Scientists are still learning about the coronavirus pandemic that has killed more than 5,00 people and infected more than 136,000 worldwide. One of the biggest mysteries is why so few children have gotten sick.

A recent study of 745 children in China identified only 10 children with the virus. Seven of those children developed a fever, one of the virus' most common symptoms, and a few experienced coughing, a sore throat, and nasal congestion. None of the children had clear signs of pneumonia in their chest X-rays — a prominent sign of the virus in adults.

Taken together, these symptoms indicate that the children's cases were mild.

Similarly, a January study in the New England Journal of Medicine found that "children might be less likely to become infected or, if infected, may show milder symptoms" of the coronavirus than adults.

Another study of coronavirus cases in China also identified only nine infants under one year old that had been infected with the virus from December 8, 2019, to February 6, 2020. None of the infants had severe complications or required intensive care, so the researchers suggested that some infants might be contracting cases of the virus that don't always manifest in physical symptoms. 

"From everything that we've seen, and for reasons that are unclear to us, it does seem that this is primarily impacting adults," Richard Martinello, an associate professor of infectious disease at the Yale School of Medicine, told Business Insider in early February.

A low case count among kids is a good thing, according to health experts, because children are less likely to wash their hands, cover their mouths, and refrain from touching others — behaviors that can spread germs.

"If we can protect kids — one, it's good for them, but two, it's good for the population," Aaron Milstone, an epidemiologist and professor of pediatrics at Johns Hopkins University, told Business Insider. "If it does penetrate the pediatric population, that might amplify the outbreak."

Few children got SARS

Symptoms of the novel coronavirus are similar to those associated with pneumonia or the flu: fever, coughing, and, difficulty breathing.

The coronavirus bears some similarities to SARS, which killed 774 people and infected more than 8,000 between November 2002 and July 2003. There were also very few cases of SARS among children: only 80 laboratory-confirmed cases and 55 probable or suspected cases.

Most of those children developed a fever, and some had coughs or vomited as well. 

In a 2007 report, experts from the Centers for Disease Control and Prevention determined that children 12 or younger displayed milder symptoms of SARS than adults did. No children or adolescents died from the virus, and there was only one instance in which a child transmitted SARS to another person.

Scientists still aren't sure why that was the case.

Beijing children coronavirus
A Beijing subway station.
Roman Balandin/TASS/Getty Images

In the current coronavirus outbreak, there are two possible explanations for why so few children have gotten sick: They've either been less likely to be exposed in the first place, or there's something different about how their bodies respond to the virus. 

If the coronavirus spreads among kids, the outbreak could get even worse

It's possible that adults aren't spreading the virus to kids because people are being careful about washing their hands, covering their mouths, and self-isolating if they feel sick. The virus can spread between people through respiratory droplets such as saliva and mucus, so good hygiene is critical to prevent transmission.

"I'm a pediatrician — I love kids — but kids really fuel the spread of respiratory viruses," Milstone said. "It's much easier to tell adults to practice common-sense good behaviors. If kids are sick, they still want to go snuggle with mommy or play with their siblings."

wuhan coronavirus infant
A baby in Hong Kong.
Anthony Wallace/AFP/Getty

As the virus continues to spread, however, countries could start to report more cases among children.

"When we see any new virus, the whole population is susceptible," Milstone said. "We don't know that this virus tickles the respiratory tract of adults more than it does kids."

Weber said the low number of kids who've gotten the coronavirus is one factor that distinguishes it from the flu (as well as a higher death rate and more severe symptoms). 

"In many outbreaks of flu, it's the children who spread it more than the adults," he said.

Children younger than 5 are at high risk of developing serious complications from the influenza virus, such as pneumonia and respiratory and kidney failure, according to the CDC. Scientists aren't sure why that is, but some suspect it's because children have a greater immunological response, so they're more likely to run a fever or experience tissue damage as their bodies try to fight off infection. 

The majority of flu-related deaths in the US — between 70 and 85% — occur in people 65 years and older. According to the World Health Organization, about 80% of the deaths from the new coronavirus have occurred among people ages 60 and older. 

Unlike the flu, however, there is no vaccine to protect people from this new virus. 

"If we can keep this virus away from the pediatric population, it will help stem this," Milstone said. 

Morgan McFall-Johnsen contributed reporting.

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