are especially risky and lead to especially high rates of infection,” said David Grusky, one of the researchers behind a recent Stanford University study that modeled the coronavirus’s spread in indoor spaces. “One of those types of places is full-service restaurants.”

How COVID-19 droplets can travel long distances in restaurants
The illustrated restaurant below is based on a Korean restaurant where COVID-19 spread among patrons.The grey boxes represent air conditioners; the blue arrows, the air flow patterns that the cool air followed.

Research has linked indoor dining to increased risk of COVID-19 spread. A number of factors make restaurants: Relatively high density, how long patrons linger and go without masks, and, in many cases, poor ventilation.

A study published in the Journal of Korean Medical Science shows one real life example of just how quickly COVID-19 can spread in a restaurant.

Person A sat down to a meal with a companion at 5:15 pm one day in June. They did not know it yet, but they likely already had COVID-19, and would start to feel sick the next day.

Person B left 5 minutes after A arrived. The two never crossed paths, and they sat more than 20 feet apart, but the blue arrows show how the ventilation system circulated droplets from A in B’s direction. B started to feel sick four days later.

Person C arrived last, a couple of minutes after B left. They were in A’s direct airflow for about 20 minutes, but seated almost 16 feet away. C tested positive for COVID-19 eight days later.

In a short time, and even with tables more than six feet apart, this restaurant became the site of a COVID-19 outbreak. Science says it’s probably not the only one.

How COVID-19 droplets can
travel long distances in restaurants

The illustrated restaurant below is based on a Korean restaurant where COVID-19 spread among patrons.
The grey boxes represent air conditioners; the blue arrows, the air flow patterns that the cool air followed.

Research has linked indoor dining to increased risk of COVID-19 spread. A number of factors make restaurants: Relatively high density, how long patrons linger and go without masks, and, in many cases, poor ventilation.

A study published in the Journal of Korean Medical Science shows one real life example of just how quickly COVID-19 can spread in a restaurant.

Person A sat down to a meal with a companion at 5:15 pm one day in June. They did not know it yet, but they likely already had COVID-19, and would start to feel sick the next day.

Person B left 5 minutes after A arrived. The two never crossed paths, and they sat more than 20 feet apart, but the blue arrows show how the ventilation system circulated droplets from A in B’s direction. B started to feel sick four days later.

Person C arrived last, a couple of minutes after B left. They were in A’s direct airflow for about 20 minutes, but seated almost 16 feet away. C tested positive for COVID-19 eight days later.

In a short time, and even with tables more than six feet apart, this restaurant became the site of a COVID-19 outbreak. Science says it’s probably not the only one.

Illustrations by Ryan Huddle/Globe staff; Animation Brendan Lynch/Globe staff

Even before states embarked on the long road toward reopening their economies, epidemiologists warned that the virus was likely to spread most rapidly indoors. Many listed indoor dining among the potentially high-risk activities that should only be resumed with extreme caution, if at all.

Since spring, Grusky and others said, emerging science has only reinforced disease experts’ initial fears. Studies have drawn on mathematical modeling, cellphone data, physics, and epidemiology to confirm over and over that dining out during the pandemic carries tremendous risk.

But even as hospitalizations climb and officials across Massachusetts and the country roll back reopening, restaurant dining rooms in many places remain conspicuously, and often controversially, open.

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Restaurant owners and the associations that represent them have fiercely defended their businesses’ safety, emphasizing the care and cost it takes to follow restrictions, including, in Massachusetts, spacing tables at least 6 feet apart, seating only parties of six people or fewer, limiting patrons’ dining time to 90 minutes, and closing by 9:30 p.m.

And some owners and patrons alike have pointed to data they feel is on their side: the state’s contact tracing reports.

Massachusetts contact tracers have linked 84 COVID-19 clusters to restaurants, a small fraction of the state’s 23,888 known case clusters.

But scientists urge a careful interpretation of that data. Of identified clusters, nearly all — 22,487 as of Thursday’s report — are classified as “household spread,” meaning two or more people living together became infected.

But that information “doesn’t really help us,” said Samuel Scarpino, a Northeastern University epidemiologist. We know that people who live together are likely to spread the disease to one another, he explained, but the first household member to be infected had to get COVID-19 somewhere else, whether at work, running errands, or out and about in their community.

“What we have to do is figure out how to stop [COVID-19] from getting into the households,” Scarpino said.

“It’s kind of scandalous that we have so little information [from contact tracing] about the venues of transmission this late in the pandemic,” said Marc Lipsitch, a professor of epidemiology at Harvard T.H. Chan School of Public Health. Lipsitch said contact tracing’s limitations are a national and even international problem, not one specific to Massachusetts.

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But even with state contact tracing data offering little evidence of how and where coronavirus spreads, Scarpino and Lipsitch both felt confident in saying that restaurants, and indoor dining in particular, are risky.

Lipsitch stressed that several types of research help locate the most likely sources of COVID-19′s spread: modeling that uses real-world data to predict how many people will be infected under different reopening scenarios; case-controlled studies that compare behaviors of COVID-positive people to people who are not infected; and basic physical and epidemiological facts about how the virus itself is transmitted.

All of those types of research point clearly to restaurants as one particularly high-risk activity, Lipsitch said. And the evidence is mounting.

In September, the Centers for Disease Control released a case-control study in which symptomatic people seeking COVID-19 tests were asked to list the types of places they had visited in the two weeks before getting tested.

Researchers found that those who tested positive were two times as likely to report having eaten at a restaurant, whether indoors or outdoors, compared with people who ended up testing negative. No other setting showed as strong a correlation with positive cases as did restaurants, a finding researchers attributed to the fact that eating and drinking require people to take off their masks, while most other activities do not.

In November, a group of Korean researchers took a closer look at how transmission passes from person to person in restaurants. The study, led by Dr. Ju-Hyung Lee of Jeonbuk National University Medical School, found that one restaurant outbreak was seeded in five minutes, with spread occurring between two patrons sitting more than 20 feet apart. A third person was also infected.

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None of the three patrons with confirmed cases interacted directly, but researchers diagramed air flow in the restaurant and found that the ventilation system likely carried droplets directly from table to table.

Add to that the Stanford model, also published in November. Grusky and his fellow researchers used cellphone data to map how 98 million people moved through 10 major US cities. They combined that data with reported daily case counts from each city to model how people’s visits to certain types of settings predicted coronavirus spread.

The model found that two mobility factors — how densely populated a given setting is and how long visitors stay — are closely associated with increased risk of COVID-19 transmission. It also predicts that a city’s decision to open one type of setting in particular — full-service restaurants — increases case counts more than any other setting the researchers studied.

High-density and long stays are “structural features” of restaurants, Grusky said, which helps explain why they account for more cases under the model than other high-risk settings, including fitness centers, cafes and snack bars, and hotels and motels.

Models, case studies, and ventilation diagrams aside, Lipsitch said the reason indoor dining spreads COVID-19 is simple — and specific to the coronavirus rather than restaurants themselves.

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“Uncovered, especially open, mouths, loud talking, and proximity and poor ventilation are all contributors to coronavirus transmission. All of that is really clear,” he said.

That the risk factors for this particular pandemic map so perfectly onto indoor dining is little more than a coincidence — a deeply unfortunate one for an industry struggling to stay afloat.

“There’s nothing mystical about restaurants,” Lipsitch said. “Whether it’s a restaurant, or a gym, or a house of worship, or a room you go into with your best friends and yodel, . . . it’s just that certain activities spread the virus more effectively.”



Dasia Moore is the Globe Magazine's staff writer. E-mail her at dasia.moore@globe.com. Follow her on Twitter @daijmoore.