The trial included 2,268 children, two-thirds of whom had received two doses of the vaccine three weeks apart. The remaining volunteers were injected with two doses of saltwater placebo. Regulators over the summer asked for the trial size to be expanded to 3,000 children.
At a virtual panel on Covid-19 last week, Norman Baylor, the former director of the Food and Drug Administration’s vaccines office, said that the number of participants in Pfizer pediatric study was noticeably small. The adult trial involved about 44,000 people.
“It does beg the question of the size, given what we have for the adults: Would one expect more for the pediatric population?” he said. “They may be thinking, ‘Well, we know the vaccine is safe, because look at how many people we had in the adults.’ But as we know, things may shift in that pediatric population.”
The Pfizer-BioNTech and Moderna vaccines have been tied to increased risks of myocarditis, or inflammation of the heart muscle; and pericarditis, inflammation of the lining around the heart, particularly in younger men. In August, the Food and Drug Administration published results from an analysis of Pfizer-BioNTech’s vaccine that used a U.S. health care claims database and found that the risk of the conditions in 16- and 17-year-old vaccinated boys could be as high as 1 in 5,000.
The cases in the database were unconfirmed, but they were considered a reasonable estimate of possible risk, the agency wrote.
A lower dose of the vaccine for children could alleviate those concerns.
Officials in a number of countries have recommended a single dose of the Pfizer-BioNTech vaccine for children ages 12 and older, which would provide partial protection from the virus, but without the potential effects occasionally observed after two doses. Sweden and Denmark joined those countries, announcing on Wednesday that adolescents should get only one dose of the Moderna vaccine.
Amy Schoenfeld Walker contributed reporting.