What’s the Deal With Kids and the Coronavirus: Five Leading Theories

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One of the biggest enigmas since the beginning of the pandemic has been how kids respond to the novel coronavirus. Children, particularly those under the age of 10, don’t appear to be as vulnerable to the virus as adults are, and scientists and pediatricians aren’t sure why. For one thing, this observation conflicts with the fact that children are typically more susceptible to respiratory infections. “In my field, almost everything infects kids more than it does adults,” says Alfin Vicencio, MD, chief of the division of pediatric pulmonology at the Icahn School of Medicine at Mount Sinai. “This is an unusual situation.”

Early data from Europe and Asia provided hope that children were nearly immune to the virus. In Iceland, zero out of 848 randomly selected children under the age of 10 tested positive for the coronavirus, and children who were tested because of suspected exposure to the virus were half as likely to be positive as adults were. An early study from China reported similar numbers, with only 1% of all Covid-19 cases occurring in children under the age of 10, and another 1% of cases in kids aged 10 to 19. Recent statistical models based on global Covid-19 data back up these optimistic observations, proposing that children are half as susceptible to being infected with the virus as adults are.

“Kids are not superheroes; they’re not covered in a weird plastic antivirus shell. But also to say they’re exactly as infectious as adults when we know that they are less likely to be symptomatic also seems wrong.”

In the United States, encouraging anecdotal news stories in June reported that there had been virtually no cases in YMCA day camps and daycare centers that had remained open since the beginning of the pandemic, despite caring for 40,000 children. But come July the narrative changed, with a fivefold increase in cases in kids under the age of 10 in Oregon, multiple outbreaks in Texas daycare centers, and a damning story out of Florida that one-third of children tested were positive for the virus. The latest outbreak reported at a camp in Georgia involved 232 children — 45% of the attendees — between the ages of six and 17, with 26% of the positive cases being asymptomatic. As a result, just how much kids really are immune has been brought into question.

“The rhetoric on this has gone in one of two extremes,” says Emily Oster, PhD, a professor of economics at Brown University who’s been tracking Covid-19 cases in camps and daycares. “[Some] people say, ‘You can’t get it from kids.’ That’s obviously crazy. That shows a very poor understanding of viruses. Kids are not superheroes; they’re not covered in a weird plastic antivirus shell. But also to say they’re exactly as infectious as adults when we know that they are less likely to be symptomatic also seems wrong.”

Without a clear picture of the likelihood of infection in children, an important question to ask is why they might be less susceptible. Is there really something biological that would make kids less likely to get sick if they catch the virus, or are there behavioral differences that mean they’re less likely to come into contact with it in the first place?

Insight into this issue could provide guidance on if, when, and how to reopen schools in the fall. Here are the latest theories as to why kids seem less susceptible.

1. The coronavirus has trouble entering a child’s body

Looking at the biology of coronavirus infections, one theory that has gained support is the finding that children have fewer ACE2 receptors than adults do. These receptors, which the novel coronavirus uses to gain entry into cells, are present throughout the body, but a high concentration of them line the airways from the nose down into the lungs.

In a study published in the Journal of the American Medical Association in May, asthma researchers in New York found that children had significantly fewer ACE2 receptors in the nose than adults did. “It was an age-related increase, meaning that the younger kids had less and older patients had more,” says Vicencio, who conducted the study. “From a very rudimentary standpoint, it is possible that less receptor means less [virus] entry into the body.”

George Rutherford, MD, a professor of epidemiology and biostatistics at the University of California, San Francisco, says that this study was the first he’s seen to provide a concrete explanation for the lower infection rates in children. “All of the sudden it becomes clearer that, okay, there is a reason that kids may not get infected as much, and they may not excrete as much [virus],” he says. “Because they have fewer of these receptors, they’re going to have fewer respiratory cells infected, and they’re going to have fewer of these respiratory cells turned into little virus factories.”

A recent study from Switzerland, however, showed that kids who tested positive for the virus have as many virus particles present in their nose as adults do, suggesting they may be just as contagious. New research published last week by scientists in Chicago backs up this finding, revealing that children have as much — and in some cases up to a hundred times more — virus RNA present in their nose and throat. In other words, despite there being fewer ACE2 receptors along the upper respiratory tract to infect, the virus was still able to get into enough cells to turn them into highly productive factories to produce just as much virus.

2. A child’s immune system is primed by the common cold

Another prominent theory is that children have partial immunity to the novel coronavirus because of recent exposure to other coronaviruses that cause the common cold. Several studies in adults have found that roughly 50% of the people tested have immune cells that respond to the novel coronavirus, even if they’ve never been infected with it. It’s possible that those immune cells, which help produce antibodies, were programmed to recognize a similar coronavirus during a past infection, and now they can mount a stronger, faster defense against the new threat. This rapid immune response could potentially result in either asymptomatic infections or even no infection despite contact with the virus.

“In some cases, the fact that you’re exposed to something similar means that you have some memory response, meaning that you respond faster and maybe you clear the virus faster,” says Alba Grifoni, PhD, a scientist at the La Jolla Institute who published the findings in June. “If that hypothesis is true, then that would make sense that you have more mild and asymptomatic cases.”

Children, particularly young ones, are notorious for having a new cold seemingly every other week, so if other coronaviruses really offer partial immunity to the novel coronavirus, many kids could be protected. However, Audrey Odom John, MD, PhD, chief of the division of pediatric infectious diseases at Children’s Hospital of Philadelphia, says, “I’m not sure I buy that as an explanation.” She points out that exposure to those other circulating coronaviruses doesn’t protect kids from getting colds year after year. What’s more, she says, presumably adults, who have been exposed to many more circulating coronaviruses over the years, would have greater preexisting immunity than children.

