A study has found the coronavirus can indeed cause severe or critical illness in children, and killed a 14-year-old boy last month.

New data from coronavirus-infected children in Wuhan, the birthplace of the pandemic, has found they can indeed get seriously ill and confirms the first known child death from the disease.

Much has been made of the fact youngsters are the least likely age group to develop the deadly Covid-19 pneumonia caused by the SARS-CoV-3 virus.

It's one of the reasons — along with the disruption to working parents and the economy — many governments have decided to keep schools open.

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But now the biggest study of coronavirus-infected children to date has found the disease does indeed make some youngsters seriously ill — around 6 per cent — with babies and preschoolers most vulnerable.

Alex Lewis says she and her partner Ed were turned away from being tested for coronavirus. Video / ABC Australia

A teenager confirmed to have the virus died in China on February 7, with details of his case coming to light for the first time via the paper, published online by the American Academy of Paediatrics.

In response to the new data, the World Health Organisation acknowledged the teen's death and the potential of the virus to make some children every sick.

Researchers looked at 2143 youngsters from Wuhan, where the pandemic began, and surrounding areas within Hubei province. Their ages ranged from one day to 18 years old.

They found a staggering 90 per cent were asymptomatic, moderate or mild cases. However, 125 children (5.9 per cent) went on to develop serious or critical illness, including a 14 year-old boy who died, compared to 20 per cent in adults.

Of the 125, thirteen were considered "critical" and on the brink of respiratory or organ failure. The rest were classified as "severe" because they had dire respiratory problems.

Two children wear masks as they visit a tourist lake in Wuhan, China. Photo / Getty Images
Two children wear masks as they visit a tourist lake in Wuhan, China. Photo / Getty Images

The rate of severe and critical cases was 10.6 per cent in children under the age of 1; 7.3 per cent in those aged 1 to 5; 4.2 per cent in 6 to 10-year-olds and three per cent of those aged 16 to 18.

More than a third — about 39 per cent — became moderately sick, with additional symptoms including pneumonia or lung problems revealed by CT scan, but with no obvious shortness of breath.

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About half of the children had mild symptoms, such as fever, fatigue, cough, congestion and possibly nausea or diarrhoea, the study found.

WHY CHILDREN RESPOND DIFFERENTLY TO THE VIRUS

As to why the virus was more dangerous for adults than children, researchers said it was possible youngsters had more antibodies because they commonly suffered seasonal respiratory illnesses.

In addition, their immune systems were still developing, meaning their bodies responded less violently to the virus than adults, thereby causing less damage.

"Why most of the children's Covid-19 cases were less severe than adults' cases is puzzling," the study's lead researcher, Professor Shilu Tong, wrote.

"This may be related to both exposure and host factors. Children were usually well cared for at home and might have relatively less opportunities to expose themselves to pathogens and/or sick patients.

Air New Zealand CEO Greg Foran discusses the airlines latest response to coronavirus. Video / Supplied

"It is speculated that children were less sensitive to (coronavirus) because the maturity and function (e.g., binding ability) of ACE2 in children may be lower than that in adults.

"Additionally, children often experience respiratory infections in winter, and may have higher levels of antibody against virus than adults. Furthermore, children's immune system is still developing, and may respond to pathogens differently to adults."

The findings matched research by University of Texas virologist Vineet Menachery, who infected old and young mice with the coronaviruses' cousin Sars.

He found the virus caused immune systems in older mice to wildly overreact, comparing it to "police responding to a misdemeanour with a Swat team".

"It's the aggressive response from their immune system that is damaging them, even more than the infection itself," Prof Menachery told The Washington Post.

"It's like the police responding to a misdemeanour with a Swat team crashing through the door."

Frank Esper, a US paediatric infectious disease specialist, said youngsters may have better protection against the disease because they're likely to have chronic or pre-existing conditions.

"Maybe it has nothing to do with the virus and has to do with the host, like underlying conditions in the lungs, diabetes or hypertension," he told The Post.

"After all, few 7-year-olds or newborns have hypertension. Figuring out what's at play here could be helpful in so many ways."