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What scientists are learning about kids and Covid-19 infection

Several new studies are deepening our understanding of infection and transmission in kids, but there’s still a lot we don’t know.

An adult and child wear protective face masks in the Bethesda Arcade in Central Park on July 29, 2020, in New York City.
Noam Galai/Getty Images

In mid-June, a sleepaway camp in Georgia opened for its first camp session of the summer, welcoming 363 campers and 234 staffers and trainees back for what was supposed to be a fun summer outdoors. The camp followed most of the Centers for Disease Control and Prevention (CDC) guidelines, including keeping campers in cabin groups as much as possible. But the campers were not required to wear masks for camp activities, including singing and cheering.

Six days in, a teenage staff member left camp after developing chills, and tested positive for Covid-19. Eventually, 49 percent of the campers, even children as young as age 6, were found to have been infected, while 19 percent of trainees and 56 percent of staff were infected. Notably, 26 percent of the cases reported no symptoms.

A CDC report on the Georgia camp joins a rapidly growing body of desperately needed evidence of how the coronavirus impacts children and young people. In July alone, several studies of note were published, including new findings about infected children’s viral loads, lessons from large-scale contact tracing in South Korea, and associations of transmission with US school closures.

The same CDC report showed that the virus spreads efficiently even in groups of young children in an overnight setting, resulting in rapid transmission in all age groups — despite efforts by the camp to reduce the spread. It also finds that asymptomatic infection was common in children, and “potentially contributed to undetected transmission.”

These findings come as some school districts are still debating reopening, and the US battles a surge in Covid-19 cases. Although the new daily case average has started to decline, most states are reporting uncontrolled transmission, with alarmingly high test positivity rates, and increasing hospitalizations. Meanwhile, data discrepancies after the Department of Health and Human Services took over reporting Covid-19 data are also making it harder to track the impact of the virus.

Yet we do know substantially more about kids and Covid-19 than we did in March. And even though we’re learning at a furious pace, it’s still not enough.

“While it may feel like we’ve been in the midst of this forever, we’ve only been studying this virus for six months,” says Megan Ranney, an emergency physician and the director for the Center for Digital Health at Brown University, who researches pediatric mental health. Scientists like to talk about making evidence-based decisions, but right now, there are enough inconclusive or contradicting studies about children that people are able to cherry-pick the ones that support their preexisting opinion. So, Ranney says, “You have to be suspicious of anyone who is dogmatic.”

While many have strong opinions about priorities and how much risk is acceptable, it’s hard to make evidence-based decisions when we still don’t even definitively know how likely children are to transmit the virus. With that in mind, here’s a dive into what we do — and don’t — know about Covid-19, children, and classrooms.

Why researchers think Covid-19 is generally mild in children

While children can get Covid-19, the illness is generally less severe than in adults. The CDC says that children under the age of 18 account for less than 7 percent of US Covid-19 cases and less than 0.1 percent of the deaths.

Most children with Covid-19 have mild symptoms, the most common being fever and cough, according to a comprehensive National Academies of Medicine report from mid-July on setting priorities for reopening schools.

This is supported by one of the largest-scale pediatric studies to date, which appeared in late June in Lancet Child & Adolescent Health. It looked at data from 582 children under the age of 18 in 21 countries and found that “Covid-19 is generally a mild disease in children.” But, it acknowledged, kids can sometimes get seriously ill: More than half of the children in the study were admitted to a hospital, and four died.

That hospitalization rate may be higher than average because kids were included in the study only if they were sick enough to be tested or admitted to a hospital. The American Academy of Pediatrics estimates that somewhere between 0.6 percent and 9 percent of pediatric Covid-19 cases result in hospitalization. A preprint study that has not yet been peer-reviewed of 31 household clusters in five countries found that 12 percent of children had severe cases.

