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COVID-19 case clusters offers New warnings for reopening

COVID-19 case clusters offers New warnings for reopening

Outbreaks in restaurants, offices and other venues could guide strategies for lifting social distancing guidelines.


everal months into the COVID-19 pandemic, countries are looking for ways to restart their economies, public health officials are looking to guide safe reopening and people are eagerly looking to escape cabin fever. But tough lessons have surfaced from countries that were hit early in the pandemic and have already reopened.

Consider South Korea: In April, after new cases had steadily declined to single digits, the country began easing lockdown restrictions. But that respite was short-lived. On May 6, a 29-year-old man tested positive for SARS-CoV-2, just a few days after visiting five dance clubs in one night in Seoul’s Itaewon district. On May 8, South Korea responded quickly, postponing plans to reopen schools and urging bars and clubs to shut down again for a month. As of June 8, the Korean Centers for Disease Control had linked the sick man to 96 other clubgoers who got COVID-19, plus 178 people with whom those clubgoers came into contact.

That wasn’t the only cluster that put the brakes on South Korea’s reopening plans. Soon more clusters popped up in an online retail center, a theme park, a table tennis club and a handful of churches.

Other countries should expect similar starts and stops upon relaxing stay-at-home rules. “Reopening is not a one-way street, and we may need to make a U-turn,” says Andrew Noymer, an epidemiologist at the University of California, Irvine.

Studying these kinds of transmission clusters as well as common environments where COVID-19 moves easily from person to person provides a glimpse of how to avoid the U-turns. To that end, epidemiologist Gwenan Knight and her colleagues at the London School of Hygiene and Tropical Medicine compiled a massive database of worldwide COVID-19 case clusters based on media accounts, published scientific studies and government health department reports.

As of June 10, their database included 231 cluster events, or groups of cases tied to the same place. The data are limited to known clusters and to what the patients involved could recall and what they told investigators.

Some interesting trends have emerged. Indoor settings dominate, partly because the virus hit during winter, when people spend most of their time indoors, and partly because in outdoor settings an abundant flow of fresh air helps dissipate virus particles exhaled by an infected person. Ultraviolet rays from the sun might also help kill virus particles.

Households were the most common place for transmission, accounting for 15 percent, or 38 of 231, cluster events. And mealtime was prime time. “It’s not the eating. It’s the sitting around and talking,” Noymer suspects. SARS-CoV-2 primarily spreads via respiratory droplets and direct contact. While sharing food or utensils could theoretically pose a risk for infection, no clusters have been linked to eating itself. A study published June 3 in Emerging Infectious Diseases found traces of SARS-CoV-2 genetic material on chopsticks used by patients in Hong Kong, but whether virus particles could survive on a chopstick and actually infect someone remains unknown.

The largest known clusters occurred in settings that are by now well publicized: ships (the Diamond Princess cruise ship and the USS Teddy Roosevelt aircraft carrier), food packing plants and prisons. Dormitories in Singapore, where migrant workers live in cramped conditions, also saw large numbers of infections. In all of these places, people live or work in close quarters over long periods of time.

Restaurants and workplaces can take steps when they reopen to try to limit transmission, as some have already done. Installing plexiglass barriers between tables and improving ventilation by opening windows, serving customers outside, turning on exhaust fans and using HEPA filters could help lower transmission risk. Keeping tables or desks at least six feet apart is also a good idea. The CDC has released reopening guidelines for everything from restaurants to water parks.

Whether more outdoor cases will emerge as temperatures rise and people spend more time outside is an open question. Fresh air and sunlight could limit cases in outdoor settings, or clusters could emerge in newly opened outdoor settings, such as swimming pools or summer camps. Data on how the virus behaves in summer just don’t exist yet.

If South Korea’s thwarted first attempt at loosening restrictions has taught us anything, it’s that reopening and lifting social distancing practices comes with one certainty: New COVID-19 infections will happen. Hugs and high-fives are still a long way off. “This isn’t a marathon,” Noymer says. “It’s a 26.2-mile sprint.”

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