Children under 5 can carry just as much of the coronavirus in their noses as older children and adults, researchers at Lurie Children's Hospital of Chicago reported Thursday.
The study, published in the journal JAMA Pediatrics, raises the possibility that young kids may be able to spread COVID-19 as easily as adults, even if they aren't that sick.
Dr. Taylor Heald-Sargent, a pediatric infectious disease specialist at Lurie Children's, and her colleagues analyzed data from the diagnostic tests of 145 COVID-19 patients who had mild to moderate cases of the illness. The tests look for pieces of the virus's RNA, or genetic code, to make a diagnosis.
The 145 patients were split into three groups: those under 5, those ages 5 to 17, and adults ages 18 to 65.
"Children had equal — if not more — viral RNA in their noses compared to older children and adults," Heald-Sargent said.
Compared to adults, the young kids had anywhere from 10 to 100 times the amount of viral RNA in their upper respiratory tract, the study authors wrote.
"This supports the idea that children are able to get infected and replicate virus and therefore shed and transmit virus just as much as older children and adults," she said, noting that more research is needed to confirm this.
Indeed, "you can have somebody who has high viral load in the nose, but that doesn't mean necessarily that they're going to spread more than somebody who has a little less," said Dr. Rick Malley, a senior physician in pediatrics in the division of infectious diseases at Boston Children's Hospital.
"We don't know that for sure," Malley, who was not involved with the new study, said.
Still, the findings add another layer to the complex question of whether schools should reopen their doors for the fall semester, and if so, how do to so safely.
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"We don't have the evidence that children will play the same role with this virus as they do, say, with the flu virus, where it's pretty clear that kids with flu are main drivers of spread," Malley said.
However, he added, COVID-19 is "behaving in an unpredictable way."
Some young people have developed a potentially deadly condition called multisystem inflammatory syndrome in children, or MIS-C, that's believed to be linked to COVID-19.
The condition is relatively rare; the Centers for Disease Control and Prevention reported that as of July 15, 342 cases of MIS-C had been diagnosed in the country. Six children died.
Overall, though, children have largely been spared the most severe consequences of COVID-19.
In Heald-Sargent's study, patients' ages ranged from less than 1 month old to age 65. Those who needed help breathing were excluded from the study. All were diagnosed in March and April.
Lurie Children's required all hospitalized patients to undergo a COVID-19 test, and some cases were discovered, even if children had minimal to no symptoms.
"We were catching kids who came in with a broken arm who happened to test positive," Heald-Sargent said.
It remains unclear how prevalent COVID-19 is among children, in part because testing is limited, especially for those without symptoms. And schools have largely been closed since spring, making it difficult to ascertain how widely kids can spread this virus.
There are some theories for why children may not spread the coronavirus as easily as adults: Their lung capacity is smaller, so they may not be able to cough or sneeze with the same force as adults. Also, whatever respiratory droplets they emit may fall to the floor because their bodies are simply closer to the floor.
Heald-Sargent, who has young children of her own, dismissed the latter idea. "We have to remember that COVID-19 can be shed in the stool, it can be in the mouth and the nose. Kids touch that. They are little germ factories."
This story was originally published on NBC News.