(CNN) -- As more young children are getting their second doses of COVID-19 vaccine, another question is popping up: When will kids and younger teens need a booster shot?
Dr. Sean O'Leary, a professor of pediatric infectious disease at the University of Colorado School of Medicine who works with Children's Hospital Colorado, said his fellow pediatricians are starting to hear from more parents, "particularly for the older kids who were vaccinated several months ago."
The US Centers for Disease Control and Prevention now recommends all adults get a COVID-19 vaccine booster. Adults who have had the Pfizer or Moderna COVID-19 vaccines are eligible to get a booster six months after their second shot. Those who got the Johnson & Johnson one-shot vaccine are eligible for a booster of any authorized vaccine two months after their first dose.
On Thursday, the US Food and Drug Administration authorized the Pfizer vaccine for use as a booster in people ages 16 and 17. Just like with adults they are eligible for a booster six months after they've had their second dose. It's the first COVID-19 booster authorized for this group.
But there's no ask yet for younger kids and no official timeline for when they might need to roll up their sleeves.
Most experts say it may be a while -- if younger children get boosters at all.
Will immunity wane in children and teens?
In the fully vaccinated adult population, data from several studies showed there was waning immunity over time, and booster helped restore it. The science is still not yet clear if younger children will needs another shot. Researchers are still gathering data, and some answers should be available next year, scientists have said.
First, they will determine if kids are experiencing waning immunity and starting to see an increase in breakthrough infections.
"Probably most importantly, are we seeing any severe outcomes in children who've been previously vaccinated with two doses?" said O'Leary, who is also vice chair of the committee on infectious diseases of the American Academy of Pediatrics.
While adults have been eligible for COVID-19 vaccines since last year, younger kids got a later start. The emergency use authorization of the Pfizer vaccine wasn't expanded to include children 12 through 15 years of age until May 10. For children 5 to 11, it was October 29.
That means the research on the kids is running behind the adults.
"Most people really aren't talking about this yet because we don't know how long kids respond to the vaccine," said Dr. Claudia Hoyen, director pediatric infection control at UH Rainbow Babies and Children's Hospital in Cleveland.
Scientist can't substitute the adult research for the kids, because a child's immune system is different. Typically, children's immune systems are stronger, and they may be protected longer than an elderly person whose immune system is not as robust, for example.
"Kids' immunity may last longer and that would mean they may be on a different schedule than the adults, that's if they need one at all," Hoyen said. She thinks it's possible. "Even kids can get a common cold, which is another coronavirus, so they may need them. We really just don't know yet."
In November, Dr. Anthony Fauci, director of the National Institutes of Allergy and Infectious Diseases, told CNN's John Berman that adolescents might need boosters at some point, but "it is less likely that they will, because your healthy, strapping teenagers have a much better and stronger immune response than I do as an elderly person. I gulp when I say elderly, but that is the truth."
Would a booster dose be safe for kids?
Second, scientists would also have to determine if a third dose would be safe for these younger age groups.
"Part of that actually, it's going to be informed as we're learning more and more as more doses are given," said O'Leary.
They'll want to know, for example, if more cases of myocarditis -- a kind of heart inflammation -- show up as more children are vaccinated. "We know that rarely happens after vaccination and so that will play a role -- is the vaccine with a third dose safe?"
For now, scientists are monitoring the youngest group to be vaccinated so far, those ages 5 to 11. Asked about vaccine safety in younger children on Tuesday, CDC Director Dr. Rochelle Walensky said the agency is combing the data for signs of a safety issue.
"We haven't had anything that's come to us as a signal, and we continue to watch that carefully," she said at a White House COVID-19 briefing.
Should boosters for kids be a priority?
Aside from questions around waning efficacy and safety, vaccine companies would also have to address whether a booster provides enough benefit for this group.
"Is it worth allocating vaccines to this group of patients to keep them safe, or are they safe enough, given that they're younger, given that they're at a low-risk category?" said Dr. Claire Boogaard, the medical director of the COVID-19 vaccine program at Children's National in Washington, D.C. Kids can catch COVID, but compared with adults, they typically handle it better, and are far less likely to be hospitalized or die from COVID-19.
"They don't want to stress a system that is already stressed to vaccinate another 2.3 million people if they don't need to," Boogaard said.
One of the best ways to keep all kids safe from COVID-19 -- even without a booster -- is to make sure that as many people are fully vaccinated as possible, Boogaard said.
"That will not only keep yourself safe," Boogaard said, "the more people that (are fully vaccinated and boosted who are eligible), the less likelihood this virus will have the advantage of replicating and causing something more dramatic."
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