The Pfizer/BioNTech’s Covid-19 vaccine that could soon be authorized for younger children still requires two shots given on the same schedule as the vaccine for adolescents and adults — but the doses will be a third of the size.

For 5- to 11-year-olds, Pfizer has requested US Food and Drug Administration authorization for a 10-microgram dose; the dose used for people 12 years and older is 30 micrograms. Moderna this week released initial results for a two-dose Covid-19 vaccine for children ages 6 to 11 that’s half the size of the company’s vaccine for adults.

So why is there a difference? And what should parents of 11-year-olds do, especially if the child is approaching 12?

Finding the right dosage

One goal of vaccine trials for any age is to find the smallest level of antigen — the part that triggers an immune response — to maximize protection without side effects.

“We think that we have optimized immune response and minimized reactions,” Pfizer Senior Vice President Dr. William Gruber told the FDA’s vaccine advisers’ Tuesday about the company’s Covid-19 vaccine for younger children.

It’s not about the size of the child. Rather, it’s that little kids are still developing, and the immune system weakens with age.

“Kids actually tend to have very robust immune responses,” said Dr. Kari Simonsen who has been leading the trial of the Pfizer vaccine at Children’s Hospital & Medical Center in Omaha, Nebraska. “In some cases, they can actually create strong responses to smaller amounts of vaccine antigen.”

For some vaccines, adult and child doses can be the same, but in other cases, like with the hepatitis A vaccine, adults get a higher dose than children.

“As we are fond of saying in pediatrics: Children are not small adults. Children are children,” said Dr. James Versalovic, Texas Children’s Hospital interim pediatrician-in-chief. “Their bodies are developing and will react differently, and we need to treat them differently.”

That was a consideration as Pfizer tested vaccines in younger children.

“We took a step back after we did the adolescents, and we looked at the dosing, because we thought that we may be able to use a lower dose and be able to get the same immune response,” said Dr. Bob Frenck, director of the Vaccine Research Center at Cincinnati Children’s Hospital.

After testing, “we got just as good an immune response as the 30-microgram dose and there were less side effects.”

According to data from a Phase 2/3 trial Pfizer submitted in September, the two-dose, 10-microgram vaccine generated a “robust” antibody response in younger children. In a document posted last week, Pfizer said its vaccine is safe and 90.7% effective against symptomatic Covid-19 in children ages 5 to 11.

At higher doses tested in the trials, scientist saw a few more minor side effects, nothing severe. With the 10-microgram dose, researchers saw fewer issues with chills and fever than they saw in the 16- to 25-year age group that was tested.

The lower dose should also reduce the theoretical risk of myocarditis, the inflammation of the heart muscle that has been seen in a small number of people after they got the Pfizer and Moderna vaccines. No cases of myocarditis were seen in the younger children in the trial, but not enough children were tested to show whether they are also at risk. Scientists will be watching for cases closely.

“It is reassuring to me that we are giving a lower dose,” said Dr. Paul Offit, a member of the independent FDA vaccine committee who directs the Vaccine Education Center at Children’s Hospital of Philadelphia.

Should an almost-12-year-old wait for a larger dose?

Some parents of 11-year-olds may be wondering if they should wait to get the higher dose shot, but Dr. Bernhard Wiedermann, an infectious disease specialist working on the Pfizer trial at Children’s National in Washington, DC, said parents shouldn’t wait for the child to turn 12.

“If I had a family member in that situation, I would advise them just to get the dose that’s authorized for their current age,” Wiedermann said. “I think we still have reasonable risk that we’re going to see cases start to increase again sometime in the next few months.”

Pfizer’s Gruber said dose levels for 12- to-15-years old could be reduced at some point, but there’s no data yet to support such a change. Earlier research showed this age group had a higher antibody response with the 30-microgram dose.

“We are thinking about that as a potential option, particularly as we move out of the pandemic period,” Gruber said, referring to consideration of a smaller dose. “The key goal right now is obviously providing protection with a safe and effective vaccine.”

Wiedermann said doctors should not take that idea and run with it now for 12- to 15-year-olds.

“I’ve been telling our providers you should always stay within the lines when you’re coloring,” Wiedermann said. “The immune response for this virus is very complex. A practitioner might think, whether it’s giving a higher dose or a lower dose, that they’re helping a particular child, but if it hasn’t been studied don’t do it, because we really don’t know what the effects of that will be. This is not the time to be messing with anything outside of what’s been authorized.”

What happens next?

The FDA will take its vaccine advisers support for the vaccine under consideration and will likely extend the emergency use authorization to children ages 5 to 11 in the next few days.

Next, the CDC’s independent vaccine advisory committee meets November 2, and will vote on whether to recommend it for use.

Finally, CDC Director Dr. Rochelle Walensky will decide whether to accept or modify the CDC committee’s recommendation. Once a recommendation is final, younger children could start getting vaccinated immediately.

Some 28 million children would be eligible for the Covid-19 vaccine, and plans are already underway to help them get it. Getting this group vaccinated could get the country a step closer to ending the pandemic, experts have said.

“If we can create a situation where more of these kids are not getting infected, we should be able to drive this pandemic down, which is what we really hope to do even as we face the cold weather and other concerns about whether we might see another surge,” Dr. Francis Collins, director of the National Institutes of Health, said on ABC’s Good Morning America on Tuesday. “We don’t want that, and this would be one significant step forward in getting our country really in a better place.”

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