New research adds to evidence that children infected with COVID-19 have less severe symptoms than adults with the coronavirus.

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Researchers analyzed the cases of 731 children in China with laboratory test-confirmed cases of the coronavirus and 1,412 children who were suspected of having COVID-19.

Most of those 2,143 cases were mild, and only one child died. Close to 6% of the children's cases were severe or critical, compared with 18.5% of cases in adults.

The study was published online March 16 in the journal Pediatrics.

It's not clear why children with COVID-19 tend not to become as ill as adults.

"There could be a number of reasons," said Dr. Bonnie Maldonado, chair of the AAP Committee on Infectious Diseases.

"It's possible that the receptor for the virus may not be in the same configuration in children as adults. It's possible that there were just more adults who were tested because that has been the focus," said Maldonado, who was not involved with the latest study. "However, there have been other studies of pregnant women and children who did not have as severe disease.

"I suspect the immune response of children is different," she said. "Their immune system is young and evolving."

The coronavirus "especially seems to be sparing young children," Maldonado noted.

The researchers also said their study provides strong evidence of person-to-person transmission of the coronavirus.

More than 90% of all the children in the study were asymptomatic or showed mild or common forms of illness—and about 13% of the children who tested positive for the virus didn't have symptoms of illness, a rate that "almost certainly understates the true rate of asymptomatic infection, since many asymptomatic children are unlikely to be tested," Dr. Andrea Cruz and Dr. Steven Zeichner wrote in an accompanying editorial.

Many questions need to be answered, according to Maldonado.

"Studies need to be done as soon as possible to understand the differential impact between adults and children, because that information could be helpful," she said.

When people have symptoms of the disease, it might be possible to contain it faster, but it's unknown if children with no symptoms of illness can spread the coronavirus to others, Maldonado said.

A different study from China recently published in the Journal of the American Medical Association found that while infants can become infected with the coronavirus, their illness is milder than in older people and in those with chronic health problems.

Another Chinese study found that even though children infected with the coronavirus typically have only mild symptoms, they can shed the coronavirus long after they no longer have symptoms. The research was published March 13 in Nature Medicine.