F.D.A. advisers will debate the long-awaited Covid vaccines for young children.

Outside advisers to the Food and Drug Administration on Wednesday will consider applications from Moderna and Pfizer to vaccinate the youngest American children against the coronavirus, a much-anticipated review that could clear the way for regulators to grant emergency authorization this week for one or both vaccines.

The F.D.A. has said that clinical trial data from the companies shows that each vaccine met the criteria for safety and effectiveness in the age group. The agency has sent 230 pages of documents to the outside advisers to review. Votes on whether to recommend authorization of Pfizer’s vaccine for children 6 months through 4 years old, and Moderna’s for children 6 months through 5 years old, are expected by the end of Wednesday.

More than two years into the coronavirus pandemic, no vaccine has been authorized for children under 5 in the United States, an unmet need that has dismayed many parents. If the advisory panel recommends one or both vaccines, the F.D.A. is poised to clear them as early as Friday.

A similar advisory committee for the Centers for Disease Control and Prevention is scheduled to meet on Friday and Saturday to issue its own recommendation, which could lead to children starting to get vaccinated next week.

Some difficult questions remain. Perhaps most important is that neither vaccine has been tested against the subvariants that are now pervading the United States. The clinical trials were largely conducted when the Omicron variant prevailed, before the emergence of subvariants. Two of them, BA.4 and BA.5, could become dominant within a month.

“We’re really trying to predict the future,” said Dr. Paul Offit, a vaccine expert with Children’s Hospital of Philadelphia and an F.D.A. advisory panel member. “This is a problem.”

Advisory committee members may also zero in on how the two vaccines differ, and whether one is better than the other. Pfizer is seeking to offer the youngest children its vaccine in three doses at one-tenth the strength of adult shots. Moderna’s would be two doses at one-fourth the strength of adult shots.

Both appear significantly less effective against symptomatic infection than the adult vaccines when they were first rolled out in December 2020. The F.D.A. attributes that to the fact that Omicron is far more adept at evading the vaccines’ defenses against infection than the original version of the virus was.

Given the waning of protection observed among adult recipients and the virus’s rapid evolution, regulators have said that pediatric recipients of Pfizer’s and Moderna’s vaccines will both most likely require a booster dose. That would mean Pfizer’s vaccine, developed with the German firm BioNTech, could wind up as four doses, while Moderna’s could be three.

Both vaccines provoked levels of neutralizing or virus-blocking antibodies among children comparable to those in teens and young adults 16 to 25. Pfizer’s vaccine has been authorized for children 6 to 17 since last year, and regulators said it helped prevent hospitalization and deaths.

Although studies showed that the adult versions of both vaccines were more than 90 percent effective in preventing Covid when they were first rolled out in December 2020, the variants have weakened their potency.

After two doses, Pfizer’s vaccine was about only 28 percent effective in preventing symptomatic infection in children aged 6 months through 4 years old. Pfizer suggested the vaccine was 80 percent effective after a third dose, but the finding was based on only 10 cases out of a subset of the 1,678 trial participants. The trial protocol specified that vaccine efficacy assessments be based on at least twice that many cases.

Moderna found its vaccine to be 51 percent effective in preventing symptomatic infection in children 6 months to 2 years old, and 37 percent effective in children 2 through 5. Those results appear consistent with those reported in studies of adults during the Omicron era, the F.D.A. said. Still, immunization continues to offer strong protection against severe disease that leads to hospitalization and death.

Although Moderna’s efficacy data appeared somewhat stronger than Pfizer’s, its vaccine also seems to provoke more fevers that could concern parents. But scientists strongly cautioned against such comparisons.

“These are different populations. The studies were done at different times. The results are reported in different manners, with different endpoints,” said Dr. Jesse L. Goodman, a former chief scientist for the F.D.A. “Scientifically, we don’t have a direct comparison.”