The World Health Organization said parents need to prepare their kids to guard against COVID-19 after a new study showed that babies and very young children can sometimes develop severe symptoms.

A recent study showed that a number of children in China have developed severe or critical disease and one child has died, said Dr. Maria Van Kerkhove, head of WHO's emerging diseases and zoonosis unit, at a news conference on Wednesday. "What we need to prepare for is the possibility that children can also experience severe disease."

The new study, which was published online in the journal Pediatrics, looked at 2,143 cases of children with confirmed or suspected COVID-19 that were reported to the Chinese Centers for Disease Control and Prevention between Jan. 16 and Feb. 8. More than 90% of the cases were asymptomatic, mild or moderate cases. However, nearly 6% of the children's cases were severe or critical, compared with 18.5% for adults.

"There could be a number of reasons," Bonnie Maldonado, an infectious disease expert at Stanford who was not involved with the study, said in a statement. "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."

Early in the outbreak, researchers said the new virus appeared to be sparing children while being particularly severe in the elderly and those with underlying health conditions. Symptoms can include a sore throat, runny nose, fever or pneumonia and can progress to multiple organ failure or even death in some cases, they said.

A small study published Jan 30 in the peer-reviewed medical journal The Lancet found that the average age of coronavirus patients was roughly 55 years old. The study looked at 99 patients at Jinyintan Hospital in Wuhan, China, from Jan. 1 to Jan. 20.

"Increasing age increases the risk for death," Kerkhove said in February. "It appears even over 80 is the highest risk factor."

Marc Lipsitch, a professor of epidemiology at the Harvard School of Public Health, told CNBC last month that the apparent lack of children among confirmed coronavirus cases could also be because they are getting infected but developing more mild symptoms and aren't being reported to local authorities.

"The data is coming out in so many places and so many forms," he said at the time.

The differences in symptoms among different age groups are seen in other respiratory illnesses as well. The seasonal flu, which infects millions in the U.S. each year, is usually more severe in adults than children.

Thousands of children are hospitalized each year from the flu, but death is rare, according to the Centers for Disease Control and Prevention. However, between 50% and 70% of flu-related hospitalizations in the U.S. occur in people 65 years and older, and between 70% and 85% of deaths occur in the same age group, the CDC says.