WASHINGTON — A booster shot of the coronavirus vaccine made by Pfizer-BioNTech significantly increased the level of neutralizing antibodies against both the original version of the virus and the Omicron variant in a small trial of children ages 5 to 11, the companies announced on 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, which had been announced in December, included 140 children who had received a booster six months after their second shot. The companies described the findings in a new release.

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 study did not show how long the antibodies last or test effectiveness against Covid-19. The data was not published or peer-reviewed, and 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. Their level is expected to rise after an additional dose of vaccine; how rapidly that protection wanes has been an enduring concern for vaccine experts, regulators and manufacturers.

Pfizer and BioNTech 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.

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. Studies suggest that 5- to 11-year-olds may particularly need a booster.

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 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 the 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 F.D.A., questioned whether the sixfold increase in antibody levels in the overall group was the result of the additional shot or of coronavirus infections among the participants in the six-month period between the second and third doses.

“There are a lot of very important details that are missing,” he said of the companies’ announcement.

Dr. Ofer Levy, a vaccine expert at Boston Children’s Hospital and a member of the F.D.A.’s vaccine advisory board, said that the results were helpful, but that the trial was modest in size and that 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.”

The companies’ announcement comes as new U.S. virus cases are again ticking up after two months of sustained declines. The upswing has been particularly noticeable in the Northeast, where the Omicron subvariant known as BA.2, now the dominant version of the virus in the United States, first took hold.

Dr. Anthony S. Fauci, President Biden’s chief medical adviser, warned in recent days that the nation could see a significant increase in infections over the next several weeks. But he has said the rate of hospitalizations is unlikely to rise in tandem because so many Americans have a degree of immunity, either from vaccines or prior infections.

Several hundred children ages 5 to 11 have died of Covid since the pandemic began, according to the Centers for Disease Control and Prevention, but pediatric shots have been a hard sell for many parents. Only about 28 percent of children in that age group have received two doses and would be eligible for a booster. Roughly 7 percent have received one dose, according to agency data.

There was an initial rush for shots after they were first offered for children ages 5 to 11 in November, but the increase in the vaccination rate then slowed to a crawl. In the past month, it rose by a single percentage point.

Dr. Edwards said some parents felt that the chances were low that their children would get seriously ill, while the shots were an unknown. Some research indicates that 45 percent of children who get infected have no symptoms, she said.

“The problem is that we can’t predict who is going to get sick and who is not,” Dr. Edwards said. And among those who do get sick, she said, “there will be kids that are going to be hospitalized, and there will be a few deaths.”

Dr. Sally Goza, a pediatrician in Fayetteville, Ga., and former president of the American Academy of Pediatrics, said some parents saw no reason to act because they viewed the pandemic as having been quelled. “I’ve had parents come into my office and say, ‘Covid’s over. I don’t need to worry about that,’” she said.

To some extent, she said, parents have also been numbed by surge after surge of infections. “People are tired of dealing with it,” she said. “They are just like, ‘We are just going to take our chances.’”

The share of children ages 5 to 11 with at least one dose varies starkly by region, according to a Kaiser Family Foundation study. Five of the top 10 states with the highest rates were in New England, while eight out of the 10 states with the lowest rates were in the South.

Even though more than 250 million Americans have been safely vaccinated since the pandemic began, pediatric experts say many parents see the vaccines as brand-new. By comparison, shots that protect against diseases like measles and mumps have been around for decades.

The study done by New York researchers, posted online in late February, found that for children ages 5 to 11, the Pfizer vaccine’s effectiveness against infection fell to 12 percent from 68 percent within 28 to 34 days after the second dose.That was a steeper decline than for older adolescents and teens who received a much stronger dose. Some experts suggested that the difference in dosage explained the gap in protection, while others blamed the Omicron variant that was prevalent during the study.

Another C.D.C. study stated that two Pfizer doses reduced the risk of Omicron infection by 31 percent among those ages 5 to 11, compared with a 59 percent reduction in risk among those age 12 to 15. Pfizer’s vaccine is the only one that has been authorized for those younger than 18.