In the heated debate over reopening schools, one burning question has been whether and how efficiently children can spread the virus to others.
A large new study from South Korea offers an answer: Children younger than age 10 transmit to others much less often than adults do, but the risk is not zero.
And those between the ages of 10-19 can spread the virus at least as well as adults do.
The findings suggest that as schools reopen, communities will see clusters of infection take root that includes children of all ages, several experts cautioned.
"I fear that there has been this sense that kids just won't get infected or don't get infected in the same way as adults and that, therefore, they're almost like a bubbled population," said Michael Osterholm, infectious diseases expert at the University of Minnesota.
"There will be the transmission," Osterholm said. "What we have to do is accept that now and include that in our plans."
Several studies from Europe and Asia have suggested that young children are less likely to get infected and to spread the virus. But most of those studies were small and flawed, said Dr Ashish Jha, director of the Harvard Global Health Institute.
The new study "is very carefully done, it's systematic and looks at a very large population," Jha said. "It's one of the best studies we've had to date on this issue."
Other experts also praised the scale and rigor of the analysis. South Korean researchers identified 5,706 people who were the first to report COVID-19 symptoms in their households between Jan. 20 and March 27, when schools were closed, and then traced the 59,073 contacts of these "index cases." They tested all of the household contacts of each patient, regardless of symptoms, but only tested symptomatic contacts outside the household.
The first person in a household to develop symptoms is not necessarily the first to have been infected, and the researchers acknowledged this limitation. Children are also less likely than adults to show symptoms, so the study may have underestimated the number of children who set off the chain of transmission within their households.
Still, experts said the approach was reasonable. "It is also from a place with great contact tracing, done at the point interventions were being put in place," said Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.
Schools reopening and risk of new infections
Children under age 10 were roughly half as likely as adults to spread the virus to others, consistent with other studies. That may be because children generally exhale less air - and therefore less virus-laden air - or because they exhale that air closer to the ground, making it less likely that adults would breathe it in.
Even so, the number of new infections seeded by children may rise when schools reopen, the study authors cautioned. "Young children may show higher attack rates when the school closure ends, contributing to community transmission of COVID-19," they wrote. Other studies have also suggested that a large number of contacts for schoolchildren, who interact with dozens of others for a good part of the day, may cancel out their smaller risk of infecting others.
The researchers traced the contacts only of children who felt ill, so it is still unclear how efficiently asymptomatic children spread the virus, said Caitlin Rivers, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
"I think it was always going to be the case that symptomatic children are infectious," she said. "The questions about the role of children are more around whether children who don't have symptoms are infectious."
Rivers was a member of a scientific panel that on Wednesday recommended reopening schools wherever possible for disabled children and for those in elementary schools because those groups have the most trouble learning online. She said the new study does not alter that recommendation.
Middle and high school more 'worrisome'
The study is more worrisome for children in middle and high school. This group was even more likely to infect others than adults were, the study found. But some experts said that finding may be a fluke or may stem from the children's behaviors.
These older children are frequently as big as adults, and yet may have some of the same unhygienic habits as young children do. They may also have been more likely than the younger children to socialize with their peers within the high-rise complexes in South Korea.
"We can speculate all day about this, but we just don't know," Osterholm said. "The bottom line message is: There's going to be the transmission."
He and other experts said schools will need to prepare for infections to pop up. Apart from implementing physical distancing, hand hygiene, and masks, schools should also decide when and how to test students and staff - including, for example, bus drivers - when and how long to require people to quarantine, and when to decide to close and reopen schools.
But they face a monumental challenge because the evidence on transmission within schools has been far from conclusive so far, experts said. Some countries like Denmark and Finland have successfully reopened schools, but others, like China, Israel and South Korea, have had to close them down again.
"People, depending on their ideology on school opening, are choosing which evidence to present - and that needs to be avoided," said Jeffrey Shaman, an epidemiologist at Columbia University's Mailman School of Public Health in New York.
Although the new study does not offer definitive answers, he said, it does indicate that schools can increase virus levels within a community.
"So long as children are not just a complete dead end - incapable of passing the virus on, which does not seem to be the case - putting them together in schools, having them mix with teachers and other students will provide additional opportunities for the virus to move from person to person," he said.
At the same time, Shaman said, it's important for children not to miss out on critical years in education and socialization, and school districts have the unenviable task of choosing between those options: "It's hard trying to find the right balance."