Children ages 12 to 15 in the United States are now eligible to receive a Pfizer COVID-19 vaccine booster shot, according to the Centers for Disease Control and Prevention (CDC) .
Dr. Rochelle Walensky, director of the CDC, approved the expanded eligibility Wednesday, a few hours after a CDC panel of experts voted to recommend the booster shot after reviewing data. The Food and Drug Administration (FDA) authorized the Pfizer boosters on Monday.
The new eligibility for 12- to 15-year-olds come as millions of children are returning to school across the U.S. following the holiday break and as pediatric cases of COVID-19 are increasing amid the omicron surge.
Because the CDC no longer requires vaccinated and boosted people to quarantine after exposure, the availability of booster shots to 12- to 15-year-olds could help keep kids in school during the winter surge.
The CDC panel on Wednesday though emphasized that more children still need to get their initial vaccines. Nearly one-third of 12- to 17-year-olds have yet to receive their first shots of the Pfizer vaccine.
Here are nine questions answered about the COVID-19 vaccines and kids as families seek to make the best decisions.
1. What is the science behind the COVID-19 vaccine?
Both the Pfizer and Moderna vaccines use mRNA technology, which does not enter the nucleus of the cells and doesn't alter human DNA. Instead, it sends a genetic "instruction manual" that prompts cells to create proteins that look like the outside of the virus -- a way for the body to learn and develop defenses against future infection.
The Johnson & Johnson vaccine uses an inactivated adenovirus vector, Ad26, that cannot replicate. The Ad26 vector carries a piece of DNA with instructions to make the SARS-CoV-2 spike protein that triggers an immune response.
This same type of vaccine has been authorized for Ebola and has been studied extensively for other illnesses and for how it affects women who are pregnant or breastfeeding.
Neither of these vaccine platforms can cause COVID-19.
2. What is the status of vaccine eligibility for kids?
Children ages 5 and older are now eligible to receive Pfizer's COVID-19 vaccine.
Timing on a vaccine for children younger than 5 is less certain.
Pfizer announced in December that it plans to amend its ongoing clinical trials for children under age 5 to add a third dose, which means a vaccine for younger children will not be available until well into 2022.
The two other vaccines currently available in the U.S., Moderna and Johnson & Johnson, are currently available only for people 18 years and older.
Moderna filed for emergency use authorization with the FDA for its vaccine in adolescents in June but is still awaiting a decision.
Johnson & Johnson announced in April that it had begun vaccinating a "small number of adolescents aged 16-17 years" in a Phase 2a clinical trial.
As of April, the trial was enrolling participants only in Spain and the United Kingdom, with plans to expand enrollment to the U.S., the Netherlands and Canada, followed by Brazil and Argentina.
3. Why do kids need to be vaccinated against COVID-19?
While there have not been as many deaths from COVID-19 among children as adults, particularly adults in high-risk categories, kids can still get the virus and they can also transmit the virus to adults.
As of late December, more than 2,100 children were currently hospitalized with confirmed or suspected cases of COVID-19, according to federal data.
According to the CDC, unvaccinated 12- to 17-year-olds had an 11 times higher risk of hospitalization than fully vaccinated adolescents.
"We know that COVID does not spare kids," ABC News medical contributor Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital, said in December. "Maybe it’s less severe than their adult counterparts but we also know that the virus has had real significant impacts on morbidity and mortality in kids."
"We also know that kids play an important role as vectors of spread," he said. "And especially in light of increases we’re seeing right now, with increases of cases in kids in record numbers, infections among kids further perpetuate community transmission and further create risks for those who would be the most vulnerable of the virus."
4. Will kids experience the same vaccine side effects as adults?
Pfizer has conducted clinical trials with its COVID-19 vaccine on kids ages 5 to 11 since last year, and the company's most recent data shows that it was nearly 91% effective against symptomatic illness.
The vaccine also appeared safe. None of the children in the clinical trials experienced a rare heart inflammation side effect known as myocarditis, which has been associated with the mRNA vaccines in very rare cases, mostly among young men.