Flipping the theory on its head, John says it’s possible that children are the ones who respond normally to the novel coronavirus and adults with severe infections are reacting abnormally, precisely because of past infections by other coronaviruses. There are a few examples of viruses, such as dengue, where multiple exposures to slightly different variations of the pathogen can cause an extreme immune response. It could be that the novel coronavirus should result in a very mild infection, but in adults who have had recent exposure to another virus, the immune system overreacts, resulting in inflammation and more damage to the body.

“One of the compelling hypotheses is that maybe some individuals are predisposed to having severe infection because they’ve previously seen some other infection, whether that is a different coronavirus or maybe they saw the flu at exactly the right time in their life, and are basically primed to have a disordered immune response,” John says. “So that is one hypothesis that, essentially, some individuals have preexisting immunity that is essentially disordered in the setting of the novel coronavirus.”

John says there is evidence that this overactive immune response occurred with the original SARS, but she’s quick to clarify that there hasn’t been any evidence in animal studies or early vaccine trials for Covid-19.

But come July the narrative changed, with a fivefold increase in cases in kids under the age of 10 in Oregon, multiple outbreaks in Texas daycare centers, and a damning story out of Florida that one-third of children tested were positive for the virus.

3. Risk factors increase with age

Another possibility is that there is something different in the immune systems of young children that scientists haven’t accounted for yet. As kids age and their bodies go through puberty and look more like adults, they start to respond to the virus more like adults, too. For instance, in countries that have reopened schools, several outbreaks have occurred in high schools, but there have been relatively few cases in elementary schools.

“We know that older kids are more susceptible because they’re physiologically more like adults,” says Thomas Murray, MD, PhD, a pediatric infectious disease specialist at the Yale University School of Medicine. “Additionally, some of the risk factors like obesity and diabetes can also start to show up in adolescence, and in some cases, that can also increase your risk of becoming sicker.”

4. Social isolation keeps young children safe

Without a clear biological explanation, and in light of the recent outbreaks, some scientists wonder if the answer may lie in children’s behavior and environment. The lack of childhood infections could in large part be due to a lack of exposure to the virus in the first place. With schools closed, kids were no longer intermingling with other children, and they never had to leave the house to go grocery shopping or to a job as an essential worker like some adults did. In short, young children would not have had the same level of exposure as adults, especially during the strictest phases of lockdown. However, children still could have been infected by their parents who had to venture out, which would explain the limited number of cases.

In one study from Switzerland that looked at the prevalence rates of antibodies to the virus, children between the ages of five and nine and adults over the age of 65 were both significantly less likely to be positive for antibodies than adults between the ages of 20 and 49. The different antibody rates are likely due to differences in behavior, specifically not going out in public, that protected the very old and the very young from coming into contact with the virus.

Hypothetically, teens would have experienced a similar drop in their potential exposure, except, John says, “I think it’s pretty reasonable to assume that teenagers have managed to get together far more often. Your average six-year-old does not sneak off to go down the street to hang out with friends.”

“Because their movements are controlled by their parents, I think it’s much easier to socially distance and enforce that in the very young elementary school-age kids,” she adds. “I think they’ve had their worlds collapsed, probably more than the teenagers have.”

Now that restrictions have started to lift and children are allowed out of the house again, their infection rates have started to pick up too.

5. Children aren’t tested as often

Another possibility is that children actually do have similar infection rates as adults, they’re just not tested as often because they’re more likely to be asymptomatic. Supporting this theory, a study out of China that relied on contact tracing found that children were as likely to contract the virus as adults were, but they were more likely to be asymptomatic. This suggests that the low number of cases in children may be due to insufficient testing — the children get the virus, but they don’t have any symptoms, so they’re not tested for it.

“It’s clear that there are a lot of asymptomatic infections in children, and so I think in large studies, it’s clear that children are relatively underrepresented if there’s not widespread contact tracing and screening, including of asymptomatic individuals,” John says.

If young children are infected at similar rates, it’s still possible that they don’t spread the virus as easily because of kinetic differences. Kids have less lung capacity than adults do, so they don’t breathe or cough as forcefully. As a result, they may expel fewer viral particles into the air, thereby infecting fewer people. A recent study from South Korea backs this up, showing that older and younger children differ in their transmissibility of the virus. Older children aged 10 to 19 spread the virus to other people as often — and in some cases more frequently — as adults, but children under the age of 10 were half as likely to infect someone else.

While the debate about children’s risk for contracting and spreading the virus is unresolved, experts do know how to stop the spread of the novel coronavirus, and the rules are the same for kids as they are for adults: masks, distance, good ventilation, and good hand hygiene. These prevention measures, as well as taking into account local community spread, are recommended in recent guidelines about reopening schools from the U.S. Centers for Disease Control and Prevention, the American Academy of Pediatrics, and the National Academies of Sciences, Engineering, and Medicine.

John says that if they can stop transmission in a hospital setting with these steps, they can avoid outbreaks in a school. It’s not about children being magically immune to the virus, it’s about diligent prevention measures. “It requires a lot of will, [and] it requires a lot of resources,” she says. “That’s probably the piece that is going to be the hardest for schools, is that this is not a cheap thing to do. So is it feasible? In theory. Is it practical for most schools? That’s really going to be on a case-by-case basis.”

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental