The outbreak of a new virus always breeds confusion. Where did it come from? How does it spread? How dangerous is it? Ten weeks into the Covid-19 epidemic, enough information has emerged to start filling in some of these gaps. Scientists believe the virus that causes the respiratory disease is likely to be transmitted in droplets through coughing. The available data suggests that a single contagious person will infect about 2.2 others, on average. Globally, 3.4 percent of reported Covid-19 patients have died, though that fatality rate is likely inflated, since people with mild symptoms are probably not being diagnosed and counted in the overall patient pool.

But as the weeks have gone by, one mystery has remained: Where are all the kids?

In a recent analysis by a team of researchers at Johns Hopkins and in China of more than 72,000 confirmed cases from China, children under the age of 10 accounted for less than 1 percent of all infections. Of the 1,023 deaths recorded in China at the time, not a single child was among them. “We see relatively few cases among children,” World Health Organization director general Tedros Adhanom Gheberyesus told reporters in mid-February. “More research is needed to understand why.”

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“It’s really very weird,” says Buddy Creech, an infectious disease pediatrician at the Vanderbilt University Medical Center. Mortality patterns associated with most respiratory pathogens traditionally form a U-shaped curve, reflecting more severe disease in the very young and the elderly, says Creech. Respiratory viruses flourish in bodies where the immune system is either still developing or has started to wear out. These include the four coronaviruses that cause the common cold, which tend to be way more common in children than adults. “The new virus that causes Covid-19 appears to flip that,” says Creech.

Now, a detailed new study helps to explain what’s been going on. It turns out, it’s not that kids are somehow immune to SARS-CoV-2. They’re just not getting very sick.

“Kids are just as likely to get infected as adults,” says Justin Lessler, an infectious disease epidemiologist at Johns Hopkins Bloomberg School of Public Health, who co-led the new study with epidemiologists at the Harbin Institute of Technology in Shenzhen and the Shenzhen Center for Disease Control and Prevention. “If you only have data on cases, which is most of what’s out there, you can’t figure out whether an exposed kid is more likely to get infected than an exposed adult. Nobody besides us has been able to look at data on groups of exposed people to see who got sick and who didn’t.”

A few weeks ago, Lessler was contacted by a Johns Hopkins alumnus currently working in China about collaborating on disease modeling using data collected by the Shenzhen CDC. Once he got a look at the data, Lessler realized it could help answer this question about how the virus acts in different age groups.

On January 8, the Shenzhen CDC identified its first local case of Covid-19 and began monitoring travelers from Hubei province, the center of the outbreak 700 miles to the north. Over the next few weeks, the agency expanded its surveillance, testing people with fevers in hospitals and local clinics. Altogether, public health officials identified 391 Covid-19 patients and 1,286 people who’d been in close enough contact to these patients to be exposed to the virus. They followed these contacts, testing them regularly to see whether they contracted the disease, even if they never showed symptoms. The team found that children ages 9 and younger who had been exposed to the virus were just as likely to be infected as other age groups—between 7 and 8 percent of the time. But they were much less likely to have the kinds of severe symptoms seen in older groups. In fact, many kids never showed symptoms at all. The researchers posted their analysis to the medRxiv preprint server on Wednesday.