For Justin Do, getting a COVID-19 vaccine seemed to be no big deal. He knew millions had received it before him, and he figured the experience would be a lot like what happens when he gets the flu shot every year.

That was true, yet his June 10 appointment to get the first of two injections was nevertheless unusual: He was 8 years old.

Justin, now 9, and his brother, Joey, 4, of East Brunswick, N.J., are among thousands of children under 12 to enroll in the latest stages of the vaccine trials. The boys’ parents signed them up for the chance to get the vaccine made by Pfizer Inc. and BioNTech SE — though as in the adult and teen trials, it is possible they got a placebo.

If all goes well, the betting is that the U.S. Food and Drug Administration could authorize vaccines for this age group by midwinter.

In the eyes of many public-health experts, that decision can’t come soon enough, with another pandemic-scarred school year about to get underway. While it remains true that children are at less risk of severe consequences from infection than adults, they can get very sick. Since the start of the pandemic, hundreds of children have died of COVID-19 and thousands have been hospitalized, some with a dangerous inflammatory syndrome called MIS-C.

Yet with infections once again on the rise and the contagious delta variant widespread, children under 12 are the only people for whom vaccines are not yet available.

Minh-Tu Do, the boys’ father, knows the risks first-hand. A physician who specializes in pediatric emergency medicine at Robert Wood Johnson University Hospital in New Brunswick, he has treated children with COVID-19 for the last year and a half, some of them sick enough to need a ventilator.

So when he heard that one of the Pfizer vaccine trial locations was Rutgers University, with which the hospital is affiliated, he was eager for the boys to have a chance at being protected.

“It really was a no-brainer,” he said.

Beyond ensuring the health of children, the broader goal is protecting society at large. Youths can spread the coronavirus to those who are older and more vulnerable, even to those who are vaccinated themselves — as evidenced by the recent emergence of the word “breakthrough” in everyday conversation.

It remains unclear what percentage of the population must be vaccinated to reach the elusive goal of herd immunity, or even to bring the pandemic down to a manageable level. Early estimates were in the range of 70% to 80%, but with delta on the rise, some infectious-disease experts worry that the threshold is now even higher. With just half the United States fully vaccinated as of Aug. 1, the country is well short of the goal.

And without expanding eligibility to the more than 43 million Americans under age 12, epidemiologists say it will be harder to get there.

For children enrolled in the Pfizer vaccine trial, the process is different from how the study was conducted in adults:

  • The doses are smaller, corresponding to smaller bodies. Justin Do received two doses of 10 micrograms each — one-third of the 30-microgram doses given to adults — while his younger brother received two doses of just 3 micrograms each.

  • The primary “endpoints” that researchers are tracking include the children’s antibody levels and any signs of side effects. The children also are being monitored for what the vaccines are designed to prevent — COVID-19 — but that is a secondary measure, given the overwhelming earlier evidence of efficacy. With just a few thousand children in the trial, it is unlikely enough of them would become infected in the short term for statisticians to draw additional conclusions about how well the shots work.

  • Two-thirds of the young trial participants were chosen at random to get the vaccine and one-third received a saline injection, as opposed to the 50-50 split in earlier adult trials. In both cases, participants were not told at first whether they got the real thing. But eventually (in the case of the children, after six months), those who got a placebo are given the chance to get the vaccine.

  • For legal and ethical reasons, the process of enrolling minors involved additional safeguards. Children are not considered able to give informed consent, so their parents had to give permission. Children 7 and up also were asked to give “assent” after having the process explained to them in simplified form.

Justin Do said that before he received his first injection, he was shown a series of easy-to-understand, illustrated explanations on an iPad.

“It would give you little cartoons when telling about what it was,” the boy said. “At the end, I signed my name.”

After each dose, Justin Do experienced temporary side effects, including arm pain, a slight headache, and a bit of nausea, leading his father to hope that he got the real vaccine. His younger brother, on the other hand, reported no issues. Yet he, too, might have gotten the vaccine, as plenty of vaccinated people report no reactions.

» READ MORE: As delta variant spreads, more kids need hospital care

Side effects will be tops on the minds of FDA regulators when considering whether to authorize the vaccines for emergency use in children, likely later this year, said George Karavetsos, a former assistant chief counsel at the agency, now practicing in the Miami office of Buchanan Ingersoll & Rooney.

Temporary arm pain and headache would not raise any red flags. But recently, the agency asked Pfizer and Moderna, the maker of the other vaccine now being tested in children, to add more trial participants to make sure the risk of serious consequences is low. (Pfizer’s original plan was to enroll 4,500 children, while Moderna aimed for nearly 7,000; the exact numbers have yet to be announced.)

Chief on the FDA’s radar screen is a condition called myocarditis — inflammation of the heart muscle — that has been reported in a small number of adult vaccine recipients.

In mid-June, the U.S. Centers for Disease Control and Prevention estimated that this reaction occurs in 1 out of 200,000 vaccinated people, more often in younger men. In August, a study in JAMA found the rate might be twice that high.

But such cases generally have involved mild symptoms and smooth recovery, said Sunanda Gaur, a professor of pediatric infectious disease at Rutgers Robert Wood Johnson Medical School.

And if those reactions are indeed the result of the vaccines, the risk appears far lower than that of the illness that the injections prevent, said Gaur, who is helping to run the Rutgers trial site. COVID-19 itself can cause myocarditis, along with the well known array of more serious symptoms, including death.

Minh-Tu Do, the father of the two young vaccine volunteers, said he came to a similar conclusion when weighing the risks and benefits of signing up his sons for the injections.

He acknowledged that some parents have been hesitant about vaccinating their children, as evidenced by the low rate in those 12 to 15, for whom the Pfizer vaccine was authorized on May 10. As of Aug. 3, just 29.1% of children in that age group had been fully vaccinated, according to the CDC.

But with the eventual findings from his sons and thousands of others in their age group, he is optimistic that acceptance will continue to build.

“We’re getting there,” he said. “As parents are learning more about it, they’re becoming more open to it.”

If all goes well, perhaps they will think of it like the annual flu shot. That’s the comparison that Justin Do made, before running off to play with his scooter:

“It’s like what we do every year.”