NEW YORK • The new coronavirus has infected more than 37,000 people and more than 800 have died.

But relatively few children appear to have developed severe symptoms so far.

The median age of patients is between 49 and 56 years, according to a report published last Wednesday in the Journal Of The American Medical Association. "Cases in children have been rare."

So, why are not more children getting sick?

"My strong, educated guess is that younger people are getting infected, but they get the relatively milder disease," said Dr Malik Peiris, chief of virology at the University of Hong Kong, who has developed a diagnostic test for the new coronavirus.

Scientists may not be seeing more infected children because "we don't have data on the milder cases", he said. "If this coronavirus spreads worldwide and it spreads as widely as the seasonal flu does, probably we'll see more," he added.

In one published case, a 10-year-old travelled to Wuhan, China, the epicentre of the outbreak, with his family. Upon returning to Shenzhen, the other infected family members, ranging in age from 36 to 66, developed fever, sore throat, diarrhoea and pneumonia.

The child, too, had signs of viral pneumonia in the lungs, doctors found - but no outward symptoms. Some scientists suspect that this is typical of coronavirus infection in children. "It's certainly true that children can be either asymptomatically infected or have a very mild infection," said Dr Raina MacIntyre, an epidemiologist at the University of New South Wales in Australia who has been studying the spread of the new coronavirus.

In many ways, this pattern parallels that seen during outbreaks of the severe acute respiratory syndrome (Sars) and Middle East respiratory syndrome (Mers), also coronaviruses.

The Mers epidemics in Saudi Arabia in 2012 and in South Korea in 2015 together claimed more than 800 lives. Most children who were infected never developed symptoms.

No children died during the Sars epidemic in 2003 and the majority of the 800 deaths in the outbreak were people older than 45, with men more at risk.

Among the more than 8,000 cases of Sars, researchers at the United States Centres for Disease Control and Prevention were able to identify 135 infected children in published reports.

Children under age 12 were much less likely to be admitted to a hospital or to need oxygen or other treatment, researchers found.

Children over age 12 had symptoms much like those of adults.

"We don't fully understand the reason for this age-related increase of severity," Dr Peiris said.

It is not unusual for viruses to trigger only mild infections in children and much more severe illnesses in adults. Chickenpox, for example, can be largely inconsequential in children yet catastrophic in adults.

Influenza infects millions worldwide each year. Still, even though thousands of young children end up in the hospital each year with influenza, just a small percentage of them die, Dr Peiris noted.

Adults may be more susceptible because they are more likely to have other diseases, such as diabetes, high blood pressure or heart disease, that weaken their ability to stave off infections.

The body's innate immunity, which is critical for fighting viruses, also deteriorates with age and particularly after middle age.

"Something happens at age 50," Dr MacIntyre said. "The body declines exponentially, which is why for most infections we see the highest incidence in the elderly."

A key question about the new coronavirus is whether children who are infected and asymptomatic are able to pass the virus to others.

Young people who do not realise they are sick may contribute to the epidemic's momentum, she said.

Dr Mark Denison, a paediatric infectious diseases specialist at Vanderbilt University in the United States, said he does not expect a sudden uptick in infected children.

"It's hard for me to imagine that there's such a degree of underreporting of clinical illness in children that we're only hearing about two or three cases," he said.

He added: "I think it means that there are many, many less children (who are infected in China) and they're not as much at risk."

NYTIMES