Federal regulators have decided to authorize a second omicron-specific coronavirus vaccine booster shot for people who are at least 65 or have weak immune systems – an effort to provide additional protection to high-risk individuals, according to several officials familiar with the plan.

The Food and Drug Administration is expected to announce the step in the next few weeks, and the Centers for Disease Control and Prevention is expected to move quickly to endorse it, said the officials, who spoke on the condition of anonymity because they were not authorized to publicly discuss internal discussions.

Eligible individuals will be able to receive the dose as long as it is at least four months since their first shot of what’s known as the bivalent booster, which targets omicron subvariants BA.4 and BA.5 as well as the original novel coronavirus. The expectation is that consumers will consult with their health-care providers about whether to get the extra booster, the officials said.

The FDA’s policy change will be “permissive” – people may get the shot but will not be told they should get it, the officials said. It’s not clear whether the CDC’s vaccine advisory committee would meet to discuss the change before CDC Director Rochelle Walensky makes a final recommendation.

Doctors and other experts have expressed mixed views about a second bivalent booster. Some say there is little data to justify it, while others argue it would benefit high-risk individuals who received their first omicron-targeting shot last fall and probably have reduced protection as the effects fade.

Some anxious patients have been “really clamoring” for a second omicron booster, said Camille Kotton, clinical director for transplant and immunocompromised host infectious diseases at Massachusetts General Hospital in Boston. At a recent meeting of the CDC vaccine advisory panel, she said she would support allowing additional booster doses for high-risk groups, especially for the most significantly immunocompromised patients.

Jamie Loehr, a family medicine doctor in Ithaca, N.Y., who is also a member of the vaccine advisory panel, said there is evidence that older people and those with weak immune systems don’t produce especially robust responses to vaccines in general, and to the coronavirus vaccine in particular. It would seem reasonable to give them more frequent boosters, he wrote in an email, but he wants to see data before deciding whether he would support a more frequent booster for these groups.

John P. Moore, a professor of microbiology and immunology at Weill Cornell Medicine in New York, said an extra booster could benefit people who are in poor health or have an impaired immune system. But he was skeptical everyone older than 65 needs it. Boosters lead to “a short-term boost against mild infection but protection against severe disease is still pretty robust” because of previous shots, he said.

Moore said it is a mistake to think “everyone over a certain age is in the same health bracket,” when in fact health status varies widely. He said he is older than 65 and healthy – and “not giving a moment’s thought about getting another booster, though I might next winter if infections tick up.”

Administration officials acknowledge there is not extensive data on the bivalent vaccine, which was first authorized in August. But they said real-world data and smaller studies are consistent with large studies on the original vaccine showing that its protection against symptomatic infection fades after several months.

In addition, unpublished data presented at the CDC’s vaccine advisory panel meeting in February confirmed earlier real-world reports that bivalent vaccines are providing protection against serious illness – emergency room visits and hospitalizations – in adults, compared with people who received previous doses of the original vaccine and no omicron-targeting dose.

Other studies also suggest older people might be better shielded from serious illness with an additional booster, the officials said. They noted they are not advocating the second omicron booster for young people, who might experience rare heart-related side effects.

The Wall Street Journal and NPR previously reported that the FDA was considering a second bivalent booster for high-risk groups.

Only about 42 percent of people 65 and older have received the first bivalent booster dose, according to the CDC.

A second booster will not affect the FDA’s plan to move to a once-a-year coronavirus vaccine booster for most Americans – a strategy announced by the agency in January, the officials said. This summer, the agency and its advisers will select a retooled formula for an updated vaccine to be deployed during a campaign planned for next fall. The formula will be based on which coronavirus strain scientists think will be circulating in the fall and winter.

In a statement, the FDA said: “We continue to closely monitor the emerging data in the United States and globally, and we will base any decision on additional updated boosters upon those data. Importantly, individuals who have not yet received an updated (bivalent) booster are encouraged to speak with their health care provider and consider receiving one.”

The updated shot will be free of charge, regardless of someone’s insurance coverage, because the government has an ample supply of boosters.

Even after the federal supply of vaccines is gone, shots will continue to be free of charge to most people with private and public health insurance. But once federally purchased doses are depleted, uninsured and underinsured adults may have to pay, and privately insured people may need to confirm their provider is in-network, according to an analysis by the Kaiser Family Foundation.

Peter Hotez, dean of the National School of Tropical Medicine and professor of pediatrics and molecular virology and microbiology at Baylor College of Medicine, said he is a strong proponent of making a second omicron-targeting booster available for everyone 50 and older.

Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, agreed, saying a second booster would provide an additional tool for people who have been conscientious about reducing their risks of contracting covid.

An omicron subvariant, XBB.1.5, now accounts for almost 90 percent of the cases in the United States, according to the CDC.

The Washington Post’s Dan Diamond contributed to this report.