“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. 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 who are more likely to have a severe illness.
The reasons for the apparent protective effect of youth are not 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 mathematical 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.
The new report, however, estimates that children are only half as likely to become infected. When they do, they usually remain asymptomatic, or have mild, “subclinical” symptoms. Among people between the ages of 10 and 19 infected with the virus — SARS-CoV-2 — only 21 percent show symptoms, compared with 69 percent among people older than 70, according to the authors.
“We 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: “School 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.”
The researchers found that children were 35 to 60 percent less susceptible to coronavirus infection than adults 20 and over. That statistical range reflects uncertainty in the data that is fed into the mathematical model, he said.
The findings come as states and school systems consider when and how to reopen schools this fall.
What the study does not answer is the extent to which children, including ones with no symptoms, can transmit the virus. That is a major factor in school closings: Even if children as a group are less likely to become seriously ill from the disease, their teachers and other adult staffers at school are in higher-risk age cohorts.
Officials have also been concerned that children could pass the virus on to a family member at home. Any research showing children are at relatively low risk from the virus could give school districts comfort as they work to reopen buildings in some form this fall, as most hope to.
Many systems are considering a hybrid model, with some children in the building and others learning from home in a staggered schedule, to minimize the numbers on site at any time.
Some parents have expressed concern about sending their children into school buildings before there is a widely accepted treatment or vaccine for the virus. But many others are eager for in-person learning to resume, both because the parents need to be at work and because remote education has fallen far short of meeting children’s academic and social-emotional needs.
The science, meanwhile, remains far from clear-cut.
“While the evidence is mixed, children clearly have some role and can be infected. I think that opening schools and daycare facilities is very risky,” Jeffrey Shaman, an epidemiologist at Columbia University, said in an email Tuesday.
Davies noted that he and his colleague found large variations among countries in the apparent susceptibility to infection across different age groups. Some of that might simply be “noise” in the data, he said, and the researchers combined the data from various countries to get their estimates. It’s possible, though, that there may be real differences among countries, he said, noting that a recent study from the United Kingdom found no difference in infection rates among children and adults.
“It just emphasizes that more research would help to clarify this question,” he said.
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