There’s growing evidence children with preexisting conditions like cardiac disabilities have an increased risk of severe cases. Rarely, children with Covid-19 are also developing a severe, multi-system inflammatory syndrome that causes a high fever and a rash and can be deadly. Out of the 342 children in the US with the syndrome, 71 percent were in Hispanic and non-Hispanic Black children — more evidence of Covid-19’s disturbingly disproportionate impact on communities of color in the country.

We’re also still learning about long-term consequences and prolonged symptoms of Covid-19, which appear to affect as many as 87 percent of adults, and we don’t know what that might look like in kids. Ranney says, “We just don’t know. It would be disingenuous of me to say there’s no long-term risk, just because we don’t have data. It’s also not fair to say kids are at high risk of long-term symptoms just because adults are.”

Does the age of kids matter when it comes to Covid-19 risk?

There does appear to be a difference between younger children and adolescents, both in terms of their likelihood of getting infected with Covid-19 and the likelihood of more severe disease.

One study from late July of 16,025 people across the US showed that children over age 10 may get infected after exposure at rates on par with adults. A study in Iceland also found that 10 years old seemed to be the threshold when incidence rates changed. According to the CDC, nearly one-third of US pediatric Covid-19 cases were between the ages of 15 and 17, and the median age was 11. (As in adults, boys are slightly more likely to get sick than girls.)

This all suggests that middle school and high school may pose different risks for children than elementary schools — although there’s still a risk to teachers and other adult staff. (The Kaiser Family Foundation found that about one-quarter of US teachers are at higher risk of severe Covid-19 cases because of their age or preexisting conditions.)

It’s still unclear why younger children may be less susceptible. One possibility is they are more frequently exposed to related coronaviruses, like the common cold, and since immunity from these exposures lessens over time, kids who have recently been infected might have some protection that adults don’t.

Or, another new study suggests that the gene for a receptor the virus attaches to in the upper airway is expressed less in children than adults. Generally, the immune system becomes less robust as you age, so it’s also possible children’s immune systems just mount a better response to the virus — not too much and not too little. Or, because younger children don’t generate as much force when coughing or speaking to aerosolize the virus, they may be less likely to transmit the virus to others in indoor spaces, even if they are sick. But overall, the jury is still out.

If kids can transmit Covid-19, how often do they?

If children are exposed to the virus, it appears they may be at a lower risk of developing Covid-19. One study published in Nature in June with data from six countries suggests kids under the age of 20 are about half as likely to get sick after exposure as adults; other studies in Israel, the Netherlands, and Switzerland consistently report children get infected less easily than adults.

But children can certainly transmit the virus both to each other and to adults. The question is how often they do.

Scientists have repeatedly found infected children have similar viral loads to adults, and a new study out July 21 in JAMA found that young children may have even more of the virus. It looked at different ages and found that children under age 5 had very high levels of the virus in their nose and throat, compared to adults. (Some experts speculated there may have been sampling bias in the study — testing primarily children with symptoms, when children having symptoms may not be the norm.) An important caveat, Ranney says, is this doesn’t necessarily mean the virus is infectious — the next step will be actually trying to culture live virus from swabs of children.

And again, the age of the child likely matters when it comes to their ability to transmit the virus: A study in South Korea followed the contacts of 5,700 Covid-19 patients and found that children between ages 10 and 19 spread the virus at a similar rate as adults, while children under the age of 10 transmit much less. A limitation of the study is that they looked at transmission in households, where masks and social distancing were less likely. And a preprint — a study that hasn’t yet been peer-reviewed — from Italy that also came out in late July found that children under age 14 are slightly less likely to be infected than adults, but actually 9 percent more likely to transmit the virus overall.

Multiple studies suggest that children are rarely the first person in a household to get sick, meaning they’re more likely to get it from their parents than to give it to their parents. But household studies are often biased because researchers are looking for who got sick first via reported symptoms — and kids are more likely to be asymptomatic.

