As schools reopen in parts of the United States, a study published Thursday found that some children have high levels of virus in their airways during the first three days of infection despite having mild symptoms or none at all — suggesting their role in community spread may be larger than previously believed. One of the study’s authors, Alessio Fasano, a physician at MassGeneral Hospital for Children, said that because children tend to exhibit few if any symptoms, they were largely ignored in the early part of the outbreak and not tested. But they may have been acting as silent spreaders all along.

“Some people thought that children might be protected,” Fasano said. “This is incorrect. They may be as susceptible as adults — but just not visible.”

The study in the Journal of Pediatrics comes on the heels of two others that offer insights about children and coronavirus transmission. On July 30, researchers reported in JAMA Pediatrics that children younger than 5 with mild or moderate illness have much higher levels of virus in the nose, compared with older children and adults. Shortly before that, investigators in South Korea published a household study that some believed implied older children could spread the virus as readily as adults, while younger children less so. But researchers later clarified that it was unclear whether the transmission came from the older children or from contacts that they shared with other family members.