Usually, it's infants, individuals in old age, and immune-compromised people who are most vulnerable to new diseases. The new coronavirus is no different, with one exception. For some mysterious reason, infants aren't getting as sick as other groups, and scientists can't explain why.

Research published in JAMA in February that surveyed those sick from the virus in China found only nine hospitalized cases of infants under 1 year of age. That's stunning, considering that more than 65,000 people had been reported infected in mainland China at the time. None of those nine cases developed severe complications, reporting only mild fevers or respiratory symptoms, reports Science News.

The report comes on the heels of study conducted by Northwestern University researchers that found no evidence that COVID-19 can be transmitted from an infected pregnant woman to her unborn fetus. This study, published in The Lancet, also looked at umbilical cord blood and the mothers' breast milk, both of which tested negative for the virus in every case. Doctors did not test whether the virus might be transmitted via vaginal delivery, though given the complete lack of reported cases of such transmission, it's safe to say that it's rare if it occurs at all.

What's protecting some babies from coronavirus?

A children's play area sits empty in Tokyo, coronavirus and infants A children's play area sits empty in Tokyo. (Photo: Carl Court/Getty Images)

This might mean babies are more resistant to the virus, which could offer clues on how the virus works and how health officials might be able to treat it. Right now, it's unclear why young children are at such low risk, but it's possible the virus simply effects infants differently, similar to how children with the chickenpox virus show milder symptoms than adults do.

Officials note that while they could only identify some cases of infants hospitalized with coronavirus, this doesn't mean children aren't susceptible to catching it. In fact, a more recent breakdown of more than 72,000 cases compiled by the Chinese Center for Disease Control and Prevention showed 416 of those were in children under the age of 10. Those numbers are still low, but they raise new theories. It's possible that they can catch it at a normal rate, and even spread it, without showing any severe symptoms. All of the infants had at least one infected family member and became sick after their relatives fell ill.

Interestingly, low rates of infant hospitalization were also reported during the previous outbreaks of SARS and MERS, which are also types of coronaviruses similar to COVID-19.

This doesn't mean children aren't at risk

While most children who contract the virus only develop mild symptoms, a select group of youngsters have had serious complications. Researchers with the Chinese Centers for Disease Control and Prevention looked at more than 2,000 cases of children younger than 18 and found that 6% of cases were serious, with symptoms such as shortness of breath and low levels of oxygen in body tissues. The study has been accepted for pre-publication and peer-reviewed for the journal Pediatrics.

Researchers found that among the serious 6% — about 125 children — one 14-year-old boy died, 13 were considered "critical" (on the brink of respiratory or organ failure) and the others were classified as "severe" because they had dire respiratory problems, according to The New York Times.

"What this [study] tells us is that hospitals should prepare for some pediatric patients because we can't rule out children altogether," Dr. Srinivas Murthy, an associate professor of pediatrics at University of British Columbia who was not involved with the study, told the Times.

What role do cytokines play?

One of the more alarming complications of COVID-19 is acute respiratory distress syndrome, a type of autoimmune response called a cytokine storm, and this detail may offer clues to children's strengthened immune response, according to New Scientist.

"Cytokines are proteins that help rally immune cells around an injury or infection, but this beneficial mission can go awry when they accumulate at high levels for too long," MNN's Russell McLendon explained in a story about awe and health and the role of cytokines in that context.

When they go into overdrive, as they do in a cytokine storm, cytokines have the opposite effect, raising the risk of inflammation and increasing the chances of a wider range of other illnesses. Young children are not having this response to the new coronavirus, though they are not protected from this response in acute cases of the flu. That variation in behavior may offer clues for researchers.

Though the reasons remain mysterious for why infants appear resilient to getting sick from coronavirus, it's welcome news in the aftermath of a disease that has proven deadly to other vulnerable populations.

Editor's note: This story has been updated with new information since it was first published in February 2020.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

No age group is immune to coronavirus, but why are so few infants getting sick?
Scientists are at a loss to explain why infants seem largely resistant to the coronavirus, but new studies are providing clues.