For children themselves, Covid-19 is not a big threat. They usually have mild symptoms or none at all. Among children with symptoms, only 0.1 percent of those younger than ten and 0.3 percent of those aged between ten and 19 end up in hospital, a study from Britain shows. For school-age children, a Covid-19 infection is less deadly than most flu infections.

The big worry is that children may spread the virus through school. Studies in households where someone introduced the infection usually find that younger children are much less likely to catch the virus than adults. The evidence for older children is mixed, with some studies concluding that they are as susceptible to infection as adults.

But even if children are less easily infected at home, when they mingle a lot, chances are that many of them will pick up the virus. In an overnight summer camp in the state of Georgia in June, at least half of the 346 children attending were infected.

Whether the sort of mingling that happens at school is also a recipe for disaster is best judged by looking at countries where schools have reopened. Data from England published on August 23 is encouraging. Its schools reopened in June for some students before closing for the summer a month later. In that period, only 0.01 percent of preschools and primary schools had Covid-19 outbreaks, affecting 70 children and 128 staff — out of 25,470 infections recorded in England as a whole. Of the 30 school outbreaks involved, the probable source in 20 was a staff member. That teaching is not exceptionally risky is also the conclusion from Sweden. Staff at its nurseries and primary schools, which never closed, were no more likely to become infected than in other jobs.

Less clear is the role of secondary schools in infections. They have stayed shut almost everywhere. Outbreaks in France and Israel suggest that the virus could spread more easily in secondary — than in primary — schools. Older students may be easier to keep apart in classrooms, but good luck trying to stop them congregating afterwards.

America will struggle to contain school outbreaks as much of Europe has done, because infection rates in many states are too high and health officials are overwhelmed. Tough choices may be necessary. Britain’s prime minister, Boris Johnson, has warned that pubs might have to close (to keep infections down) so children can go to school. In America, where any constraint on freedom goes against the grain, such trade-offs may be an even tougher sell.