Are 42 percent of younger children really immune to the coronavirus?

Most likely, no. The C.D.C. estimate was based on tests of a small number of children who had blood drawn for routine medical care or other illnesses. That is not a representative sample of the general population, said Akiko Iwasaki, an immunologist at Yale University.

For example, the sample may have included children who have cancer or other conditions that weaken the immune system. “Usually, kids don’t get blood drawn for any routine medical care unless they have some reason to,” Dr. Iwasaki said.

Studies based on blood samples at clinics or by recruiting volunteers notoriously overestimate how many people have been infected, said Deepta Bhattacharya, an immunologist at the University of Arizona.

“If you’re not careful about doing a random sample, then the seroprevalence numbers can get pretty wacky,” he said.