The coronavirus seems to be mild in most children, but scientists are not sure why

Among 44,627 people confirmed to have been infected in China, 2.3% have died, none of them under age 10.

In this Sunday, Feb. 16, 2020, photo, a nurse prepares medicines for patients at Jinyintan Hospital designated for new coronavirus infected patients, in Wuhan in central China's Hubei province. China reported thousands new virus cases and more deaths in its update Tuesday, Feb. 18, 2020 on a disease outbreak that has caused milder illness in most people, an assessment that promoted guarded optimism from global health authorities. (Chinatopix via AP)Read moreAP

Published Feb. 19, 2020, 12:16 p.m. ET

Some good news on the coronavirus front: So far, it has not killed any children under age 10.

Yet scientists are not sure why.

Chinese health officials provided details this week on 44,672 patients whose illnesses had been confirmed with laboratory testing as of Feb. 11, including 1,023 deaths. Just one of the people who died was under age 20 — a slightly different picture from what is usually seen with a far more common respiratory illness, the flu.

The flu can occasionally cause severe complications in the young, and in the United States, it kills 200 or so children under age 18 every year, such as a Marple Township second grader who died this month.

The impact of the coronavirus is a reminder that the immune system is a complex, many-layered series of defenses, and the biochemistry of why it may be more effective in different phases of life — and against different diseases — is not fully understood.

The reasons why very young children can be hit hard by the flu make sense. Their immune systems have not fully matured, and they do not have prior exposure to most strains of the flu, said Andrew Pavia, chief of pediatric infectious diseases at the University of Utah. That is why vaccination is recommended.

On the other hand, most young children do not develop severe complications from the measles and chickenpox, whereas those illnesses can be “disastrous” in adults, Pavia said. (Vaccination is recommended for those illnesses, too, but no vaccine is 100% effective.)

“This is where we really don’t have great answers,” Pavia said.

One theory is that when faced with certain unfamiliar viruses, children can fight them off because they have a robust layer of defense called the “innate” immune system. This consists of white blood cells called macrophages, which can detect and gobble up foreign particles in the bloodstream regardless of their identity.

Adults have innate immune systems as well. But against some viruses, they may rely more on the “adaptive” immune system: targeted, infection-fighting cells that have “learned” to attack specific kinds of viruses after being exposed to them in the past.

That is how vaccines work. The shots contain a dead or inactivated form of the virus in question, or even a fragment of it, enabling the immune system to form a cellular memory should the person ever be exposed to the real thing.

Yet in response to some viruses, the adult immune system can overreact, unleashing a “storm” of harmful, inflammatory chemicals, said Akiko Iwasaki, a professor of immunobiology at the Yale School of Medicine.

Older people have an added deficit: The various layers of their immune systems start to become less effective. Many also have underlying conditions, such as heart disease. And increasingly, the bodies of older people suffer from chronic, low levels of age-related inflammation, nicknamed “inflammaging.”

So when older people become infected with a virus, the combination of even more inflammation and a weaker immune system can be deadly, Iwasaki said.

“You have this perfect storm of not having the right antiviral defenses and having elevated inflammaging,” Iwasaki said.

Every year, most flu-related deaths in the U.S. occur in older people. The same pattern appears so far with the new coronavirus, according to the summary of 44,672 confirmed cases from the Chinese Center for Disease Control and Prevention.

Among 1,408 patients who were at least 80 years old, 208 died, for a mortality rate of 14.8% — higher than any other age group.

The mortality rate for those ages 70 to 79 was 8%, and the rate for patients in their 60s was 3.6%.

The overall mortality rate for the 44,672 patients was 2.3%, but Iwasaki and Pavia cautioned that the true percentage is almost certainly lower. Many infected patients with milder symptoms have not had their illnesses confirmed with laboratory samples — pushing down the apparent number of those affected, and making the death rate look higher than it is.

The rate of deaths has been dropping steadily since the virus first struck in December. Among 757 confirmed cases through Jan. 10, 117 died, for a mortality rate of 15.4%.

But among 12,030 confirmed cases identified from Feb. 1-11, just 102 died — a mortality rate below 1%.

That decline could be due to a more complete accounting of cases, or perhaps the virus has shifted in some way. And as with any invader that challenges the human immune system, some of the details may remain a mystery.