The coronavirus raging around the globe has tended to tread gently with children, who account for the smallest percentage of the tens of thousands of 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.