Vaccinating children is crucial to building up population levels of immunity and curtailing the spread of the coronavirus. Though children spread the virus less efficiently than adults do, they make up about 23 percent of the population.

Experts have said that the country is unlikely to reach the “herd immunity” threshold — the point at which virus transmission essentially stalls — but vaccinating children will be important for getting as close as possible.

Ty Dropic, 14, one of the trial participants, urged others his age to be vaccinated so they could build up widespread immunity and protect themselves. He had no side effects, leading him to suspect that he got the placebo. If that turns out to be the case, he plans to be immunized as soon as possible.

“I know it can be kind of scary, but it’s really not as bad as it seems,” he said. “If you do get Covid, it’ll be a lot worse than getting stuck with a needle for, like, two seconds.”

Ty’s three siblings, ages 8, 10 and 16, are also enrolled in vaccine trials for their age groups. Their mother, Dr. Amanda Dropic, a pediatrician in northern Kentucky, said that in her practice, most parents were eager to have their children vaccinated so they could regain some semblance of normalcy.

“The anxiety and depression that we’re seeing with kids, the social delays, has been tremendous,” she said.

Dr. Dropic said her children understood the risks and were willing to volunteer because they saw it as a civic duty. Every medicine available today came to be because “somebody was willing to go first,” she added.