When it comes to the coronavirus and children, looks may be deceiving.

In a study of children under five who show mild to moderate symptoms of COVID-19, those kids were found to contain higher concentrations of the virus compared to older children, teens and adults, according to researchers at a Chicago pediatric hospital and Northwestern University.

The findings come as parents, educators and policymakers around the world grapple with the question of whether it's safe to reopen day-care centers and schools in the coming weeks.

The study, which was released Thursday in the journal JAMA Pediatrics, did not test the transmission rate of children—but does raise the prospect that children could be just as, or even more, prone to COVID infection and transmission than adults, although symptoms in the vast majority of children are comparably milder, the researchers found.

"One of the things that’s come up in the whole school reopening discussion, is: since kids are less sick, is it because they have less of the virus?," said Taylor Heald-Sargent, the lead author and a pediatric infectious diseases specialist at Lurie Children's Hospital of Chicago, and assistant professor of pediatrics at Northwestern University's Feinberg School of Medicine.

"And our data does not support that,” she told Fortune.As a result, "we can't assume that kids aren't able to spread the virus."

The study used clinical data collected during the treatment of 145 COVID-19 patients in Chicago with mild to moderate symptoms, who had begun to display those symptoms over the previous week.The sample was roughly divided into three equal groups: children under five, children between five and 18, and adults from 18 to 65.The sample excluded patients who were seriously ill, asymptomatic patients, and patients whose symptoms had been present for more than a week, or whose symptoms couldn't be tracked.

Nasal swabs taken from those patients indicated that the viral load in children under five was "statistically significant" compared to older children and adults, the paper found.

The trend was first noticed during clinical observations, Heald-Sargent said, when she and her co-authors first noticed that nasal samples from kids who were not particularly ill contained unusually high levels of the virus.

"We were surprised," she added.

There's still much to be learned about how most children become infected with the virus, how their immune systems respond to COVID-19, and how they transmit it, she added.What's better understood is that the symptoms shown in young children tend to be less severe than those of adults—that's despite the fact that young children are broadly more vulnerable to respiratory infections at a younger age.

The study also warns that because schools and day-care closed early in the lockdown, children broadly were less exposed to the virus than adults.Other studies have also tackled this observation—with mixed results.

Even with the new data, Heald-Sargent herself warns that reopening schools and day-cares is a "complex and nuanced" subject.Other pediatricians have warned that the impacts on children's mental health, education, and development could be damaged far more by long breaks from school and time spent in lockdown than by the virus itself, complicating the arguments for and against a great reopening.