How likely are kids to get Covid-19? Scientists see a ‘huge puzzle’ without easy answers

In January, when it became clear that a new coronavirus was transmitting with ease among people in central China, one of the top questions scientists who study disease dynamics wanted answered was this: What role are children playing in the spread of the new disease, now known as Covid-19?

Five months later, they and the rest of us would still like to know.

There is some evidence that kids are less likely to catch the virus and less likely to spread it, but it’s not clear exactly how strong that evidence is. Much of it was generated at a time when children were caught up in the topsy-turvy world of Covid-19 transmission suppression, with schools closed and families cocooned, limiting their chances of catching or spreading the SARS-CoV-2 virus. In reality, it may take reopening schools and returning children to a closer-to-normal life for the picture to come into clearer focus.

If we’re lucky, and the hints in the data hold true, a normalizing of children’s lives — and the lives of their parents — could be safely achieved. If we’re not lucky, and the hints were false harbingers, more disruption likely lies ahead.

“We are going to find out,” said Sean O’Leary, a pediatric infectious diseases specialist at Children’s Hospital of Colorado and vice chair of the infectious diseases committee of the American Academy of Pediatrics.

With some respiratory illnesses — influenza in particular — young children play an amplifying role. Because of their limited immunological experience, they don’t carry the years’ worth of antibodies that adults have amassed. As a result, they are more susceptible to many of the bugs that cause colds and flu, which circulate more actively as the cold weather sets in.

Kids get sick, pass the viruses among themselves, and infect the adults in their lives — teachers, day care attendants, parents, and grandparents. It’s not a coincidence that elementary school teachers are often sniffling and sneezing within a month of the start of school, or that flu season often hits in earnest after Thanksgiving or Christmas, when multiple generations share holiday cheer and the occasional germ.

If children play the same role with the SARS-CoV-2 virus, closing schools and restricting the access of children to each other and the older adults in their lives could be important ways to suppress transmission of the new disease. But if they play a less active role, as studies seem to suggest, then the high cost of these restrictions — interrupted education and socialization, stress as parents juggle working from home with child care, lack of access to school meals — aren’t warranted by the small benefits gained.

The World Health Organization does not currently see clear answers in the data that have been collected to date.

“I think the evidence remains weak — and that’s the complexity of answering that question properly,” said Olivier le Polain, an epidemiologist in the Geneva-based agency’s health emergencies program.

Le Polain said that large serology studies — in which researchers collect blood samples from a cross-section of people to get a better sense of how deeply the virus has penetrated the population of a city, a region, a country — should help shed light on how likely children are to be infected in the first place.

An early look, from Geneva, suggests when we’re talking about children we need to distinguish between young children on the one hand and tweens and teens on the other. Published last week in the journal the Lancet, the study found very little evidence of prior Covid-19 infection among children ages 5 to 9 years (the youngest included). But children ages 10 to 19 were as likely to have antibodies to the infection as adults ages 20 to 49 — and more likely than adults older than that.

One thing that is clear is that the disease is generally far less severe for children than it can be for older adults. Deaths among children have been few and the percentage of children who need hospitalization for the illness is substantially lower than it is among adults.

But teasing out whether kids are as likely to catch the virus and spread it has been exceedingly difficult at a time when children are spending far less time mixing with others than they normally do.

“There is a huge puzzle over the dynamics in kids and what happens with kids,” said Nick Davies, an epidemiologist and mathematical modeler at the London School of Hygiene and Tropical Medicine.

“We don’t really have that one great database, piece of evidence, or experiment that has really settled this question,” he said.

Davies is the lead author of a study published this week that suggested children are less susceptible to the virus than adults. Published in Nature Medicine, it used data from China, Italy, Japan, Singapore, Canada, and South Korea to model whether children contract the virus at the same rate as adults. He and his co-authors concluded the risk of catching Covid-19 for children and teens (under age 20) was half that of people older than 20. They wrote their article in March, based on data collected in the first couple of months of the pandemic.

Davies said there are confusing differences seen across countries that muddle the picture. A large national serology survey in Spain found that children were less likely to have been infected than adults — but the same was not true in Britain.

Disparities continue. Denmark reopened schools in mid-April, without serious disruptions. But when classes resumed in Israel in May, infections among children and teachers flared. In China, where a cluster of cases in Beijing has everyone on edge, authorities moved quickly this week to preemptively close schools.

In the United States, overnight summer camps may help to illuminate the risks of allowing children — albeit generally teens — to resume activities that allow them to congregate together, O’Leary suggested. Colorado is home to a lot of these camps; kids come from across the country to attend. While they aren’t yet open, word has come down they will be allowed to operate starting in July, said O’Leary, who admitted that the idea of the overnight camps makes him nervous, keen as he is to get kids back into school.

The movement restrictions many countries adopted earlier in the spring didn’t just make it difficult to gauge what role children are playing in the pandemic. The all-at-once layering of multiple actions — closing schools, workplaces and non-essential businesses, issuing shelter-in-place orders — made it challenging to assess the contribution each individual measure had on the tamping down of transmission of the virus.

“In most places, those interventions have been put into place at pretty much the same time, with maybe one or two weeks in most places,” WHO’s le Polain said.

The stepwise approach to reopening that many jurisdictions have adopted could afford an opportunity to try to tease out some information on the contributions of the various interventions. Having a good sense of the scope of the benefit of placing restrictions on children would help policy experts as they look to the next school year.

Marc Lipsitch, an epidemiologist with Harvard’s T.H. Chan School of Public Health, said someone should be studying the issue, though he acknowledged that after the past few brutal months, he doesn’t have the energy to do it.

But he said the timing of the start of the school year in the United States is so variable that the early start states could provide evidence for those that wait until after Labor Day.

“The good thing about school opening is that it’s a point change that’s big,” said Lipsitch. “So if there’s an effect, it will be a little bit delayed — but it’s not fuzzy and it’s not subtle.”