Adolescents experienced a similar range of side effects to the vaccine as seen in older teens and young adults -- generally seen as cold-like symptoms in the two to three days after the second dose -- and had an "excellent safety profile," Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said in August.
Moderna has said its COVID-19 study with teens ages 12 to under 18 identified no "significant safety concerns." The most common side effects from the vaccine were injection site pain, headache, fatigue, muscle pain and chills, according to the company.
The FDA will scrutinize Moderna’s clinical data before authorizing the use in anyone under 18.
5. Is there data showing COVID-19 vaccines are safe for kids?
The CDC released three studies in December showing COVID-19 vaccines are safe and effective for children.
One study, which evaluated the safety reports of more than 42,000 children ages 5 to 11 who received a Pfizer shot, found the side effects from the Pfizer vaccine were mostly mild and temporary. It also found that myocarditis, a heart inflammation side effect that has been associated with the mRNA vaccines in very rare cases, does not appear to be a risk.
A second study, which looked at data from 243 children ages 12 to 17 in Arizona, found the Pfizer vaccine was 92% effective at preventing infection. The study, conducted between July and December when delta was the dominant variant in the U.S., also found that adolescents who developed COVID-19 reported a lower percentage of time masked in school and time masked in the community.
The third study, also conducted when delta was dominant, found that among children ages 5 to 17 hospitalized due to COVID-19, less than 1% were fully vaccinated against the virus.
6. How effective are the vaccines in children?
Pfizer announced in late March that its clinical trials showed the vaccine was safe and 100% effective in children ages 12-15, similar to the 95% efficacy among adult clinical trial participants.
Marks confirmed on May 10 that after a trial with more than 2,000 children, Pfizer found no cases of infection among the children who had been given the vaccine and 16 cases of infection among the children who received a placebo.
No cases of COVID occurred in the 1,005 adolescents that received the vaccine, while there were 16 cases of COVID among the 978 kids who received the placebo, "thus indicating the vaccine was 100% effective in preventing COVID-19 In this trial," said Marks.
Moderna's COVID-19 vaccine is 100% effective in children ages 12 to under 18, the company said last month, in announcing results of their latest clinical trials.
In addition to its efficacy, the vaccine showed "no significant safety concerns" in the trial of more than 3,700 adolescent participants, according to Moderna.
7. Will kids get the same dose of the vaccines as adults?
Kids ages 5 to 11 are given a 10-micrograms dose of the Pfizer vaccine, one-third of the adolescent and adult dose. Like with adults and adolescents, the pediatric vaccine is delivered in two doses, three weeks apart.
For 12-to-15-year-olds, the FDA has authorized the same dosing as adults with the Pfizer two-dose vaccine.
The FDA and CDC have recommended the Pfizer booster shots now available for kids ages 12 and older be administered five months after the primary vaccine series.
8. Could COVID-19 vaccines impact puberty and menstruation?
There is currently no clinical evidence to suggest the vaccines can have long-term effects on puberty or fertility, according to Ashton, a practicing, board-certified OBGYN.
"Women can have changes in their menstrual cycle and also have gotten the vaccine. That does not mean that one caused the other," said Ashton. "Right now there is no puberty concern. There is no fertility concern."
A possible explanation for menstrual changes may have to do with how the body responds to physical and emotional stresses. Prior studies indicate that COVID-19 itself can be a stressor, leading to irregular menstrual cycles for some people. However, research hasn’t shown that COVID-19 vaccines cause issues with menstruation.
9. Where can kids get vaccinated against COVID-19?
Vaccines are accessible at pediatricians' offices, children's hospitals, pharmacies like CVS, Walgreens and Rite-Aid and school and community-based clinics.
Parents can search for appointments at Vaccines.gov to find a local provider.
ABC News' Sasha Pezenik, Anne Flaherty, Eric Strauss, Cheyenne Haslett and Jade A. Cobern, MD, a member of the ABC News Medical Unit, contributed to this report.