Here’s how the doses of the two vaccines for young children differ.

Federal health officials are poised to take the final steps on Saturday to clear the Moderna and Pfizer-BioNTech coronavirus vaccines for very young children —  the only age group in the United States that does not yet have a vaccine option available.

If the two vaccines are cleared, as expected, doses could be rolled out as soon as Tuesday. Vaccinating the youngest children would provide relief for many families who have had to deal with disruptions to day care and preschool during the pandemic, as well as worries that their children might be among the few who become seriously ill with Covid-19.

Vaccines from Pfizer and its partner BioNTech have already been available for Americans 6 and older, and Moderna’s vaccines for adults.

On Friday, Dr. Peter Marks, who heads the Food and Drug Administration’s vaccine division, said that parents should feel comfortable vaccinating their very young children with either the Pfizer-BioNTech or the Moderna vaccine, whichever one was available.

Here are some key facts about the pediatric vaccines.

Pfizer-BioNTech

• Number of shots: Three.

• Spacing of shots: Three weeks between the first and second shots, and at least eight weeks between the second and third.

• Dosage: Three micrograms in each shot. For comparison, children 5 to 11 years old are given two 20-microgram shots; adolescents and adults get two 30-microgram shots.

• Efficacy: Pfizer’s scientists say the vaccine has an overall efficacy of 80 percent in children under 5, but that calculation was based on an extremely small sample of 10 children.

Moderna

• Number of shots: Two.

• Spacing of shots: Four weeks apart.

• Dosage: Twenty-five micrograms in each shot — half the size of the shots for adults.

• Efficacy: Moderna estimated the vaccine’s efficacy against symptomatic infection at about 51 percent among children ages 6 to 24 months, and 37 percent among children ages 2 through 5.