Currently, a second booster shot is available only to those 50 and older, as well as to those 12 and older who are immunocompromised. But administration officials are concerned by data that suggests immunity wanes within several months of the first booster shot. Swiftly expanding access to booster shots also would enable people who are boosted now to receive reformulated shots that target newer virus variants, when those become available, probably later this year. In addition, officials want to use vaccine doses that are reaching their expiration dates and would otherwise be discarded.

While the booster plan still needs formal sign-off from regulators and public health officials, it has the backing of White House coronavirus coordinator Ashish Jha and Anthony S. Fauci, the government’s top infectious-disease expert, according to five officials who like others interviewed in this report spoke on the condition of anonymity because they were not authorized to discuss the plan.

The Centers for Disease Control and Prevention recommends that all Americans age 5 and older receive at least one booster shot several months after undergoing their initial vaccinations, although the timing varies based on which vaccine was originally administered. Regulators in March also said adults over age 50 and some immunocompromised people should receive a second booster.

While about 67 percent of all Americans have been fully vaccinated against the virus, only 34 percent of eligible Americans have received a first booster dose, according to federal data.