Erica Kensek, a 33-year-old hospital administrator in Columbia, S.C., got vaccinated for the coronavirus as soon as she was eligible in January. Her husband, a manager at an auto service store who has an autoimmune disease, got vaccinated in March.

But when vaccination became available for their 13-year-old daughter, Abby, the couple opted not to sign her up.

“The threat to children [from covid-19] has been so minimal, and we feel like there just has not been enough long-term research on this vaccine and its effect on children,” Kensek says.

Federal health officials agree that approved vaccines are safe for teens, and that although most cases of covid-19 are very mild in children, in rare instances the effects of infection can be severe. From a public health standpoint, vaccinating eligible adolescents is a priority: Teens and kids who are unvaccinated may drive the spread of the virus, extending the pandemic and giving the virus more opportunities to mutate in ways that could erode the effectiveness of the vaccines for everybody.

For individual parents looking at their own kids, however, the choice doesn’t always seem so clear-cut. Abby had a seizure last year that was never fully explained by the slew of medical specialists the family visited, says Kensek, and she occasionally suffers from high blood pressure. It makes Kensek nervous about signing her up for a relatively new vaccine, despite assurances of its safety in general.

“I don’t see the necessity of poking that beast,” she says. “There’s just not enough [data] out there for us yet. The CDC says it’s safe, and that’s great. But how many times have they gone back on their suggestions?”

She’s hardly alone. A majority of American parents have chosen not to have their adolescent children vaccinated for the coronavirus — at least, not yet. More than two months after the U.S. Food and Drug Administration expanded its emergency-use approval of the Pfizer-BioNTech vaccine to include children ages 12 to 15, the Centers for Disease Control and Prevention last week reported that less than a third of eligible adolescents are fully vaccinated. Rochelle Walensky, director of the CDC, said low uptake among eligible teens influenced the agency’s decision to recommend a return to mask-wearing as the highly contagious delta variant drives a surge in infections across the country, especially among unvaccinated people.

“It is concerning” that so few adolescents have gotten the shots, says Jennifer Lighter, pediatric infectious-disease specialist at New York University Langone Health. “It’s rare, but we do see severe cases of covid” in children, she says, “and it’s very difficult treating a patient in the hospital knowing that their hospitalization and illness could have been prevented.”

Vaccination hesitancy among adults has dogged the country’s mass-vaccination efforts, with the Biden administration missing its goal of getting at least one dose into 70 percent of adults by July 4. But with cases surging, the school year looming, and health officials increasingly certain that the delta variant might cause more-severe illness, the focus has expanded to include adults who are vaccination-hesitant with regard to their kids.

Elizabeth, a freelance writer in St. Louis, has agonized over the question of whether to vaccinate her 14-year-old son to a degree she hadn’t expected.

Elizabeth, who asked to be referred to only by her middle name to protect her son’s medical privacy, says she and her husband have taken every precaution during the pandemic. Their son was born premature, and though he has been mostly healthy since then, they were eager to protect him as much as they could. They both received vaccinations as soon as possible, and planned to do the same for him.

Then Elizabeth heard news reports about instances of teenagers who received the shot being diagnosed with myocarditis, a condition that inflames the heart muscle.

Federal officials have repeatedly said that the benefits of vaccination for teenagers still outweigh the risks, noting that heart inflammation can also be a side effect of a covid infection. (There have been just over 1,200 reports of myocarditis out of 300 million doses of the vaccines given to both adults and adolescents, according to the CDC. But more than 4,000 children who caught covid ended up with multisystem inflammatory syndrome.) “Myocarditis is a very exceedingly rare side effect that can happen,” says Lighter, the NYU pediatric infectious-disease specialist. “But, we know how to diagnose it and treat it.”

Elizabeth says she has listened to every public CDC meeting, read scientific research papers, sought guidance from medical experts. She feels uneasy about the fact that even the experts seem still to be updating their understanding of the risks of both the virus and the vaccine. The idea that she might be proactively exposing her son to something that may cause him harm has left her feeling stuck.

“It’s just hard to feel like I could take my kid out of our bubble and go do something that could harm him,” she says. “I’ve never had a vaccination before where I thought, ‘Is he going to end up in the hospital because of this?’ ”

Even before the onset of covid-19, vaccine skepticism among American parents was on the rise. But even some families that typically vaccinate their children according to medical guidelines have struggled with the question of whether to have their kids sign up for a covid shot.