Why community transmission is so central to the question of reopening schools

Reports on how likely schools are to drive wider community outbreaks outside of households are mixed. Even if children don’t transmit the virus as readily as adults, they have as much as three times the number of contacts — meaning they have three times the number of opportunities to transmit the virus.

Because schools in the US have been closed, we don’t yet have data on transmission from American classrooms. A new study looked at how states with early closure of schools had reduced levels of Covid-19 compared to states that closed schools later — even after adjusting for other state policies like lockdown orders. This does not mean shutting schools caused lower levels of coronavirus cases, but the researchers found a strong correlation. This association would mean more if school closures hadn’t occurred within days of other measures, making it harder to measure school impact.

This is where comparing other countries’ experiences may be helpful: For example, Sweden kept elementary schools open and closed middle and high schools. Finland closed all three. When researchers compared the two Nordic countries, they found that because of limited testing, the infection rate in younger children was probably significantly underestimated. They suggest given the available data, it did not appear that keeping elementary schools open drove Swedish community outbreaks.

But there have been Swedish school outbreaks — in one school, 18 of 76 staff were infected, and several teachers have died — although the lack of testing and contact tracing makes it difficult to draw conclusions. A Swedish Public Health Survey in May found a comparatively high antibody rate in children, suggesting there may have been significant transmission in schools.

In other places, school transmission has been more clear cut. For example, one study in France found that new cases dropped when school holidays began. When Israel tried to reopen schools in stages in May, within two weeks, over 20 schools had to close again, and hundreds of students were infected. (That was when transmission rates in Israel were much lower than they are in most of the US today.) Eyal Leshem, the director of the Institute for Travel and Tropical Medicine at the Sheba Medical Center in Israel, says, “It has been known for many years that school closure is one of the highest impact interventions to stop a pandemic influenza outbreak.”

In a recent study of 727 people, the Israeli Health Ministry found 28 percent were infected in educational institutions. Leshem adds, “In retrospect, it could be expected that when you reopen schools, children are drivers of respiratory viral outbreaks.” One study goes so far as to suggest that closing schools might be the single most important non-pharmaceutical intervention in reducing rates of Covid-19. “Based on the South Korean and Israeli school outbreaks, it’s clear schools do spread Covid-19,” says Chethan Sathya, a pediatric surgeon and assistant professor of surgery at the Cohen Children’s Medical Center in New York.

The reason why some countries seem to have school outbreaks and others don’t is likely linked to how much of the virus is circulating in the communities around the school. And this brings us to a key point in thinking about kids’ risk of Covid-19: Their risk is largely driven by the extent of the community transmission.

For example, researchers from Stanford University, Georgia Tech, and the Applied Bioinformatics Laboratory have developed an online calculator that tells you what the chances are of someone in a group having Covid-19. It says that right now a group of 20 people in Miami — like you would have in a classroom — has a 98 percent chance of at least one person having Covid-19.

In the last two weeks of July, Florida saw a 34 percent increase in new cases in children and a 23 percent increase in pediatric hospitalizations. A small percentage of a big number still means a lot of kids are getting sick.

This makes families’ choices about schools even harder. “It feels like a truly impossible decision for parents to make,” says Julia Marcus, an assistant professor at Harvard Medical School. “There are so many competing priorities and so little certainty.”

Pooja Lakshmin, a perinatal psychiatrist specializing in women’s mental health and a clinical assistant professor of psychiatry at George Washington University School of Medicine, says she’s had a lot of parents ask her for a solid set of rules that can make things safe. “But this is about balancing multiple risks,” she says.

As school districts and parents around the country weigh their options, Ranney says there are no universal right answers. “The tough thing is that, like with vaccines, your decision impacts not just your kid but other families. A risk you may be able to tolerate might not be okay with your community. So you have to consider not just your family but the health of the community at large.”

Lois Parshley is a freelance investigative journalist. Follow her Covid-19 reporting on Twitter @loisparshley.

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