(CNN) -- Several months after older children became eligible to get vaccinated against COVID-19, the United States might be just days away from offering vaccines to those younger than 5.
The US Food and Drug Administration's independent Vaccines and Related Biological Products Advisory Committee is set to meet Tuesday and Wednesday to discuss amending the emergency use authorization (EUA) of Moderna's and Pfizer/BioNTech's COVID-19 vaccines to include younger ages.
Children under 5 -- about 18 million people -- are the only US age group that isn't eligible to get a COVID-19 vaccine.
The Moderna vaccine is authorized for use in adults, but on Tuesday, the advisers will discuss including children 6 through 17; on Wednesday, they will consider expanding the authorization to infants and children ages 6 months through 5.
The Pfizer/BioNTech vaccine is approved for people 16 and older, and it's authorized for use in children as young as 5. At Wednesday's meeting, advisers will discuss amending the EUA to include child-size doses of the vaccine for infants and children ages 6 months through 4.
According to FDA briefing documents posted Sunday, an agency review found that data supports the effectiveness of the Pfizer/BioNTech COVID-19 vaccine, given as a three-dose series, in preventing the disease in children 6 months through 4 years old. According to another document, a FDA review found that Moderna's COVID-19 vaccine is also safe and effective in younger kids.
In their meetings Tuesday and Wednesday, VRBPAC members will vote on whether the benefits of the Moderna and Pfizer/BioNTech vaccines outweigh risks for younger children.
What's next after vaccine advisers meet
After the FDA vaccine advisers vote, the agency will consider their decision and weigh whether it should authorize the vaccines.
However, shots can't be given until the US Centers for Disease Control and Prevention's vaccine advisers have voted on whether to recommend them and CDC Director Dr. Rochelle Walensky has signed off on the recommendation.
The White House has said vaccines for the youngest children could start next week.
The rollout of vaccines for this group could mirror that for older children, as pediatricians' offices will play a significant role in helping get shots into arms.
Some in-pharmacy clinics also are preparing to administer more vaccines.
"We plan to provide COVID-19 vaccinations to children 18 months through four years of age at our national network of 1,100 MinuteClinic locations once the FDA and CDC have provided guidance and inventory is received," CVS spokesperson Matt Blanchette said in an emailed statement Monday.
"Our MinuteClinic clinicians comprised of board-certified Family Nurse Practitioners, Physician Associates, and Nurses, have significant experience providing vaccinations to a younger population as well as private exam rooms, which will make the process easier for kids, parents, and guardians," the statement says.
Children under 5 typically have lower COVID-19 case rates than other age groups. But in the first week of June, hospitalization rates among these kids were slightly higher than among adults under the age of 50 and four times higher than those of other children.
At least 481 US children under the age of 5 have died of COVID-19, according to the CDC.
People 5 and older are already eligible for Pfizer/BioNTech's two-dose COVID-19 vaccine and a booster dose -- but in Wednesday's meeting, VRBPAC members will discuss whether to add younger children to the emergency use authorization.
According to a briefing document, Pfizer's three-dose COVID-19 vaccine for children younger than 5 appears to be safe and generated an immune response in trials that's comparable to the response in older people.
One trial included more than 4,500 children ages 6 to 23 months and 2 to 4 years. Some got three 3-microgram shots, with the second shot coming three weeks after the first and the third shot given at least eight weeks after the second. The rest got a placebo.
The FDA briefing document says the Pfizer vaccine for this age group induced responses comparable to what has been seen in 16- to 25-year-olds. There weren't enough COVID-19 cases to establish vaccine efficacy in the study, but the FDA noted that this vaccine is known to be less protective against the Omicron variant.
A preliminary analysis put vaccine efficacy for children younger than 5 at 80.4%, with three COVID-19 cases in the vaccine group and seven in the placebo group. All the cases happened while the Omicron variant was dominant in the United States.
"Given the uncertainty of the COVID-19 pandemic and likelihood of continued SARS-CoV-2 transmission during the ensuing months, deployment of the vaccine for use among children 6 months through 4 years of age will likely have a beneficial effect on COVID-19 associated morbidity and mortality in this age group," the FDA said in its analysis.
There were no cases of anaphylaxis. There were also no cases of myocarditis or pericarditis -- inflammation of the heart muscle and inflammation of tissue surrounding the heart -- reported among more than 3,000 vaccine recipients in the trial, but it wasn't a large enough group to rule out the risk.
The most common adverse reactions among children 6 to 23 months were irritability, drowsiness, lack of appetite and tenderness at the injection site. For children 2 to 4 years old, the most common adverse reactions were pain or redness at the injection site and fatigue.
It's not clear how long the vaccine will be effective; the agency noted that the evaluation period was limited and that protection is known to wane over time in older ages. The agency also said it's "likely that a booster dose will be needed in addition to the three-dose primary series to increase robustness, breadth, and duration of protection."
The immune response and safety profile of Moderna's COVID-19 vaccine also appear to be favorable for the youngest children, according to the FDA briefing documents.
VRBPAC members will discuss Moderna's vaccine on Tuesday and Wednesday, evaluating whether it should be authorized for younger ages.
In trials evaluating more than 6,000 children 6 months to under 6 years old, Moderna scientists found that two 25-microgram doses of vaccine, given 28 days apart, yielded a similar immune response as the two-dose vaccine series does in adults 18 to 25.
For children and teens ages 6 to 17, Moderna found that two doses of its vaccine provided a similar immune response as two doses in adults.
The vaccine was also found to be safe in all age groups, with adverse reactions described as "mostly mild to moderate in severity, generally of short duration," and happening more frequently after the second dose than the first, according to the document. Injection site pain was the most commonly reported adverse reaction. As for serious adverse events, the document described them as infrequent and didn't raise any concerns. No deaths were reported.
Although there were no known cases of myocarditis or pedicarditis associated with the vaccine, it is acknowledged as one of the known risks and has been reported mostly in males ages 18 to 24.
The FDA did not require the vaccine makers to submit vaccine efficacy data for emergency use authorization for young children, but Moderna's vaccine has been estimated to be 93.3% effective against symptomatic COVID-19 among teens ages 12 to 17 when the original coronavirus and the Alpha variant were dominant.
The vaccine was estimated to be 76.8% effective against symptomatic COVID-19 for children ages 6 to 11 when the Delta variant was predominant. However, the FDA also noted that for children in that age group, the "vaccine efficacy could not be reliably determined due to the small number of COVID-19 cases accrued during the study."
The vaccine was tested during a time when the Omicron variant was dominant. It was estimated to be 36.8% effective against symptomatic COVID-19 for children ages 2 to 5 and 50.6% protective for children 6 months to 23 months.
Vaccine efficacy estimates "were generally consistent" with what has been seen in adults, according to the FDA briefing document.
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