THURSDAY, June 12, 2025 (HealthDay News) — A newly approved shot could soon help protect babies from respiratory syncytial virus (RSV), the top cause of hospitalization in U.S. infants.

The U.S. Food and Drug Administration (FDA) has approved a monoclonal antibody called Enflonsia, the drugmaker Merck announced June 9.

Given as a single 105-milligram injection, Enflonsia is designed to protect newborns and infants throughout their first RSV season — which usually runs from fall through early spring, CNN reported.

In a clinical trial, Enflonsia cut RSV-related hospital stays in infants by more than 84% compared to a placebo. It also reduced serious lower respiratory infections, like pneumonia, by more than 60%.

“Enflonsia provides an important new preventive option to help protect healthy and at-risk infants born during or entering their first RSV season,” Dr. Dean Li, president of Merck Research Laboratories, said.

“We are committed to ensuring availability of Enflonsia in the US before the start of the upcoming RSV season to help reduce the significant burden of this widespread seasonal infection on families and health care systems,” he added in a news release.

The most common side effects identified in the study were mild and included swelling at the injection site and rash. Merck says the safety of the shot was similar to that of a placebo.

Before Enflonsia can be used, it must be recommended by the U.S. Centers for Disease Control and Prevention (CDC).

The CDC’s Advisory Committee on Immunization Practices was expected to review the shot this month — but that process has been delayed, as U.S. Health and Human Services Secretary Robert F. Kennedy Jr. removed and replaced members of the panel this week. The new members include some vaccine critics.

Doctors say another tool to fight RSV can’t come soon enough.

RSV is very common — most kids get it by age 2, according to the CDC. For older children and adults, it often feels like a cold.

But for infants, RSV can be much more serious. Their small airways can become inflamed, making it hard to breathe. In some cases, the virus leads to bronchiolitis or pneumonia and even death.

Each year, 2% to 3% of U.S. infants under 6 months of age are hospitalized with RSV. Between 58,000 and 80,000 kids under age 5 are hospitalized with the virus annually.

There’s no specific treatment for RSV, Dr. Amy Edwards, a pediatric infectious disease expert at UH Rainbow Babies and Children’s Hospital in Cleveland, told CNN.

Doctors basically support them with oxygen and wait for them to recover, Edwards added.

“I hate RSV,” said Edwards, who was not involved in the Merck trial.

“Just to watch them struggle to breathe, and then they get scared, and then they cry, which of course makes the breathing worse, and their little lips turn blue," she said. "It’s just so hard to watch.”

Enflonsia joins a short list of new tools to prevent RSV in infants.

Another RSV antibody called Beyfortus, made by Sanofi and AstraZeneca, was approved in 2023. But there were supply shortages during last year’s RSV season.

A vaccine given during pregnancy can also protect newborns, and together, these efforts seem to be working.

A CDC study published in May found that hospitalization rates for RSV in babies under 7 months were lower during the 2024–2025 season compared to past years when no protection was available.

Edwards hopes parents take advantage of all the options.

“Every RSV season fills us to the gills,” she said. “This should theoretically empty us out, if we have good uptake.”

More information

The Mayo Clinic has more on the respiratory syncytial virus (RSV).

SOURCE: CNN, June 11, 2025