Why children aren't getting sick from coronavirus

A 10-year-old boy travels to Wuhan, the Chinese epicentre of the coronavirus outbreak, on a holiday with his family.

He returns home to Shenzhen, in south-eastern China, where his five adult family members succumb to high fevers, sore throats, diarrhoea and pneumonia.

Chinese children wear plastic bottles as protective masks while waiting to check in to a flight in Beijing.Credit: Getty

Days pass and the boy shows no symptoms. A chest scan, however, shows signs of viral pneumonia in his lungs, the study published in the prestigious medical journal The Lancet revealed.

It is the mystery perplexing scientists studying coronavirus across the globe: why aren't more children getting sick?

The novel coronavirus has infected in excess of 90,000 people worldwide and more than 3000 of them have died, with a slightly higher number of deaths among men.

Of the more than 44,000 cases in China, fewer than 1 per cent have been in children aged nine or younger. No children have died so far and the limited documented cases have shown far less severe symptoms such as a runny nose or cough.

Australia's Chief Medical Officer Brendan Murphy noted this in his update on Tuesday morning.

"I think the evidence around the world is that when children are infected, [their symptoms] are incredibly mild, in fact so mild that they almost don’t have symptoms."

Allen Cheng, director of the infection prevention unit at Alfred Health in Melbourne, has been examining the spread of coronavirus closely and suspects two possibilities.

"One is simply that the small number of cases we see in children mean that they are mainly not getting infected at all," Professor Cheng said.

"The other is that they still getting it at the same rate as adults, but they just don’t come to hospital very commonly because they have far milder symptoms than adults. These are the things we just don't know yet."

There are some infectious diseases such as chicken pox or measles, where the older you are the more severe the illness, Professor Cheng said.

"There's the possibility that may be the case with coronavirus," he said.

While children experience milder symptoms of the virus, determining whether they are potential "super spreaders" was critical.

"It's a really important question because, if children don’t really get it at all, then we don’t need to go shutting schools," Professor Cheng said. "But if children do get it, and get a milder form of the disease, then measures like closing down schools temporarily may well be on the cards."

Kanta Subbarao, the director of the WHO Collaborating Centre for Reference and Research on Influenza at the Peter Doherty Institute for Infection and Immunity in Melbourne, said this happened before in other outbreaks of coronaviruses such as SARS and MERS. Children got infected less and displayed significantly milder symptoms. No children or teenagers died from SARS.

Professor Subbarao believes children may be spared from severe disease because they are infected with lower doses of the virus. Another hypothesis is that the virus cannot replicate in children as well as it can in adults.

As Australia records its first cases of person-to-person transmission of the coronavirus, Professor Subbaro says we are shifting into new ground.

"It's a fascinating observation," Professor Subbarao said. "Unlike the SARS and MERS outbreaks where we could actually wonder if it was lack of exposure, with COVID-19 we know it is not a lack of exposure because of its far-reaching spread, yet we are still seeing less disease in children."

Others theories suggest children have healthier lungs because they have not had as much exposure to cigarette smoke and air pollution.

Raina MacIntyre, head of the Biosecurity Program at the University of NSW's Kirby Institute, suspects a child's robust immune system means their bodies can fight the virus more quickly than adults. The median age of patients with coronavirus is between 49 and 56.

The risk of dying is dramatically higher among patients in their 70s and 80s, most likely because many of those people have pre-existing health issues.

"Once you reach the age of 50, something called immunosenescence occurs, which is the gradual declining in function of the immune system and our ability to fight infections," Professor MacIntyre said, adding that underlying conditions such as diabetes, high blood pressure or heart disease also weaken our ability to stave off infections.

"It doesn't matter how fit, healthy or strong you are, this happens to everybody. The coronavirus causes a milder illness in younger people who have a stronger immune system and hits older people much harder for that reason."

So why do other respiratory illnesses, most notably influenza, peak in babies, young children and the elderly?

"I think the answer is that coronavirus' natural tendency is to cause a mild illness, but in elderly people it can be very severe," Professor MacIntyre said. "Whereas influenza's natural tendency is not to cause a mild illness. It can be very severe in every age group."

Robert Booy, an infectious diseases expert at the University of Sydney, believes the key to unlocking the mystery rests in our immune system's exposure to coronavirus.

Adults are exposed to coronavirus multiple times in their life and some experts speculate their immune system goes into overdrive.

"Instead of fighting off the virus, it damages the person by causing a severe pneumonia in both lungs," Professor Booy said. "Whereas it could be the first time a child has been exposed to coronavirus and therefore their immune system has a much milder response."

On the other side of the spectrum, he said, different strains of the highly infectious coronavirus family have been circulating widely in China and children may have unknowingly built up immunity.

"You toss a coin and one theory says adults are responding excessively and children mildly," he said. "Another says they’ve been exposed to a different coronavirus, which helps them to respond less severely. The fact remains, until we learn more about how this disease operates, there are far more questions than answers."