The vaccination drive among adolescents, a critical part of the wider effort to build society-wide immunity to Covid-19, has slowed, as many parents harbor safety concerns.

Many parents rushed to get their children inoculated in May after regulators widened use of Pfizer Inc.’s Covid-19 shot to children as young as age 12. Yet vaccinations have flagged since. Other parents have held off because of concerns about the shot’s speedy development and a rare side effect, an inflammatory heart condition called myocarditis. They are struggling with how to weigh these risks against research indicating that Covid-19 itself isn’t a significant risk for children.

Some of the reluctant parents are vaccinated themselves, a new challenge for public-health officials trying to overcome more general hesitancy about vaccines, as they race against the Delta variant and prepare for the resumption of schools.

“I’m just going to hold off,” said Jackie Gordon, of O’Fallon, Ill., who hasn’t gotten her 16-year-old son vaccinated, though she and her 18-year-old daughter have taken the shots.

Giving her pause, she said, was uncertainty whether the shot is safe for children like her son with extreme allergies. The Centers for Disease Control and Prevention recommends vaccines for people with severe allergies so long as they aren’t related to shots or injectable medications.