Children can spread the coronavirus like anyone else — but there's still a lot to learn about how infectious they are

• Some health experts have suggested that children aren't common carriers of the coronavirus.
• But a growing body of research suggests that kids can transmit the virus like anyone else.
• Researchers in Germany recently determined that "children may be as infectious as adults." They cautioned against reopening schools too soon.
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There has been much confusion and debate about the coronavirus' impact on children and their role as potential carriers — in large part due to a lack of data. Since children are likely to develop mild symptoms, or none at all, they're often excluded from clinical studies and data collected from testing centers.

Some public-health officials have taken this as an indication that children are less infectious than adults.

"Young children are not infected and do not transmit the virus," Switzerland's infectious diseases chief, Daniel Koch, said at a news conference on April 29.

Swiss children under the age of 10 are now permitted to hug their grandparents, provided that the contact is brief and the children aren't showing any coronavirus symptoms. The country reopened schools on Monday, following in the footsteps of nations like China, Norway, Denmark, Germany, and Japan.

But a growing body of research suggests that children can transmit the coronavirus like anyone else.

Researchers in Germany recently determined that "symptoms are not a good predictor of infection," since adults and children can still excrete viral particles when they're asymptomatic. Scientists also haven't found any difference in the viral loads (the amount of viral particles released into the environment) of children versus adults.

"Based on these results, we have to caution against an unlimited re-opening of schools and kindergartens in the present situation," the German researchers wrote. "Children may be as infectious as adults."

Similarly, a CDC study concluded that "persons with asymptomatic and mild disease, including children, are likely playing a role in transmission and spread of COVID-19."

Children may be prone to mild or asymptomatic cases

Children under 10 represent around 1% of confirmed coronavirus infections. But it's not clear yet whether they're less likely to contract the virus in the first place, or whether many of their cases are simply being missed because they are so often mild or asymptomatic.

The largest study of children with the coronavirus to date, from the US Centers for Disease Control and Prevention, found that 18% of those studied tested positive but didn't report symptoms. But that report only included kids with confirmed cases, so the proportions could be skewed.

Because the immune system becomes more dysregulated as a person ages, some experts think the pediatric immune system could be better at battling the coronavirus than the adult immune system. The exception to this finding, however, might be infants: 62% of infected infants in the CDC study were hospitalized, the highest percentage among the various pediatric age groups. Some scientists suspect that's because infants have a greater inflammatory response than kids of other ages, so they're more likely to experience tissue damage as their bodies try to fight off the virus.

Scientists have also found evidence that children may have less mature ACE2 receptors — the enzymes that serve as ports of entry for the coronavirus — which could make it more difficult for the virus to infect a child's cells.

A recent study in The Lancet, however, found that children in Shenzhen, China, were just as likely to be infected with the coronavirus as adults: Children under 10 had a 7.4% infection rate compared to 6.6% for the general population. By contrast, a study of 80% of the population in the Italian village of Vó found that no children ages 10 and younger had contracted the virus there — even though at least 13 children lived with infected family members.

Scientists do mostly agree, however, that an infected child does not have to be coughing or sneezing to be contagious.

"It is a scary thought, but you can still have and spread coronavirus even if you don't have symptoms," Juan Dumois, an infectious diseases physician at Johns Hopkins All Children's Hospital, wrote on the university's website.

Does low transmission among kids justify reopening schools?

Some health experts see children's low infection rates as reason to reopen schools.

In an opinion letter published Wednesday in the journal JAMA Pediatrics, two Italian pediatric doctors argued that school closures may do more harm than good by "deepening social, economic, and health inequities." The doctors noted "the very low incidence of symptomatic disease among school-aged children."

The Dutch government — which reopened primary schools on May 11 and plans to reopen secondary schools on June 2 — also reported that "children play a small role in the spread of the novel coronavirus." The conclusion was based on the government data on transmission among 54 Dutch households.

However, recent surveys conducted in Wuhan and Shanghai found that when schools were open, children had about three times as many contacts — either brief in-person conversations or physical interactions such as a handshake — as adults. The researchers determined that closing schools reduced peak infections in these cities by up to 60%.

"The risk of catching coronavirus is lower for kids who are at their home and not exposed to other kids at school or daycare," Dumois wrote on the Johns Hopkins website.

Around one-third of coronavirus transmission takes place in schools and offices, while another third occurs in the community at churches, bars, restaurants, grocery stores, and playgrounds, according to research from Imperial College London. When it comes to saving lives, however, the Imperial College models found that school closures only prevent 2% to 4% of coronavirus deaths.

"Are any of these studies definitive? The answer is 'No, of course not,'" Jeffrey Shaman, an epidemiologist at Columbia University, told the New York Times. "To open schools because of some un-investigated notion that children aren't really involved in this, that would be a very foolish thing."