Nearly 23 million eligible children have received at least one dose of a coronavirus vaccine since the most recent group of 5- to 11-year-olds got the green light late last year and started rolling up their sleeves.

But many families are anxiously awaiting a pediatric vaccine for the youngest Americans — ages 6 months through 4 years — as the omicron variant surges across the United States.

“We do need a vaccine for these kids,” said Sean O’Leary, vice chair of the Committee on Infectious Diseases for the American Academy of Pediatrics. However, a vaccine for children younger than 5 may not be available in time “to change this current wave” of infections caused by the omicron variant, which many experts say may be nearing its peak.

Pediatricians say coronavirus vaccines probably will be important well beyond this wave.

More on the COVID-19 pandemic

Here are some things we know about the vaccine for the youngest children.

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Q: When will the vaccine be available for children younger than 5?

There is no set date for when the coronavirus vaccine will be available for children younger than 5, but it is expected in the first half of 2022.

Pfizer and its German partner BioNTech recently announced that they are amending the clinical trial for children 6 months to 5 years to include a third dose of the vaccine. The companies said two doses of the vaccine — which is recommended for children 5 and older — triggered an adequate immune response in children 6 months to 2 years old. But in 2-, 3- and 4-year-olds it failed to trigger a response comparable to what was generated in teenagers and adults. The companies are also testing a third dose for other age groups.

No safety concerns have been identified, the companies said.

The decision to study a third dose in this age group “reflects the companies’ commitment to carefully select the right dose to maximize the risk-benefit profile,” Pfizer and BioNTech said in a statement.

If three doses trigger a protective immune response, the companies plan to submit the data to the Food and Drug Administration in the next several months. If it is authorized for emergency use, it will go to the Centers for Disease Control and Prevention.

Kawsar Talaat, one of the principal investigators of the Pfizer pediatric trial and a physician at Johns Hopkins Bloomberg School of Public Health, said that although the news of the amended trial may be disappointing, a third dose is expected to work well to provide protection, particularly against omicron.

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“I think that a third dose will give a nice boost, and honestly this is really exciting — as we know from the adult data, three doses is probably better for omicron. And I think it’ll be good to have similar data for children,” Talaat told The Washington Post in December.

Q: Will it be the same vaccine given to older children?

No. Children younger than 5 will receive less than one-third the dose given to 5- to 11-year-olds or one-tenth the dose given to those 12 and older.

But, at least in the clinical trial, the initial two doses are still given 21 days apart — the same amount of time for older children, teens and adults. In the trial, the third dose will be administered at least two months after the second dose in the two-dose series, Pfizer and BioNTech said in the statement.

Q: What about Moderna and Johnson & Johnson?

Moderna asked the FDA over the summer to authorize its vaccine for adolescents. In October, the regulatory agency indicated that it will take until at least January to complete a review of the company’s application for use of its vaccine in 12- to 17-year-olds, to evaluate whether the shot increases the risk of myocarditis, or inflammation of the heart muscle.

Moderna’s vaccine is being tested in children younger than 12, and the trials are ongoing.

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The company did not immediately respond to a request for comment on the pediatric trials.

Asked about a pediatric vaccine, Johnson & Johnson said in an email that trials are underway for adolescents ages 12 to 17, but that the vaccine “is not currently authorized for use in children.”

Q: Do children under 5 need to be vaccinated against the coronavirus?

Some parents have questioned whether younger children really need the vaccine, particularly given that children tend to have less severe illness than adults. But Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, said children can still “suffer and be hospitalized and die from this virus.”

Since the start of the pandemic, nearly 8.5 million children in the United States have tested positive for the coronavirus, resulting in more than 30,000 hospitalizations and more than 700 deaths, according to the most recent data from the American Academy of Pediatrics.

More than 6,000 children have suffered a rare but serious condition called multisystem inflammatory syndrome (MIS-C), which is associated with COVID-19 and can cause inflammation of the heart, lungs, kidneys, brain, eyes and other organs, the CDC said.

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The number of pediatric cases of COVID-19 is significantly lower than the more than 60 million total cases and over 800,000 deaths across the United States, but health experts said the virus is affecting children.

Offit said medical professionals are seeing a lot of children come into the hospital with COVID-19, some requiring intensive care, and “they’re invariably not vaccinated.”

“So it’s all very frustrating,” he said. “One thing is having a vaccine available, but another thing is actually giving it.”

Q: Should I be concerned about myocarditis?

There have been very rare instances of heart muscle inflammation in vaccinated adolescents and young adults. Although it has been reported after vaccination, the coronavirus-caused COVID-19 itself is more likely to cause myocarditis than the vaccine, especially in children who develop MIS-C because of the disease.

More than 245 million Americans have received at least one vaccine dose, including about 26% of people ages 5 to 11, 64% of people ages 12 to 17 and 73% of people 18 to 24, according to the CDC.

As of Dec. 16, about 1,900 recently vaccinated people under age 30 reported myocarditis or pericarditis (inflammation of the outer lining of the heart) to the Vaccine Adverse Event Reporting System. Medical investigators are assessing what the relationship to the shots might be, the CDC said.

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In a recent presentation from the CDC’s Advisory Committee on Immunization Practices, data showed that cases were significantly lower in the 12-to-15 age group, and even more so in those ages 5 to 11. As of Dec. 19, more than 8.5 million doses of the coronavirus vaccine had been administered to children ages 5 to 11, and there had been 12 reports of myocarditis that met the case definition, according to a slide show.

“There’s every reason to be confident that it wouldn’t be a problem in the less-than-5-year-olds who are getting an even smaller dose than the 5- to 11-year-olds,” Offit said.

And O’Leary said in the low number of cases reported among children, “they’ve almost all been mild, with recovery within a matter of days.”

In general, Offit, of the Children’s Hospital of Philadelphia, said parents should vaccinate their children against the coronavirus for the same reason they vaccinate them against other diseases — to protect them not only as children but also as they grow older, and to protect those around them.

“Knowing that children can suffer from this, knowing that children grow up and that they’re going to be susceptible to this virus,” he said, “we will need a high percentage of population immunity.”

Q: What should I do while waiting for the vaccine?

In the meantime, make sure your child is up to date on all routine vaccinations, experts said.

O’Leary, of the American Academy of Pediatrics, said although COVID-19 is understandably making parents nervous, “a lot of the diseases that we vaccinate kids for, like measles, are more severe in kids than COVID — and we have seen a global dip in those vaccines and we are very concerned about potential outbreaks of those diseases.”

O’Leary said that while parents and guardians are waiting for a vaccine for children younger than 5, they should make sure their kids have been vaccinated against these other diseases, including influenza.

“Those are things we can prevent,” he said.