As demand for coronavirus testing surges around the nation, laboratories that process samples are again experiencing backlogs that have left anxious patients and their doctors waiting days — sometimes a week or more — for results.

At the city and state levels, testing delays could mask persistent rises in case numbers and could cloud ways to combat the coronavirus, as health officials continue to find themselves one step behind the virus’s rapid and often silent spread, experts said.

In the heated debate over reopening schools, one burning question has been whether and how efficiently children can spread the virus to others.

A large new study from South Korea offers an answer: Children younger than 10 transmit to others much less often than adults do, but the risk is not zero. And those between the ages of 10 and 19 can spread the virus at least as well as adults do.

The findings suggest that as schools reopen, communities will see clusters of infection take root that include children of all ages, several experts cautioned.

“I fear that there has been this sense that kids just won’t get infected or don’t get infected in the same way as adults and that, therefore, they’re almost like a bubbled population,” said Michael Osterholm, an infectious diseases expert at the University of Minnesota....