Why does coronavirus seem to affect children less severely?

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Young children and older people are usually the most severely affected with flu but coronavirus seems to be a different case, leaving scientists perplexed. Children with Covid-19 generally experience milder symptoms than adults.

However, there have been some reports of young people being seriously affected by the virus as case reports and studies emerge from around the world. A 5-year-old child from the UK (with underlying health conditions) and a 12-year-old girl from Belgium died in hospital last week, making them the youngest victims in Europe. An infant in Illinois has also been reported to have died after contracting the virus, although the exact cause of death remains unknown. With many schools now closed, parents worry about the effect the new disease might have on their children.

Just as with adults, children exposed to coronavirus can get infected. Early research from China suggests children are less likely to die than adults if they catch the coronavirus and seem to experience milder symptoms. In a recent study of 44,672 people with confirmed Covid-19 infection, children under the age of 19 made up two per cent of those cases and none of the 1,023 deaths logged by 20 February. “Children don’t seem to be at that much of a risk. But that doesn’t mean zero risk from severe disease or mortality,” says Paul Hunter, professor in medicine at the University of East Anglia.

A second study, published in the journal Pediatrics, looked at 2,143 children who had contracted the virus and were reported to the Chinese Centers for Disease Control. About half of the children had mild symptoms of fever, cough, sore throat, runny nose and fatigue while a third (39 per cent) became moderately sick with signs of pneumonia but without the shortness of breath seen in more severe cases. One hundred and twenty-five children (six per cent) developed serious illness and one 14-year-old boy from Hubei province died. Ninety-four children (4.4 per cent) showed no symptoms at all.

In other parts of the world trends also seem to suggest children are less likely to be infected than adults, especially people over 70 and those with underlying health conditions, but this could be due to the fact that, in many countries, testing is only offered to those reporting to hospital with severe symptoms of Covid-19.

Children that become infected are still less likely to fall ill and die from Covid-19, a similar trend to that seen with Sars and Mers, two other severe respiratory diseases caused by coronaviruses. So what’s shielding them?

The annoying answer is that we don’t really know. Infants and young children are typically at high risk of infections caused by influenza – mainly because their respiratory tract and immune system are still immature. As for Covid-19, the virus uses a “spike protein” to bind to a receptor protein called ACE2 in human respiratory cells. One theory is that children have less of these cellular locks in their lower airways (lungs), or they might be a different shape, which is why their upper airways (nose, mouth and throat) are predominantly affected by Sars-Cov-2.

It could also be down to the way children’s immune systems respond to the virus. In some adults, an excessive immune reaction called cytokine storm is what caused multi-organ failure and led to death, not the virus itself. “Covid-19 is mostly immune-mediated, which means that disease in the body results from the activity of the immune system, so young healthy people with good immune systems can also end up with severe disease,” said Julian Tang, associate professor of respiratory sciences at the University of Leicester in a statement to the Science Media Centre.

It is possible that children’s immune systems don’t rev up to fight off the virus and leave their lungs protected from serious damage. During the Sars outbreak in 2003, studies found that, unlike adults, children produced relatively low levels of cytokine responses.

But just because children tend not to be severely affected by Covid-10, it doesn’t mean they can’t spread it. The disease spreads from person to person through small droplets from the nose or mouth, which are spread when a person infected with the Sars-Cov-2 virus coughs or sneezes. Many children could be silent carriers and spread the virus widely as they are in close contact and don’t always wash their hands or do not use tissue properly. They are often known as super spreaders in other viruses such as cold and flu. This means that children carrying coronavirus, with very mild or no symptoms, could pass it on to other, more vulnerable members of their family.

Schools are notorious places for spreading diseases but there were a lot of concerns and debate over closures in the UK. Some parents who are unable to work from home would have had to leave their children in the care of vulnerable grandparents. Since March 23, only a small proportion of children – those with special needs and those of workers in certain professions – are still able to attend. “That wasn’t an easy decision because children don’t get as severely ill,” says Hunter.

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