The Biden administration on Wednesday outlined a plan for Americans who received the Pfizer-BioNTech and Moderna coronavirus vaccines to get a booster shot eight months after receiving their second doses, starting Sept. 20.

Health care workers, nursing home residents and other older adults who were vaccinated early will be first in line, starting then, contingent on authorization by federal regulators. “We are starting to see evidence of reduced protection against mild and moderate disease,” officials of several federal agencies said in a prepared statement.

“Here’s what you need to know: If you are fully vaccinated, you still have a high degree of protection from the worst outcomes of Covid-19, severe disease hospitalization and death,” Dr. Vivek Murthy, the surgeon general, said at a White House briefing.

“We are not recommending that you go out and get a booster today. Instead, starting the week of Sept. 20,” he added, but that fully vaccinated adults should “begin getting their booster shots eight months after their second shot of an mRNA vaccine.”

Protection conferred by the vaccines against severe disease, hospitalization and death could diminish in the months ahead, especially among high-risk groups who were vaccinated early, the officials said. “For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability.”

Still, Dr. Rochelle Walensky, the C.D.C. director, sought to be reassuring. “These data confirm that while protection against infection may decrease over time, protection against severe disease and hospitalization is currently holding up pretty well,” she said.

People who received the Johnson & Johnson vaccine may also require additional doses. But that vaccine was not rolled out until March 2021, and a plan to provide boosters for those individuals will be made after reviewing new data expected over next few weeks, officials said.

Some experts immediately pushed back against the decision, saying only some older adults and people with weakened immune systems needed extra protection. The World Health Organization has asked that wealthy countries defer distributing booster shots until the end of September.

Jeff Zients, the White House pandemic coordinator, said at the briefing on Wednesday that the administration is on its way to donating more than 600 million doses of vaccines to other countries.

“We’re going to do both,” he said. “We’re going to both protect the American people and we’re going to do more and more to help vaccinate the world. ”

Before Americans can begin to receive boosters, the Food and Drug Administration must first authorize a third dose of the mRNA vaccines made by Pfizer-BioNTech and Moderna, and an advisory committee of the Centers for Disease Control and Prevention must review the evidence and make recommendations.

Federal officials plan to begin by offering booster shots directly to residents of long-term care facilities, since the vaccines were distributed to this population early in the rollout and the virus poses a particular threat to the elderly.

“We will continue to follow the science on a daily basis, and we are prepared to modify this plan should new data emerge that requires it,” federal officials said.

Still, “there’s nothing magical about this number,” Dr. Murthy said, referring to the recommendation to get a booster eight months after the second dose. “This is where judgment comes in.”