Among the most important unanswered questions about COVID-19 is this: What role do children play in keeping the pandemic going?

The answer is key to deciding whether and when to reopen schools, a step that President Donald Trump urged states to consider before the summer.

Two new studies offer compelling evidence that children can transmit the virus. Neither proved it, but the evidence was strong enough to suggest that schools should be kept closed for now, many epidemiologists who were not involved in the research said.

In one study, published last week in the journal Science, a team analyzed data from two cities in China — Wuhan, where the virus first emerged, and Shanghai — and found that children were about a third as susceptible to coronavirus infection as adults were. But when schools were open, they found, children had about three times as many contacts as adults and three times as many opportunities to become infected, essentially evening out their risk.

Based on their data, the researchers estimated that closing schools is not enough on its own to stop an outbreak, but it can reduce the surge by about 40% to 60% and slow the epidemic’s course.

“My simulation shows that yes, if you reopen the schools, you’ll see a big increase in the reproduction number, which is exactly what you don’t want,” said Marco Ajelli, a mathematical epidemiologist who did the work while at the Bruno Kessler Foundation in Trento, Italy.

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The second study, by a group of German researchers, was more straightforward. The team tested children and adults and found children who test positive harbor just as much virus as adults do — sometimes more — and so, presumably, are just as infectious.

Jennifer Nuzzo, an epidemiologist at Johns Hopkins University’s Bloomberg School of Public Health, said the decision to reopen schools cannot be made based solely on trying to prevent transmission.

“I think we have to take a holistic view of the impact of school closures on kids and our families,” Nuzzo said. “I do worry at some point, the accumulated harms from the measures may exceed the harm to the kids from the virus.”

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