But the question of how likely children are to spread it to teachers, staff and other students still hasn’t been settled. One large new study from South Korea found children under the age of 10 appear to not transmit the virus very well. While it’s not exactly clear why, the pediatric infectious disease experts contacted by WIRED say that it’s perhaps because young children expel less air that contains the virus and are shorter, so any potential respiratory droplets are less likely to reach adults. A study published in April by researchers at the Brigham and Women’s Hospital in Boston suggests that younger kids haven’t developed the molecular keys that the virus exploits to enter the body and wreak havoc on the respiratory system, microscopic structures known as ACE2 receptors.

But older students are more like adults in their ability to transmit the virus, according to the South Korea study, which makes school opening decisions tougher. Should administrators allow only elementary students to attend in person, while middle and high schoolers stay online at home? If they do, will younger children be able to keep their masks on all day or stay six feet apart? What about the psychological effects of continued isolation on teens, who many parents believe are already racking up too much screen time during the pandemic shutdown and now are facing months of online learning?

The CDC announced school reopening guidelines on Thursday that call for officials to reopen classrooms this fall, based on the idea that children do not become as sick from Covid-19 and are less likely to spread it as adults, and to belay any emotional and psychological harm from the disruption of schools staying closed. The agency issued these new guidelines after President Donald Trump attacked initial rules that called for desks being set 6 feet apart, staggered lunches, and temperature screenings, as being too costly and burdensome.

Dimitri Christakis helped draft a separate set of reopening guidelines for US schools in a report for the National Academy of Sciences released July 15. It says schools should take steps to reopen for younger students in grades K-5 and those of all ages who have special needs. Christakis says that with appropriate social distancing, hand-washing and protective masking, the risk to teachers, staff and students in a school can be reduced.

“With those additional precautions, primary school teachers should feel comfortable going to school,” says Christakis, director of the Center for Child Health, Behavior and Development at Seattle Children’s Research Institute. “Those are the kids for which in-person learning is so important. We should prioritize these kids.”

At the same time, Christakis admits there are still some unknowns about transmission among school-age children. “We don’t have the answer to what extent children transmit the virus in general, and in particular in a school setting,” he says. “This is called the ‘novel coronavirus’ for good reason. It is acting very differently than most respiratory viruses and many other coronaviruses. Children appear to be less affected directly, and potentially less likely to transmit.”