Why don't children seem to get very ill from the coronavirus?

It has been widely reported that children are less likely to get severely ill and die from the new coronavirus. A recent study of 44,672 people with confirmed covid-19 infection found that children under 10 years old made up less than 1 per cent of those cases and none of the 1023 deaths.

"This is unlike flu," says Akiko Iwasaki at Yale University. With flu, young children and older people are usually the most severely affected, so why is the new coronavirus different? It is a bit of a mystery.

A straightforward explanation would be that children are resisting infection in the first place, but that doesn't seem to be the case. One recent study even found children to be just as likely as adults to get infected.

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In any case, children that do become infected are still less likely to get sick with covid-19 and die - a similar trend to that seen with SARS or MERS, two other severe diseases caused by coronaviruses. So, what is protecting children?

"No one has a good answer to that question yet," says Iwasaki. But she and other experts suspect it may be down to the unique way children's immune systems respond to these viruses.

A common complication of covid-19, SARS and MERS in adults is acute respiratory distress syndrome, where the immune response against the coronavirus becomes overzealous and causes life-threatening damage to the lungs.

The resulting leakage of fluid and immune cells into the lungs causes big problems, says Chris van Tulleken at University College London. Even if those immune responses are trying to help by attacking the virus, they can end up blocking oxygen uptake in the lungs, he says.

Because children's immune systems are still developing, one suggestion is that they are shielded from this type of dangerous immune response - called a cytokine storm - when they get covid-19 or similar diseases. During the SARS outbreak, two studies found children produced relatively low levels of inflammation-driving cytokines, which may have been what protected their lungs from serious damage.

That doesn't explain why children's immune systems react differently to coronaviruses compared with flu. It might be due to differences in the type of cytokine response produced against each virus, says Iwasaki.

Children may also be benefiting from their lack of past exposure to coronaviruses generally. Because they have lived longer, adults are more likely than children to have encountered other coronaviruses in their lives, such as those that cause coughs and cold, and to already have antibodies against these milder viruses.

There is a suggestion that these existing antibodies could actually leave adults worse off, because they aren't exactly matched to the new coronavirus. "Sometimes unmatched antibodies can be more harmful than good," says Wendy Barclay at Imperial College London.

Understanding why children are being spared is about more than scientific curiosity. "If we can somehow mimic the children's immune system, using therapeutics or drugs, maybe it just becomes a mild infection even in adults," says Iwasaki.

And just because children aren't getting severely ill, doesn't mean they aren't contributing to the spread of the new coronavirus. "There's a danger of being complacent about the children not getting severely ill," says Iwasaki.

There is already some indication that infected adults without symptoms can spread the virus and the same could be true of children. "It may be a good preventative measure to start closing schools," says Iwasaki.

A recent case study described a young child with covid-19 who had high levels of virus but no symptoms. Whether or not children in this condition are infectious isn't yet known, but finding out will be critical to tackling this pandemic.