Early data downplayed the role of children in the coronavirus outbreak because they displayed few, if any, symptoms. But a new study adds to the growing body of evidence that children may be acting as silent spreaders of COVID-19.

The latest evidence published in the Journal of Pediatrics found that some children have high levels of the virus in their airways even if they are asymptomatic or have very mild symptoms. These levels appear to peak during the first three days of infection, and can be instrumental in spreading disease through the community, especially as schools reopen.

“Some people thought that children may be protected,” said study author Dr. Alessio Fasano, of MassGeneral Hospital for Children, according to The Washington Post. “This is incorrect. They may be as susceptible as adults—but just not visible.”

On July 30, other researchers reported that children under the age of 5 with mild or moderate illness had higher levels of the virus in their nose than older children and adults. That study was published in JAMA Pediatrics. Overseas, scientists in South Korea revealed that older children can spread the coronavirus as readily as adults. This study also found that children under the age of 9 are far less likely to transmit the disease to other members of their household than their older siblings, according to the New York Post.

Dr. Adrienne Randolph, of Boston’s Children’s Hospital, is further conducting research involving 73 medical centers across the U.S. to analyze biological samples from children in the hopes of trying to understand therapies and overcoming COVID-19.

“This disease never stops surprising us,” she said, according to The Washington Post. “I think we shouldn’t make any assumptions that minimize the virus.”