A new study is challenging the idea that younger children are somehow less susceptible to SARS-CoV-2 infection. Children under the age of five have been found to carry just as much, if not more, coronavirus in their noses and throats than older kids or adults.

The results, published Thursday (July 30) in JAMA Pediatrics, tested 145 people for evidence of the virus’s RNA. After breaking their participants down into three age categories—younger children, older children, and adults—researchers found that the youngest group harbored between 10 times and 100 times more virus than the other two. While the results cannot speak to children’s ability to transmit the disease to others, they come at a time when schools nationwide weighing the risks of opening again in the fall.

“The school situation is so complicated—there are many nuances beyond just the scientific one,” Taylor Heald-Sargent, a pediatric infectious diseases expert at the Ann and Robert H. Lurie Children’s Hospital and the lead author on the study, tells The New York Times. “But one takeaway from this is that we can’t assume that just because kids aren’t getting sick, or very sick, that they don’t have the virus.”

Heald-Sargent first became interested in the question of how COVID-19 affects children after receiving some strange test results—children’s swab samples were indicating a higher load of virus than adults, a counterintuitive observation given that kids appear far less likely to develop symptoms. To determine the viral load in patients, doctors will take a standard nasopharyngeal swab and analyze it using a method called quantitative polymerase chain reaction (qPCR). During each cycle of the qPCR, the initially small amount of viral genetic material doubles, leading to an exponential increase. If there is more genetic material on the swab at the start, fewer cycles are needed to hit what is called a cycle threshold, or CT.

Samples collected from younger children generally had lower CTs, meaning they had more virus present in their samples from the beginning. Heald-Sargent decided to look back over the several weeks, between late March and late April, to see how common this result was. Originally, she tells the Times, “it wasn’t even something we had set out to look for.”

Alongside her colleagues, Heald-Sargent analyzed samples taken from 145 people who had either visited drive-through testing sites in Chicago or visited local hospitals for any reason. Because people with severe infections would be expected to have high viral loads, the team included only individuals with either mild or moderate symptoms. In addition, they excluded people with asymptomatic infections, those who couldn’t pinpoint when their symptoms had started, and people who had experienced symptoms for more than a week before testing.

After dividing the participants into the three age groups—specifically, children under the age of five, children between five and 17, and adults aged 18 to 65—the scientists saw that the low CT results were consistent in younger patients, while the CTs of older children and adults were more comparable to each other. “It definitely shows that kids do have levels of virus similar to and maybe even higher than adults,” Heald-Sargent tells the Times. 

The results are in line with similar small studies from other countries. In Germany, 47 children between one and 11 were found to have viral loads as high as those in adults. Among 438 children in France, CT scores were similar between asymptomatic children and those with obvious symptoms, supporting the idea that children can harbor high amounts of the virus without becoming visibly ill.

Alpana Waghmare, a specialist in pediatric infectious diseases at Seattle Children’s, says the results of this latest study are a welcome addition. “The data in pediatrics has not been as robust as adults with Covid-19 so it’s really nice to have additional virologic data in pediatric patients,” she tells CNN. “It’s not surprising to find higher viral loads in children. I think the question of what that exactly means for transmission is still not clear.”  

Researchers in South Korea recently published a study looking at disease transmission in children and young adults, finding that those between 10 and 19 years old transmitted the disease just as frequently as adults, although children under nine were found to transmit COVID-19 less frequently, CNNreports.

Scientists have also been turning to other diseases to assess how likely children are to transmit the coronavirus, but evidence is still lacking. Rick Malley, a senior physician in pediatrics at Boston Children’s Hospital, tells NBC News, “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.”

Heald-Sargent says it’s likely the same could be true with COVID-19. “Any grade-school teacher or pediatrician will tell you, [young children] are pretty effective little vectors of virus transmission, because we get sick a lot in the winter from these kids,” she says to CNN. “I think looking at other viruses that are similar . . . it would seem more likely that kids will be transmitting” COVID-19 as well, she says, adding that further research will be needed to confirm it.