Those experiences and conclusions may offer hopeful guidance to societies still weighing how to get students and teachers back into primary and secondary classrooms.

Much remains unknown about the interaction between children, schools and the virus.

While documented cases of younger students transmitting the virus to their classmates or to adults so far appear rare, there is enduring worry about the susceptibility of teens, college-age students and their teachers.

And, especially in communities where the virus is still circulating widely, elaborate and expensive measures may be necessary to avoid shutting down entire schools each time a student tests positive.

Arnaud Fontanet, head of the Epidemiology of Emerging Diseases unit at the Institut Pasteur in Paris, said he “gladly” sent his four teenagers back when French schools reopened on a voluntary basis in mid-May.

“High schoolers are still contagious and primary school students are less contagious but not zero-risk,” he said.

Dig into reports of two or three elementary students with the virus, and often it turns out they’re siblings.

At the École Louis-de-France, an elementary school in Trois Rivières, Canada, almost an entire class of 12 students tested positive in late May.

And at the Cheondong Elementary School in Daejeon, South Korea, two brothers were found to have the virus on June 29, and two students who had contact with one of the brothers tested positive the next day.

In Finland, when public health researchers combed through test results of children under 16, they found no evidence of school spread and no change in the rate of infection for that age cohort after schools closed in March or reopened in May.

In fact, Finland’s infection rate among children was similar to Sweden’s, even though Sweden never closed its schools, according to a report published Tuesday by researchers from the two countries.

In Sweden, researchers also found that staff members at day cares and primary schools were no more likely than people working in other professions to contract the virus.

Fontanet, with the Pasteur Institute, was the lead author on twin studies that found the virus spread in the high school of one French town but not in its six primary schools before the country’s March lockdowns.

By early June, more than 100 schools had been shut and more than 13,000 students and teachers had been sent home to quarantine.

The most notable outbreak was tied to a middle and high school: The Gymnasia Rehavia in Jerusalem saw 153 students and 25 staff test positive.

But many countries that resumed in-person classes in May and June have already abandoned some social distancing measures, at least in primary schools.

In Japan, where schools reopened shortly after the country’s state of emergency was lifted in May, children initially attended on alternate days in some schools to allow for more space in classrooms.

When France shifted from voluntary to mandatory attendance for primary and middle school students for the last two weeks of June, a social distancing requirement of four square meters between students was reduced to one meter laterally.

Similarly, before the biggest wave of school reopenings in Belgium in early June, policymakers declared that strict physical distancing rules would not be enforced, allowing more students in each classroom at once.

Belgian schools are now closed again for the summer, but leaders are planning an ambitious reopening plan for Sept.

He said distancing is hard for high school students, but especially for younger kids.

But when schools fully reopen in September for mandatory, full-time, in-person classes, elementary school students will be in “class bubbles” of up to 30 and high school students in “year bubbles” of up to 240.

Quebec, the Canadian province hit hardest by the coronavirus, experimented with various means of social distancing when it reopened elementary schools outside Montreal in May.

Bubbles will be introduced when elementary and high schools reopen for compulsory in-class instruction in the fall.

In part because there haven’t been many outbreaks associated with schools, some students, parents and teachers who initially resisted classroom reopenings have come around.

In France, when schools reopened in May on a voluntary basis, statistics from the Education Ministry showed that only about 1.8 million out of 6.7 million nursery and primary schoolers went back, along with 600,000 out of 3.3 million middle schoolers.