Here's How Moderna and Pfizer Are Testing COVID-19 Vaccines in Babies and Young Children

Months following a call from the American Academy of Pediatrics to expand vaccine trials to include all children, both Pfizer-BioNTech and Moderna are under way with vaccine trials on the youngest age group yet.

In mid-March, Moderna officially began its study to test its COVID-19 vaccine in children under 12 years old, which includes babies as young as six months. Less than a week later, Pfizer launched its trial for the same age range.

Both drug companies are following a similar "dosing" trial structure with the goal of allowing researchers to determine how much vaccine children can tolerate and if it will still adequately protect them at that dosage.

For Moderna's clinical trial, which was expected to enroll 6,750 healthy children in the United States and Canada, each infant and child receives two shots, 28 days apart. Unlike in adult studies, this trial will be completed in two phases. In the first part, children between 2 years and 12 years will receive two doses of 50 or 100 micrograms each. Infants and children under two years old will receive two shots of 25, 50, or 100 micrograms.

The first children inoculated in this group receive the lowest doses and are be monitored for reactions. Based on findings, later participants are given higher doses. At this point, researchers perform an analysis to determine which dose is safest and most likely to be protective for each age group.

Bloodwork – taken on the day of injection, a month past injection, and five months post-injection – will be taken to determine how children respond in order to confirm the appropriate dosages.

"We're giving different doses for the kids," Dr. Steve Plimpton, the principal investigator for the Moderna trial in kids in Phoenix, said in an interview with NPR. "They'll be graduated doses because we need to also determine the dose that's most effective for the kids. So it starts out with a smaller dose. It's still the same two-injection as the adults get but different doses."

The key difference in Pfizer's phase one trial – which involves 5,000 children nationwide – is that kids receive their second dose 21 days after the first, as is the case with the adult vaccines.

Then begins the second phase of the trials, which will allow children to receive the doses selected by the analysis, or placebo shots of salt water.

According to Plimpton, the Moderna children's trial was originally projected to last 14 months, but he believes that, thanks to an overwhelming response from parents of potential participants, the time span might shorten.

"They seem very ready," he said. "They're calling literally all day long asking for when they can get their kids vaccinated . . . They're looking for protection for their kids, but indirectly, they're also going to be protecting themselves and those around the kids that might be infected if the kids actually get infected."

Pfizer anticipates results to come by the second half of 2021, or most likely this winter.

Although this trial is good news for those eager to have a safe and effective COVID-19 vaccine authorized for use in children, Dr. David Wohl, the medical director of the vaccine clinic at the University of North Carolina, questions its fast timing.

Wohl – who is not involved in the Moderna or Pfizer studies – said the trial looked well-designed and likely to be effective but wondered why the children were to be followed for only one year while adults in Moderna's study are followed for two years. He also told the New York Times he was surprised to see the vaccine being tested in children so young already.

"Should we learn first what happens in the older kids before we go to the really young kids?" he said.

However, Pfizer reported at the end of March that their vaccine is 100-percent effective in adolescents aged 12 to 15, following its most recent trial. Moderna also has an ongoing clinical trial for 12- to 17-year-olds, which began in mid-December, and the company recently revealed that age range could be approved for their vaccine by the fall, in time for the upcoming school year. Johnson & Johnson has said it would wait to test its coronavirus vaccine in babies and young children only after testing it first in older children.

Neither drug company has committed to a timeline for the availability of the vaccine for the youngest children's age group, but many experts – including Dr. Anthony Fauci, the director of the U.S. National Institute of Allergy and Infectious Diseases – predict it won't be until early 2022.