This winter, for the first time ever, there were two vaccines available to ward off respiratory syncytial virus, which is particularly dangerous to older adults and infants. Only one of them — Abrysvo, made by Pfizer — was approved for pregnant women, and neither was for young children.

The distinction apparently slipped by some clinicians and pharmacists.

At least 128 pregnant women were mistakenly given the alternative vaccine — Arexvy, by GSK — and at least 25 children under age 2 received a vaccination, the Centers for Disease Control and Prevention has warned.

Dr. Sarah Long, a pediatric infectious disease physician and an adviser to the agency, said she was “blindsided” by the reports. “It is very upsetting that this should happen,” she said.

Arexvy has not been tested in pregnant women or children, so information about its effects in those groups is limited. No serious harms from the errors have yet been confirmed, but the outcome was unknown in a majority of reported cases.

Based on available data, Dr. Long said she was more concerned about the young children who received an R.S.V. vaccine than the pregnant women who received Arexvy or their babies. Evidence from animal testing “strongly suggests” that Arexvy might exacerbate R.S.V. infection in children younger than 2, rather than mitigate it, according to the Food and Drug Administration.

To prevent that, the C.D.C. has recommended that the children who mistakenly got either vaccine also be given nirsevimab (sold as Beyfortus), a monoclonal antibody that provides strong immune protection, while the R.S.V. season lasts.

Up to 80,000 children under age 5 are hospitalized with R.S.V. infections each year in the United States, and the virus is a leading killer of children worldwide.

In 2022, GSK halted clinical trials of a version of its vaccine in pregnant women after a safety review indicated an increased risk of preterm birth. The company also found an increase in neonatal deaths, but it said they were a consequence of the preterm births.

GSK is still monitoring the participants in those trials and sharing the data with the F.D.A., Alison Hunt, a spokeswoman for the company, said.

In Pfizer’s trial, Abrysvo also showed a slightly higher risk of preterm birth up to 37 weeks of gestation, but the increase was not statistically significant.

Still, concerned about the possible risk of preterm birth, the F.D.A. approved Abrysvo only for pregnant women between 32 through 36 weeks of gestation. (The goal is to produce antibodies in the women that can be passed to infants, protecting them immediately after birth.)

The C.D.C.’s advisers narrowed the window further by recommending the shots for pregnant women only from September through January, aiming to protect babies born during peak R.S.V. season.

“We were, in reviewing the data, feeling somewhat conservative,” said Dr. Camille Kotton, a physician at Massachusetts General Hospital and one of the agency’s science advisers.

“If there were to be an issue with preterm labor, having it in the last eight weeks of pregnancy would be likely to have less of an impact than earlier in the pregnancy,” she said.

Some infants mistakenly given an R.S.V. vaccine were meant to receive nirsevimab. The GSK vaccine appears to have been administered to some pregnant women because Pfizer’s vaccine was not as widely available and pharmacists thought they were interchangeable.

Amy Gardner, 39, a former kindergarten teacher in Cleveland, Tenn., said she tried to locate Pfizer’s R.S.V. vaccine in several pharmacies for her pregnant daughter. In mid-September, on the very last day that her daughter was eligible to receive the shot, she found a drugstore that said it stocked the vaccine, Ms. Gardner said.

But her daughter received Arexvy instead.

“We’re all human, but there has got to be more checks and balances than this,” Ms. Gardner said. She believes the shot led to her daughter’s premature labor a few hours later.

The errors were reported to a federal database called the Vaccine Adverse Event Reporting System. Those administering the vaccines may have been confused in part by the similarity of the two names, experts said.

“It’s just horrible — why, why did they do this?” Dr. Long said. “Lots of people got paid a lot of money to make up these names, and I don’t get them.”

Dr. Kotton urged the F.D.A. to encourage companies to give similar products clearly distinguishable names. “When there’s going to be a concomitant release of vaccines, whenever possible, it would probably be good to have different names,” she said.

Dr. Long said she was flummoxed that young children received R.S.V. vaccines. Because they are not approved for children, pediatrician’s offices, which typically administer vaccines, should not have had any in stock.

“It’s important to find out how that happened, because we don’t want it to happen anymore,” she said.