Reopened schools in other nations have largely avoided outbreaks. They have lessons for the U.S.

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

Still, public health officials and researchers caution that most school reopenings are in their early stages. Much remains unknown about the interaction between children, schools and the virus. Schools have only reopened in countries where the virus is under better control than in many parts of the United States. And parents and teachers, especially in Europe, have been vocal about their concerns. It is premature to say, as President Trump put it this past week, that “In Germany, Denmark, Norway, Sweden and many other countries, SCHOOLS ARE OPEN WITH NO PROBLEMS.”

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. But he emphasized that was only because “the virus is not too much circulating in France.”

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

Public health officials and researchers say they have not detected much coronavirus transmission among students or significant spikes in community spread as a result of schools being in session — at least for students under 12.

Virologists warn there may be additional spread that hasn’t been recognized, since testing asymptomatic people, particularly children, remains uncommon. But in many cases, young children who test positive have gotten it from someone in their family and do not appear to have infected others in school. Dig into reports of two or three elementary students with the virus, and often it turns out they’re siblings.

There are exceptions. 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.

Such cases, though, have been rare. Before the suspected transmission in Daejeon, South Korea’s education minister had emphasized that not a single student in the country had contracted the virus at school.

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.

“It really starts to add up to the fact that the risk of transmission, the number of outbreaks in which the index is a child, is very low, and this seems to be the picture everywhere else,” said Otto Helve, who worked on the report as a pediatric infectious-disease specialist at the Finnish Institute for Health and Welfare.

He said he sent his own children back to school.

Why young children may be less susceptible to the coronavirus or less prone to exhibit symptoms of covid-19, the disease it causes, remains a topic of hot debate among scientists. Theories range from the possibility that children have fewer of the receptors that the virus uses as a gateway into the respiratory system to their having higher overall immunity because of a greater exposure to other types of coronavirus.

But the overall observation has led some to question whether school closures were warranted in the first place.

“The scientific evidence for the effects of closing schools is weak and disputed,” said Camilla Stoltenberg, director general of the Norwegian Institute of Public Health, which has advised Norway’s pandemic response.

She said that although she supported her country’s March lockdown, it was less clear that Norway needed to close schools. “We should all have second thoughts about whether it was really necessary,” she said. “We see now that, after having opened schools, we haven’t had any outbreaks.”

The calculations may be different, however, for students in their teens and older, as they are thought to be somewhat more prone to the virus and more capable of spreading it.

In Israel, where the virus has been surging again, schools at every level have been affected. 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.

Israeli health authorities said they were unsure how many of those cases were the result of the virus being passed around within school buildings.

“We just don’t have a good answer for that,” said Hagai Levine, the chairman of the Israeli Association of Public Health Physicians. Many students tend to spend time together in and out of school, Levine said, making it hard to pinpoint the actual site of transmission. “There does some to be evidence that there is less transmission in children under 10.”

Plans are uncertain for what classes will look like in Israel on Sept. 1, when the next school year begins.

In many nations preparing to reopen school buildings for the first time in the fall, social distancing concerns are dominating the debate.

The Italian government, which closed schools when the pandemic first exploded and made no attempt to restart in the spring, has pledged to restart classes in mid-September and has committed to “less-overpacked classrooms.”

“We don’t want chicken coops,” Prime Minister Giuseppe Conte said in a national address.

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. But classes are largely back to normal now, albeit with students and teachers wearing masks, washing hands regularly and taking daily temperature checks.

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.

“This allows us to accommodate all students,” Education Minister Jean-Michel Blanquer said at the time of the announcement.

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 have an ambitious reopening plan for Sept. 1. For kids under 12, classes will remain in session, full-time and full-capacity, no matter how bad the second wave of infections gets in the country. If current infection rates stay steady in Belgium, students 12 and older will attend school four days a week, with an additional half-day of virtual schooling. Officials would dial back the in-person schooling for the older children if there is a second wave.

To some extent, these shifts reflect growing confidence that bringing children together may not lead to a spike in infections.

There is also rising concern about the downsides of keeping students home.

Belgium’s reopening was accelerated by an open letter from hundreds of pediatricians arguing that the educational cost of keeping schools closed was worse than the health risk of reopening them.

