Saturday, 27 November, 2021

Why Covid-19 vaccines for younger children would come in smaller doses

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.