A booster shot of the coronavirus vaccine made by Pfizer-BioNTech increased the level of neutralizing antibodies against both the original version of the virus and the omicron variant in a small trial of children age 5 to 11, the companies announced Thursday.
If the companies' claims of a strong immune response pass muster with federal regulators, the government could broaden eligibility for booster doses to include 28 million more children.
The study by Pfizer and BioNTech, announced in December, included 140 children who received a booster six months after their second shot. The findings were described in a news release.
Pfizer tested the kid booster while omicron was surging this winter. While covid-19 cases now are at much lower levels in the U.S., in recent weeks an even more contagious version of omicron, called BA.2, has become the dominant type locally and around the world.
The children showed a sixfold increase in antibody levels against the original version of the virus one month after receiving the booster, compared with one month after receiving a second dose. Laboratory tests of blood samples from a subgroup of 30 children also showed 36 times the level of neutralizing antibodies against the omicron variant compared with levels after only two doses, according to the news release and a Pfizer spokeswoman.
The new data "reinforce the potential function of a third dose of the vaccine in maintaining high levels of protection against the virus in this age group," the companies said in a statement. They added that the dose was "carefully selected based on safety, tolerability and immunogenicity data."
The study did not show how long the antibodies last, or test effectiveness against covid-19. The data was not published or peer-reviewed, and at least one expert said it was impossible to assess the study without more information.
Antibodies are the immune system's first line of defense against infection. They are expected to rise after an additional dose; how rapidly that protection wanes has been an enduring concern for vaccine experts, regulators and manufacturers.
The companies said they would ask the Food and Drug Administration for emergency authorization of a booster for 5- to 11-year-olds "in the coming days." The agency has typically acted within a month of receiving such requests.
The Pfizer shots are the only vaccine available to U.S. children. Those ages 5 to 11 receive one-third of the dose given to everyone 12 and older.
The U.S. hasn't yet allowed vaccinations for children under 5. Results from Pfizer's ongoing test of a vaccine in children as young as 6 months old are expected in "coming weeks," according to the company. But certain 5- to 11-year-olds -- those with severely weakened immune systems -- already are supposed to get three doses, to give that high-risk group a better chance of responding.
Many scientists have said they think the coronavirus vaccine is a three-dose regimen, but it is unclear whether families will embrace more shots. Uptake of the first shots has been slow, even though the vaccine became available to school-aged children right before the disruptive omicron surge. According to data from the Centers for Disease Control and Prevention, only 28 percent of children in this age group are fully vaccinated.
Meanwhile, studies of real-world vaccine performance found the two-shot regimen from Pfizer and BioNTech provided poor protection against infections during the omicron surge, even compared with the shot's performance in older adolescents.
The vaccine has been available for 5- to 11-year-olds since November, just before the omicron variant emerged. A CDC study found the vaccine was 46 percent effective at preventing visits to emergency departments or urgent-care clinics in school-aged children. A study from New York state that tracked infections and hospitalizations during December and January found that protection against hospitalization declined from 100 percent to 48 percent -- markedly less protective than for teens.
Studies in adults show that while a third shot increases the level of antibodies, it also hones those antibodies' ability to block an array of variants -- a process called "affinity maturation."
Currently, Americans 12 and older are eligible for at least one booster, and about 30 million people age 50 or older are eligible for a second one.
Researchers in New York state recently found that while two Pfizer shots protected children in that age group from serious illness, they provided virtually no protection against symptomatic infection, even just a month after full immunization.
"I think a bottom line is that in order to protect from the omicron, we know from studies and from adults and adolescents that you need three doses," said Dr. Kathryn M. Edwards, a pediatric vaccine expert at Vanderbilt University School of Medicine. She predicted regulators would authorize the companies' request.
But Dr. Philip Krause, who recently retired as a senior vaccine regulator at the FDA, questioned whether the sixfold increase in antibody levels in the overall group was the result of the additional shot or infection with the virus in the six month interim between the second and third dose.
"There are a lot of very important details that are missing," he said of the companies' statement.
Dr. Ofer Levy, a vaccine expert at Boston Children's Hospital and a member of the FDA's vaccine advisory board, said the results were helpful but the trial was modest in size, and no outcomes of safety or efficacy were cited.
"We presume that a higher level of antibodies is better and that's probably true," he said. "But how does that translate into vaccine effectiveness? We haven't fully sorted that out."
Last month, biotechnology company Moderna reported that its pediatric vaccine triggered immune defenses in children as young as 6 months old equivalent to the response that protected adults against the original version. But the Moderna shot provided only minimal protection against infection during the omicron surge. Moderna is expected to submit that data to regulators soon.
Information for this article was contributed by Sharon LaFraniere of The New York Times, Carolyn Y. Johnson of The Washington Post and Lauran Neergaard of The Associated Press.