Children and teenagers are only half as likely to get infected with the coronavirus as people 20 and older, and they usually don’t develop clinical symptoms of covid-19, the disease caused by the virus, according to a study published Tuesday.

The findings could influence policymakers who are facing tough decisions about when and how to reopen schools. Distance learning has been challenging for teachers, students and parents, and there is pressure on officials to restart in-person schooling to free up working-age parents who have been juggling work and child care.

“These results have implications for the likely effectiveness of school closures in mitigating SARS-CoV-2 transmission, in that these might be less effective than for other respiratory infections,” write the authors, based at the London School of Hygiene & Tropical Medicine.

The study also has implications for the likely disease burden in countries with much younger populations, many of which are in the developing world. To date, most of the countries hit hardest by the coronavirus have had relatively old populations.

From the start of the pandemic, it has been known that children are typically spared the worst effects of the disease. They rarely die of it. But they can still get sick and can spread the virus, including to older family members more likely to have a severe illness.

The reasons for the apparent protective effect of youth have never been clearly understood. A common theory – noted by the authors of the new study – is that the decreased susceptibility to infection or serious illness among children could “result from immune cross-protection from other coronaviruses,” or be attributable to recent infection from other respiratory viruses such as influenza.

The study, published in the journal Nature Medicine, is based on a survey of six nations: Canada, China, Italy, Japan, Singapore and South Korea. The researchers developed mathematic models to interpret the demographic patterns of covid-19 cases in those countries.

Until now, there has been mixed evidence about infections among children. A study from Shenzhen, China published in April found that, within households, children were as likely to become infected as adults. A separate study from China found no difference between boys and girls in infection rates.

This new report, however, estimates that children are only about half as likely to become infected. When they do, they usually remain asymptomatic, or have mild, “subclinical” symptoms. Among individuals between the ages of 10 and 19 infected with the virus – SARS-CoV-2 – only 21% show symptoms, compared to 69% among patients over the age of 70, according to the authors.

“[W]e find that interventions aimed at children might have a relatively small impact on reducing SARS-CoV-2 transmission, particularly if the transmissibility of subclinical infections is low,” the researchers write.

In an email Tuesday, lead author Nicholas Davies noted that school closures are a complicated issue: “[S]chool closures still do have an effect-we’re not saying they’re completely ineffective. So really, this just highlights how difficult the question of when to reopen schools is. Like with many other policies, it’s not a straightforward question of epidemiology.”

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