The F.D.A. is expected to soon authorize Pfizer-BioNTech’s booster for 5- to 11-year-olds.

The Food and Drug Administration is expected as early as Tuesday to authorize a booster shot of the Pfizer-BioNTech vaccine for children 5 to 11, according to multiple people familiar with the plan. That would make those children the youngest Americans eligible for the additional shot.

The companies announced a month ago that the extra shot significantly increased the level of neutralizing antibodies against both the original version of the virus and the Omicron variant in a small clinical trial of 140 children. . Other research has suggested that the protection against infection provided by two Pfizer shots wanes within just a few weeks for 5 to 11 year olds, underscoring the need for a booster dose.

A meeting of outside vaccine experts on an advisory committee to the Centers for Disease Control and Prevention has been scheduled for Thursday. Dr. Rochelle P. Walensky, the C.D.C.’s director, is then expected to issue her own recommendation on the shot.

There are more than 28 million children aged 5 to 11, making up nearly 40 percent of Americans younger than age 18. But given that these children

have the lowest coronavirus vaccination rate of all eligible Americans, public health experts are not expecting a rush for the booster. Although they have been eligible for Covid shots since November, only 28.8 percent of children in this age group have received two doses. Another six percent or so have received one shot.

There is no coronavirus vaccine available yet for children younger than 5, but data is rolling in from both Moderna and Pfizer on their clinical trial results and regulators are expected to decide if the proposed regimens pass muster within about a month.

Currently, Americans 12 and older are eligible for an initial booster at least five months after their second shot, and about 30 million people 50 or older, as well as many people with immune deficiencies, are eligible for a second at least four months after that.

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 many parents have hesitated to vaccinate their children, apparently because the risk of severe illness is comparatively low. The lowest rates of vaccination have been in southern states.

In the Pfizer-BioNTech clinical trial, 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. The third dose was given about six months after the second shot.

Laboratory tests of blood samples from a tiny subgroup of 30 children also showed 36 times the level of neutralizing antibodies against the Omicron variant compared with levels after only two doses. The study did not show how long the antibodies last or test effectiveness against Covid-19.

Some experts have suggested that because children 5 to 11 received a much lower initial dose than older children or adults, they are particularly in need of a booster shot. One study done by New York researchers 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. 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.

So far, Pfizer’s vaccine is the only one that has been authorized for people younger than 18.

Moderna applied earlier this month for emergency authorization of its coronavirus vaccine for children 6 to 11.

Moderna is seeking clearance for an initial two-dose regimen, without a booster. According to a published report, after two doses, participants in that clinical trial produced antibody levels that were slightly higher than those seen in young adults, which was considered a strong immune response.

Moderna’s trial was not large enough to assess the vaccine’s ability to forestall severe disease or death. But based on a small number of infections with the Delta variant that was dominant last summer and fall, the researchers estimated that the vaccine had an efficacy of 88 percent against infection.