In Germany, some public health experts have welcomed plans to drop a 1.5-meter minimum distance rule and resume full-capacity classes after summer vacation. Policymakers fear that digital learning has put poorer students at a greater disadvantage and that there would be a rising mental health toll on students if school restrictions dragged on.

But the shift away from social distancing is also about practical concerns.

“Basically, the difficulty is enforcing social distancing among students,” said Fontanet of the Institut Pasteur. He said distancing is hard for high school students, but especially for younger kids. “People have more or less given up on that entirely at this stage,” he said.

Although schools in Israel initially resumed with strict rules about temperature checks, carefully spaced-out desks and masks, critics complained that the precautions quickly lapsed. “Within two or three days, that all fell away,” said Dan Ben-David, president of the Shoresh Institution for Socioeconomic Research.

Italy’s education minister, Lucia Azzolina, said that to keep classroom sizes at acceptable levels, districts would have to reopen shuttered school buildings and transfer some students elsewhere. She also floated the idea of holding classes in theaters, cinemas and museums — “even parks,” she said.

But countries that have resumed classes already have found that it’s easier and cheaper to welcome all students back to their classrooms than it is to devise complicated schedules with multiple shifts or to find new space.

In Israel, hypervigilant public health officials mandated that an entire school close any time a single coronavirus case was detected among students or staff.

By contrast, in Germany, when a student tested positive, that class was put into a mandatory two-week quarantine, but the rest of the school continued on.

Clearly, the German model is less disruptive. Some health experts have thus come to advocate that more important than social distancing within a classroom are efforts to create bubbles within schools, to limit potential contamination and the need to shut everything down.

England started sending some grades back on a voluntary basis in June. 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. Classes were limited to 15 students. Libraries remained closed. Recess times were staggered. Some schools painted green dots on schoolyard grounds to mark sufficient separation.

Bubbles will be introduced when elementary and high schools reopen for compulsory in-class instruction in the fall. Within classrooms, students will form groups of up to six students who won’t have to maintain social distancing. Bubbles must keep a one-meter distance from each other and two meters from teachers.

Helve, the Finnish infectious-disease specialist, noted that bubbles may be especially valuable in societies with high infection rates, such as the United States, where it may be inevitable that a student or teacher shows up with the virus at some point.

“How do you minimize the impact on the school?” he said. “The more cases you have in a society, the more likely it is that you will have an outbreak at a school, or that you will have a teacher or a parent or a child who brings the virus to the school.”

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.

One survey of French-speaking parents in Belgium found that 96 percent of respondents planned to send their children back to school in the fall.

Technically, they won’t have a choice. Education is compulsory in Belgium for children 6 and older, and although the requirement was suspended this spring, it will be back in force in September.

That’s in line with moves by many countries away from voluntary in-person attendance, which saw limited uptake.

British Prime Minister Boris Johnson, who was forced to delay plans for a full reopening of elementary schools in England after strong resistance from teaching unions and some parents, intends to forge ahead in the fall.

“We want them all back in September,” said Johnson. “We’ve got to start thinking of a world in which we are less apprehensive about this disease.”

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.

France had hoped reopening would address the inequalities evident under distance learning. But the government found that students from wealthier families were more likely to be among those who returned to their classrooms, while many poorer families continued to keep their children home. The education minister suggested the gap had to do with a lack of trust.

French officials ultimately made school attendance mandatory for the final two weeks of classes in June, before the summer holidays began. Families and teachers questioned the need for such a scramble for so little class time. Some accused the government of being more concerned about freeing parents to return to work than about the needs of students and teachers.

That’s in contrast to the United States, where a growing chorus of families complain that state and local governments are downplaying the need for kids to be in school before parents can return to their workplaces.

The French government defended its decision.

“Two weeks count; two weeks are not nothing, whether it’s out of an educational aspect or a psychological aspect,” Blanquer, the education minister, said. “School should never be considered as a day-care center of sorts.”

James McAuley in Paris, Karla Adam in London, Rick Noack in Berlin, Steve Hendrix in Jerusalem, Min Joo Kim in Seoul, Simon Denyer in Tokyo, Amanda Coletta in Toronto, Quentin Ariès in Brussels and Chico Harlan in Rome contributed to this report.