Why are children so much less likely than adults to be infected with Covid-19?


It was a big pandemic conundrum: Why are children less likely than adults to contract the new coronavirus and, if infected, less likely to get sick?

One possible reason could be that many children already have antibodies to other coronaviruses, according to researchers from the Francis Crick Institute in London. About 1 in 5 of the colds that plague children are caused by viruses in this family. Antibodies to these viruses can also block SARS-CoV-2, the new coronavirus that is causing the pandemic.

In a study published Friday in Science, the group, led by George Kassiotis, who heads the Retroviral Immunology Laboratory at the institute, reports that on average only 5 percent of adults had these antibodies, but 43 percent of children had them.

Researchers who did not participate in the study were intrigued by the discovery. H. Benjamin Larman, an immunologist at the Johns Hopkins School of Medicine, called it a “well-done study that proposes a compelling theory that is supported by their data.”

Stephen J. Elledge, a professor of genetics at Harvard Medical School and Brigham and Women’s Hospital, had a similar response. He and others found that many people have antibodies to the common cold caused by other coronaviruses; in laboratory studies, these antibodies also block the new coronavirus.

In March, as the pandemic had just begun, Kassiotis and his colleagues decided to develop a highly sensitive antibody test. To evaluate it, they examined blood samples taken before the pandemic from more than 300 adults and 48 children and adolescents, comparing them to samples from more than 170 people who had been infected with the new coronavirus.

Scientists expected samples taken before the pandemic did not contain antibodies to attack the novel coronavirus. Those were to be the test controls the scientists were developing.

Instead, they found that many children and some adults carried a particular antibody that can prevent coronaviruses, including the new one, from entering cells. This antibody attaches to a spike that escapes from the coronaviruses.

While the peak of the peak is unique to the novel coronavirus, the base is found in all coronaviruses, Kassiotis said. In laboratory tests, the antibodies at the base of the spike prevented the new coronavirus from entering the cells to reproduce.

Now the researchers are planning to expand their study to monitor thousands of children and adults. Some have antibodies that can block the new coronavirus in laboratory tests. Others don’t.

“If they have the pandemic strain, are they protected?” Kassiotis asked.

Will they get sick, he wondered, or will the infection be barely noticeable?

Elledge and his Harvard colleagues developed their highly specific, sensitive and comprehensive antibody test, VirScan. It is capable of detecting a diverse collection of antibodies directed at more than 800 points of the novel coronavirus, including the antibody studied by Kassiotis and his colleagues.

After examining blood drawn from 190 people before the emergence of the pandemic, Elledge and his colleagues concluded that many already had antibodies, including that targeted at the base of the spike, presumably from related coronavirus infections that cause colds.

But while adults might get one or two colds a year, Elledge said, children can get to a dozen. As a result, many develop floods of coronavirus antibodies that are present almost continuously; they can reduce cold symptoms or even leave children with colds that are asymptomatic but still contagious.

While adults may not have detectable antibodies to the coronavirus, many may be able to rapidly produce antibodies if they are infected with a coronavirus.

In typical viral infections, the immune system delivers antibodies to fight the virus. When the infection is repressed, the antibodies, no longer needed, decrease in number. But the body is left with so-called memory cells that allow antibody production to increase rapidly if the virus tries to invade again.

So why do we have a pandemic? Shouldn’t most of us be protected from memory cells left behind by other coronavirus infections?

“It’s entirely possible that you lose your memory over time,” Elledge said.

He suspects that the new coronavirus may interfere with the activation of memory cells capable of responding to infection.

An infection “could give you a confusing memory that fades over time,” she said. In that case, a very recent infection from a common cold coronavirus would be needed to protect against the new coronavirus, and even then protection could only last for a limited time.

The new coronavirus would have hindered the production of antibodies that specifically attack it. This may explain why children, with their seemingly constant colds, are much better off than adults.

Elledge said if he’s right about the loss of memory cells, it bodes well for vaccines. A vaccine increases the production of antibodies without the presence of a virus. So the virus “isn’t in the background, messing with the formation of memory cells,” he said.

Another possibility is that most adults are actually protected by memory cells from previous infections with the common cold. Although few have enough antibodies in their blood to protect them at any given time, they may be able to rapidly produce antibodies to reduce the impact of the new coronavirus.

This could explain why many infected adults recover quickly.

“We focus on those who really get sick, but 95 to 98 percent of those who contract the virus don’t have to go to the hospital,” Elledge said. “There are a lot of people getting better.”

It happened to Larman and his family of five. Four of them fell ill with Covid-19, the disease caused by the new coronavirus, in July. No one was seriously ill and his 4-year-old son was spared entirely.

“My son was not isolated from us and therefore heavily exposed,” Larman said. “He tested negative twice, so we suspect he had some form of pre-existing immunity.”


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