The coronavirus raging around the globe has tended to tread gently with children, who account for the smallest percentage of the infections identified so far.

Now, the largest study to date of children and the virus has found that while most develop mild or moderate symptoms, a small percentage — especially babies and preschoolers — can become seriously ill.

The study, published online in the journal Pediatrics, looked at more than 2,000 ill children across China, where the pandemic began. It provides a clearer portrait of how the youngest patients are affected by the virus, knowledge that experts say can help influence policies like school closures, hospital preparedness and the deployment of an eventual treatment and vaccine.

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The researchers analyzed 2,143 cases of children under 18 that were reported to the Chinese Centers for Disease Control and Prevention as of Feb. 8. Just over a third of those cases were confirmed with laboratory testing. The rest were classified as suspected cases based on the child’s symptoms, chest X-rays, blood tests and whether the child had been exposed to people with coronavirus.