On Feb. 16, a one-year-old baby boy was warded in isolation at KK Women's and Children's Hospital.

That day, he became the second-youngest patient in Singapore to test positive for the 2019 novel coronavirus (Covid-19).

Just two days later, though, he made a full recovery and was discharged.

And in so doing, he set a record: he was the quickest-discharged Covid-19 patient in Singapore's history.

Quite a feat for the little guy, whose recovery came four days after another very young patient was discharged.

This was on Friday, Feb. 14, when a two-year-old girl recovered from her Covid-19 infection after a four-day hospital stay.

The relatively fast recovery of both these patients might come as a surprise to those used to thinking of babies and young children, with their arguably-under-developed immune systems, as being particularly vulnerable to viral infections and disease.

It does beg questions pertinent to Singapore's many parents of babies and young children:

• How vulnerable are children to the coronavirus?
• Are Singapore's little fighters exceptional cases to the norm?
• And should we be worried about the health of the kids amidst what is at the moment still DORSCON Orange?

Number of infected children "rare"

Multiple publications that have looked into this peculiar phenomenon that appears to defy common wisdom quoted an article published in the Journal of the American Medical Association (JAMA), which showed that children were few amongst those infected with Covid-19.

The study showed that the median age of patients suffering from the coronavirus is between 49 and 56 years, with cases in children described as "rare".

American media nonprofit NPR talked to Cody Meissner, an infectious disease expert and professor of paediatrics at Tufts University School of Medicine, who cited another JAMA study in giving his opinion that the virus seems to be affecting children mildly.

In the study, researchers examined an admittedly small sample of nine infants (aged between one and 11 months) who had been diagnosed with Covid-19.

Out of the nine:

• four were reported to have a fever
• two had mild upper respiratory tract symptoms
• one had no symptoms, and
• two had no information on symptoms available.

None of the nine required intensive care or had severe complications, according to the researchers.

This may sound like a pleasant surprise, but it really isn't if you were to look at the world's last two coronavirus outbreaks — namely MERS and SARS.

According to The New York Times, most children infected with MERS never developed symptoms despite the fact that the virus claimed more than 800 lives across two outbreaks (Saudi Arabia in 2012 and South Korea in 2015).

Similarly, no children died from SARS during the 2003 epidemic that claimed 774 lives.

The New York Times reported that out of the over-8000 confirmed cases of SARS, researchers were only able to identify 135 infected children.

So why aren't more children getting sick?

No one knows for sure, but a few hypotheses have been put forth such as the...

1) Innate Immune System Theory

One theory for why we aren't seeing more cases of children with the coronavirus is the strength of their innate immune system.

Speaking to Live Science, Dr. Andrew Pavia, chief of the Division of Pediatric Infectious Diseases at the University of Utah, said:

"One hypothesis is that the innate immune response, that is the early response that is aimed broadly at groups of pathogens, tends to be more active (in children)."

Let us try and explain that sentence to you.

According to the University of Arizona, our body's immune system is broadly split into two categories — the innate immune system and the adaptive immune system.

The innate immune system refers to "nonspecific defence mechanisms" that go to work immediately when pathogens (a bacterium, virus, or other microorganisms that can cause disease) enter the body.

The adaptive immune system is a bit more complex.

This part of the immune system actually learns to recognise specific viruses. It then creates what the University of Arizona calls "an army of immune cells" that are designed to attack that specific virus.

This process of learning and designing a specific response means that the adaptive immune system usually takes longer than the innate immune system to "join the battle".

But why do children have a stronger innate immune system?

According to The New York Times, who spoke to Dr Raina MacIntyre, an epidemiologist at the University of New South Wales in Sydney, Australia, the innate immune system deteriorates as people get older.

"Something happens at age 50," she said. "It declines, and it declines exponentially, which is why for most infections we see the highest incidence in the elderly."

So back to Pavia's hypothesis.

As children have a more active innate immune system (immediate response) to the pathogens of the coronavirus, they may be fighting off the virus quicker than adults, suffering only mild symptoms in the process.

2) Theory of underreporting

This doesn't mean that children aren't getting infected by Covid-19, though.

Rather, they might be getting infected, but because their symptoms are relatively mild or don't even show up, they don't get reported.

That's one of the theories put forth by The New York Times, who quoted University of Hong Kong's Chief of Virology Dr Malik Peiris as giving his "strong, educated guess" that younger people are, in fact, getting infected.

"But they get the relatively milder disease," he said.

Peiris added that scientists may be seeing fewer infected children because of the lack of data on the milder cases.

Looking back at the JAMA published research studying nine infected infants, it isn't hard to see why similar cases might not be reported.

If a child is only exhibiting mild symptoms, his or her parents might be less likely to rush them to the doctor, and doctors seeing those who are brought to them may in turn be less likely to test for Covid-19.

All the more when a robust innate immune system sees them to a recovery in a speedier fashion than in most adult cases.

However, another doctor consulted by The New York Times seems unconvinced by the theory of underreporting.

Dr Mark Denison, a paediatric infectious diseases specialist at Vanderbilt University in Nashville, said:

"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."

Denison was of the view that children are simply less infected and they are "not as much at risk".

Babies & kids: little carriers with "most intense" contact with others in society

But that doesn't mean we shouldn't worry about children when it comes to Covid-19.

They can, of course, spread the virus and infect others who are more vulnerable.

It's worth noting that back here in Singapore, our youngest patient, a 6-month-old infant, has been hospitalised for more than two weeks (and counting) since his diagnosis on Feb. 5.

Think of the personal hygiene precautions that experts have urged Singaporeans to take: Washing your hands regularly, avoiding unnecessary contact, and refraining from touching your face.

Now think about making sure a child does all that habitually and independently.

MacIntyre told The New York Times that young people (kids, teenagers, and young adults) actually have the "most intense contact in society".

That means that infected children with unaware parents may actually contribute to the spread of Covid-19.

Infected children could easily pass the virus to the rest of the family, by means of all the physical contact they are afforded.

That means that other demographics — such as grandparents — that are much more vulnerable to the virus are at risk.

And that should be a good enough reason to continue being vigilant over children while the world battles to contain the coronavirus outbreak.

Mothership Explains is a series where we dig deep into the important, interesting, and confusing goings-on in our world and try to, well, explain them.

This series aims to provide in-depth, easy-to-understand explanations to keep our readers up to date on not just what is going on in the world, but also the “why’s”.

Top image via REUTERS/Carlos Garcia Rawlins, illustration by Tan Guan Zhen