The Food and Drug Administration authorized on Friday both the Moderna and Pfizer-BioNTech coronavirus vaccines for the nation’s youngest children, a relief to many parents anxious to protect their children 18 months after shots were first made available to adults.Regulators followed the recommendation of independent experts on an advisory panel, who voted unanimously on Wednesday in favor of both vaccines. All Americans other than the roughly 20 million children under the age of 5 have had the option of protecting themselves against Covid-19 for months and are eligible for booster shots.Moderna’s two-dose vaccine is for children 6 months through 5 years old, while Pfizer’s three-dose vaccine is for children 6 months through 4 years old.

A separate advisory panel to the Centers for Disease Control and Prevention is meeting Friday and Saturday on the matter. Afterward, the C.D.C.’s director, Rochelle P. Walensky, will issue her own recommendation, the final step before doses are rolled out, probably early next week.

The Biden administration will begin shipping the vaccines on Friday. But uptake in the coming weeks could be low, which could be particularly worrisome for children with underlying conditions that can increase the risks of severe disease.

Both Pfizer’s and Moderna’s studies showed that the Omicron variant of the virus greatly dulled the power of the vaccines to prevent symptomatic infection in young children, just as it did for adults. 

Pfizer has said that two doses of its vaccine were only about 28 percent effective in preventing disease, while three doses were 80 percent effective. But that 80 percent estimate was based on only 10 cases in a subset of 1,678 trial participants.Moderna’s vaccine was about 51 percent effective in children 6 months to 2 years old and 37 percent effective in children 2 through 5. Still, Dr. Robert M. Califf, the F.D.A.’s commissioner, said in a statement, “As we have seen with older age groups, we expect that the vaccines for younger children will provide protection from the most severe outcomes of Covid-19, such as hospitalization and death.”

Both vaccines provoked an immune response comparable to that triggered in young adults, the primary criteria by which the agency measured effectiveness. Asked to compare them at a press briefing, Dr. Peter Marks, who heads the F.D.A.’s vaccine division, said that Moderna’s vaccine might trigger an immune response “slightly more rapidly,” while Pfizer’s might “bring a greater response after the third dose.”