One mother in Evanston, Ill., was worried that her reservations about making an appointment for her 12-year-old son would lead to an argument with her husband. After all, they had both leaped at the chance to get vaccinated themselves. And they believe in vaccination, generally: Their son is up-to-date on his other shots.

As it turned out, her husband also had reservations. They agreed that there are just too many risks, particularly because their son is still small and hasn’t yet hit puberty.

The mother, who asked not to be named because she doesn’t want parents in her community to find out her son isn’t vaccinated, worries that a vaccine judged safe for 12-year-olds might be less so for her undersized son. She has questions — about the clinical trials, about how researchers are so certain of the rarity of adverse effects — that she hasn’t seen answered to her satisfaction. And she likes her son’s odds against covid without it: He’s healthy, athletic, has no preexisting conditions, wears a mask and hasn’t eaten inside a restaurant since the pandemic started, she says. Also, the rate of vaccination in their city is high.

“Fewer than 400 children have died,” she says — 406 in total, according to the CDC’s most recent figures. “That’s terrible, of course. But it’s infinitesimal.”

Would anything change her mind?

“Once there are lower dosages tested in younger children, and we wait and see if there are no side effects, I would vaccinate him,” she says. “I don’t know of any of his friends who haven’t gotten vaccinated, but most of his friends are six inches taller and their voices are changing.”

Medical experts have pointed out that vaccines work differently than, say, painkillers. A covid vaccine for younger children is being tested in lower doses — but because of differences in the immune system, not in height or weight. “There really is not a lot of weight-based vaccine dosing,” says Allison Bartlett, a pediatric infectious-disease specialist at University of Chicago Medicine. “We gave it to lots of very small adults. We didn’t have a separate dose for 400-pound linebacker and a 70-pound, 4-foot-8 grandma.”

One father of four in Charlotte, who asked we not use his name to protect his family’s privacy, said his 14-year-old daughter is eligible, but he and his wife don’t plan to get her vaccinated — even though she asked to discuss it.

He’s not vaccinated, either. (His wife, a social worker who works in an emergency room, is vaccinated.) He said their approach to covid has been to eat well, spend a lot of time outside, exercise, and build and strengthen their immune systems.

“I’m not a guy who thinks we need to live in a forest and all medical interventions are wrong,” he says, noting that his kids are up-to-date on their other vaccinations. “We’ve eradicated many, many diseases because of vaccines.”

But with the coronavirus vaccines, “the juice isn’t worth the squeeze,” he says. “I don’t know the long-term impact of this.”

There are no long-term impacts of vaccines, says Lighter, the NYU specialist. “That’s not a thing,” she says. “What can occur rarely are immediate reactions, and exceedingly rare would be an adverse effect within weeks of the vaccine. But we don’t see months-to-years-later effects of vaccines.” (That includes mRNA vaccines, she added.)

We do know that covid can have long-term effects, Lighter points out. Some adults who endured covid infections, even mild ones that didn’t send them to the hospital, have continued to experience lingering symptoms for months — including fatigue, aches, shortness of breath, trouble concentrating and depression. Researchers are trying to determine how vulnerable children are to such lasting effects.

The father in Charlotte says he thinks his daughter asked about the shot because of peer pressure: Her friends were getting it and posting on social media. He said he wanted a vaccine to prevent his kids from getting covid altogether but doesn't find it necessary to have a vaccine that simply lessens the symptoms.

The fact that vaccination reduces severe symptoms should make a vaccine seem worth it to parents worried about the health of their children, according to Lighter.

Severe illness may be rare in children, she says, but vaccination will make sure of it. With the high contagion of the delta variant, “chances are they will get covid and will be mild,” says the doctor. “But severe disease can happen, and long covid can happen and it transmits. We’re in this together, we live in communities, and are supposed to be protecting each other. We’re supposed to be getting back to normal. We can’t get back to normal until most people are vaccinated.”

As for relying on other precautions, parents shouldn’t assume that they can duck the virus forever. “People need to understand,” Lighter said, that “covid will find them.” And those who do get infected and recover probably will not get the same level of protection as those who get antibodies through vaccination.

So far, Kensek, the hospital administrator, hasn’t second-guessed her decision not to vaccinate their daughter, Abby.

Not even on Thursday, when Abby, along with several other members of her cheerleading team, tested positive for the coronavirus.

“I’m a little relieved, honestly,” Kensek said. “Especially with school about to start on August 19th. Her quarantine will be over by then and she will have the antibodies.”

(Clarification: This article has been updated to include additional information about vaccine dosages.)