WASHINGTON — The Food and Drug Administration plans to allow older and immunocompromised Americans to get a second updated coronavirus booster shot in the near future, an acknowledgment of the virus’s continuing risks to vulnerable people whose immunity might be sagging months after a previous inoculation.

Federal regulators are expected to authorize the additional dose in the next few weeks, according to people familiar with the agency’s planning. Those 65 and older would be able to receive the vaccine at least four months after their previous updated shot. Those with immune deficiencies would also be eligible, and the vaccines would be free of charge.

Regulators are expected to authorize the additional dose without explicitly recommending it for those groups, a stance that emphasizes the discretion of patients and their health providers. Dr. Rochelle P. Walensky, the director of the Centers for Disease Control and Prevention, is expected to sign off on the decision — a customary step in the regulatory process.

Michael Felberbaum, an F.D.A. spokesman, said in a statement that the agency was monitoring data on the virus and would “base any decision on additional updated boosters upon those data.”

The decision reflects the Biden administration’s ongoing concerns about the durability of protection against the virus for those Americans most at risk, even as the pandemic’s threat to younger, healthier Americans has receded. About 1,600 deaths from Covid-19 were reported for the week ending March 29, according to the most recent federal data. Those who are dying from Covid-19 are overwhelmingly 65 and older.

Looking to blunt the effects of a potential winter Covid surge, the F.D.A. authorized the retooled booster shots, which are aimed at Omicron subvariants, at the end of August. But only about 55 million Americans — less than 17 percent of the population — have received an updated booster, according to the C.D.C. Among those 65 and older, 42 percent have received one of the shots.

The Washington Post earlier reported the coming authorization.

The Biden administration is planning to roll out another reformulated booster late in the summer or early in the fall, a schedule that would align with the annual flu vaccine. In the coming months, regulators plan to select the version of the virus they want to target with that retooled booster.

In the meantime, a second booster for some groups would be in line with a proposal that regulators made at a January meeting of an independent group of vaccine advisers, at which the panel discussed offering vulnerable Americans more than one annual coronavirus shot.

At that meeting, federal health officials pointed to research that showed the reformulated boosters were still working to protect Americans against newer versions of the virus that circulated deeper into the winter.

The Biden administration has a substantial stockpile of the updated booster shots, and many of those doses could end up going to waste once a new booster is rolled out later this year. Federal health officials purchased more than 170 million doses of the updated shots last year. A senior Department of Health and Human Services official said recently that the administration was considering donating some of the doses.