In an unusual move, federal health officials narrowed their recommendations for who should receive the vaccine against the respiratory syncytial virus.

The Centers for Disease Control and Prevention advised last year that adults age 60 or older could receive a single lifetime dose of an R.S.V. vaccine, in consultation with their health care providers.

On Wednesday, scientific advisers to the agency reframed that guideline. Based on recent safety and effectiveness data, they unanimously recommended that all Americans age 75 and older receive one dose of an R.S.V. vaccine.

But for adults 60 to 74 years of age, the panel endorsed vaccination only for those with certain serious conditions, such as chronic heart or lung disease, advanced kidney disease and diabetes with organ damage.

The advisers voted not to recommend the vaccine for other adults in this age group, although individuals may still consult with their health care providers to evaluate the risk the infection poses to them.

The C.D.C.’s director, Dr. Mandy Cohen, accepted the panel’s recommendations on Wednesday afternoon.

The Food and Drug Administration has approved three R.S.V. vaccines. Two of them, GSK’s Arexvy and Pfizer’s Abrysvo, were approved in May last year and were administered during the fall and winter. The third, an mRNA vaccine called mRESVIA made by Moderna, was approved only this May.

Officials said in February that the vaccines might have caused some cases of Guillain-Barré syndrome, a rare neurological condition. Data analyses presented to the panel on Wednesday did little to clarify the concerns about the vaccine.

The uncertainty fueled much of the hesitation among advisers weighing the relative benefits and harms of the vaccines. G.B.S. “is not a small consequence,” said Dr. Jamie Loehr, one of the advisers and a family medicine practitioner in Ithaca, N.Y.

“Patients who have it often end up in the hospital for three to four months, might be intubated and might die,” he said.

Concerns about the side effect played a particularly big role when the panelists considered expanding eligibility for GSK’s Arexvy to adults age 50 to 59. The F.D.A. approved Arexvy for use in that age group in June.

But in the meeting on Wednesday, the C.D.C. advisers concluded that for most Americans in that age group, the risk of R.S.V. was not high enough to justify the potential harms or costs of vaccination.

“I don’t want even one not-at-risk person to get G.B.S. from a nonnecessary vaccine,” Dr. Camille Kotton, one of the advisers, said in an interview.

The panelists will continue to review their recommendations as more data becomes available. For now, Dr. Kotton said, “we just don’t want to rush a recommendation for a lower-risk population.”

Some panel members also noted that it was important to administer the vaccines when adults were most at risk.

The vaccines are intended to be given only once, because in clinical trials a second dose did not appear to offer much of an immune boost.

If Americans who are not at high risk receive the vaccine at a younger age, their immunity against the virus may have waned by the time they really need the protection, some advisers said.

As with other respiratory illnesses, the risks of R.S.V. infection increase sharply with age. From 2016 to 2020, the virus was associated with up to 140,000 hospitalizations each year in Americans age 65 years and older, compared with fewer than 20,000 among those 60 to 64.

Among unvaccinated adults who are hospitalized, R.S.V.’s severity is comparable to that of Covid and flu. Chronic conditions compound the risks. Among adults age 65 and older, hospitalization rates are more than three times higher among those with heart failure, according to data presented at the C.D.C. meeting.

Several advisers worried that narrowing the number of Americans age 60 to 74 who qualified for R.S.V. vaccination might send “the wrong message” about the safety of the vaccines. But ultimately, all voted to approve the new criteria.