Vaccines for respiratory syncytial virus may have caused a few cases of Guillain-Barré syndrome, a rare neurological condition, federal health officials said on Thursday.

The numbers were small, on the order of two cases per 100,000 vaccinated people or fewer, and much more data is needed to pin down the risk, the officials said. In May 2023, the Food and Drug Administration approved two vaccines against R.S.V.: Abrysvo, by Pfizer, and Arexvy, by GSK.

In June, rather than recommend the shots to all older adults, the Centers for Disease Control and Prevention recommended that adults aged 60 or older might opt to receive a single dose of an R.S.V. vaccine in consultation with their health care providers. Fewer than 10 million doses had been administered by Feb. 16.

The new safety data, disclosed at a meeting of scientific advisers to the agency, came from multiple databases maintained by federal health agencies. Still, because of the preliminary nature of the analysis, officials urged caution in interpreting the results.

“At this point, due to the uncertainties and limitations, these early data cannot establish if there is an increased risk for G.B.S. after vaccination in this age group,” Dr. Thomas Shimabukuro, director of the C.D.C.’s Immunization Safety Office, said at a meeting on Thursday.

Ongoing surveillance “will be better able to determine if an increased risk for G.B.S. after R.S.V. vaccination is present, and if so the magnitude of the risk,” he said.

In Guillain-Barré syndrome, the immune system attacks nerves. Most patients recover, but in severe cases the syndrome can lead to paralysis and death.

Experts noted that even if confirmed, the absolute risk remains low. The highest estimate from any of the databases pegs the risk at about one case of G.B.S. per 40,000 doses administered.

That rate is “very rare and needs to be considered in the context of the benefits of vaccination,” said Dr. Daniel Salmon, director of the Institute for Vaccine Safety at Johns Hopkins Bloomberg School of Public Health.

R.S.V. vaccines may prevent an estimated 120 to 140 in-hospital deaths and about 25,000 hospitalizations per million doses administered, federal officials said.

Most of the other side effects observed after inoculations with the R.S.V. vaccines were minor. But on Jan. 19, federal health officials picked up signs of an increase in Guillain-Barré syndrome after inoculation with Abrysvo.

Of 37 preliminary reports in the Vaccine Adverse Event Reporting System, officials verified 23 by medical record review, 15 with Abrysvo and eight after Arexvy, Dr. Shimabukuro said. There were nearly three additional cases of G.B.S. per million doses of Abrysvo than would be expected in the population of older Americans.

A separate database identified four cases of G.B.S. linked to Arexvy, translating to an estimated 14 cases per million doses administered. That system did not pick up any cases after shots of Abrysvo. But the vaccine accounted for only about 10 percent of the total doses recorded in the database.

“I will say that these rates are higher than rates that we’ve observed for high-dose influenza and for Shingrix,” Dr. Shimabukuro said. (Shingrix is a shingles vaccine.)

Additional data from the Centers for Medicare & Medicaid Services indicated that the incidence of Guillain-Barré syndrome after vaccination with Abrysvo was roughly five times higher than would be expected. The incidence after vaccination with Arexvy was not statistically significant.

“These data are preliminary and there are several limitations to take into consideration,” said Dr. Patricia Lloyd, a health statistician at the F.D.A.

G.S.K. is planning to study a possible link, said Alison Hunt, a spokeswoman for the company. “There are limitations to all of these data, and further analysis by F.D.A., C.D.C. and the vaccine manufacturers are needed to confirm and quantify any potential risk,” she said.

Pfizer did not immediately respond to a request for comment. A representative at the meeting on Thursday said the company was conducting four safety studies that monitor G.B.S.

Rare cases of Guillain-Barré syndrome have been linked to other vaccines, including those against influenza and shingles. A few cases were observed in the clinical trials of the two R.S.V. vaccines, but the numbers were too small to be certain of an association.

R.S.V. is particularly dangerous for those with other chronic conditions.

During the 2017-18 respiratory season, hospitalizations related to R.S.V. were about 6.5 times higher for adults with chronic kidney disease, according to data presented on Thursday. Those with other respiratory conditions, severe obesity or heart disease were also at increased risk.

By the end of December, about one in four Americans ages 60 and older with a chronic condition had received a dose of R.S.V. vaccine, federal health officials said.

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