Newly approved antibody could offer another option for protecting infants from RSV, a common infection that can be deadly

By Jen Christensen, CNN

(CNN) — The United States could soon have another tool in the fight against respiratory syncytial virus, an illness that’s the No. 1 cause of hospitalization in infants.

The US Food and Drug Administration has approved a new monoclonal antibody to help prevent infection, according to an announcement late Monday from drugmaker Merck.

Enflonsia is designed to be given in a single 105-milligram shot to protect newborns and infants from mild, moderate or severe RSV through all five months of their first virus season, which typically starts in the fall and goes through the next spring.

Study materials that Merck submitted to the FDA for its approval showed that the antibody had a similar safety profile as a placebo.

The most common adverse reactions from Enflonsia were mild and included injection-site swelling and a rash in a small number of infants.

In a mid- to late-stage trial, Enflonsia reduced RSV-associated hospitalizations in infants more than 84% compared with a placebo. RSV can sometimes turn into serious lower respiratory infections like pneumonia, but the shot also reduced lower respiratory infections that needed medical attention by more than 60% compared with a placebo.

“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 Y. Li, president of Merck Research Laboratories, said in a news release. “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.”

Merck says it hopes Enflonsia will be available before the start of the 2025-26 respiratory virus season.

First, it needs to be recommended by the US Centers for Disease Control and Prevention, and that means going in front of the agency’s Advisory Committee on Immunization Practices. It’s on the agenda for the panel’s meeting this month, but US Health and Human Services Secretary Robert F. Kennedy removed all the members of that committee Monday. He says he will appoint new ones, but it’s unclear how long that process will take.

Doctors say another tool to prevent RSV cannot come soon enough.

RSV is ubiquitous, one of the most common causes of childhood illness. Most kids will catch this highly contagious respiratory virus at some point before they turn 2, according to the CDC.

For many healthy adults and older kids, RSV causes a mild illness like a cold. Typically, symptoms can be managed at home, and they often go away on their own. But for infants and the elderly, it can be a different story.

Very young children’s immune systems are just starting to learn how to fight infections, and infants have tiny airways. RSV inflames those airways, making it difficult to breathe, and can turn into a serious lower respiratory illness like bronchiolitis or pneumonia. Some of these RSV infections can be deadly.

Two to three percent of infants under 6 months are hospitalized with RSV in the US every year, according to the CDC. Among children younger than 5, about 58,000 to 80,000 are hospitalized due to RSV.

There’s no specific medicine to treat RSV. Doctors can give an infant supportive care and oxygen, and then they essentially wait until their oxygen levels get back to normal, said Dr. Amy Edwards, director of pediatric infection control at UH Rainbow Babies and Children’s Hospital in Cleveland.

“I hate RSV,” said Edwards, who was not connected with 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. It’s just so hard to watch.”

Enflonsia joins a handful of other tools recently made available to protect babies from, although the FDA put RSV vaccine trials involving infants and young children ages 2 to 5 on hold last year after some developed severe illness.

To prevent RSV in infants, the CDC currently recommends an RSV antibody made by Sanofi and AstraZeneca, called Beyfortus, which was approved in 2023. It was in short supply during that year’s RSV season, although Edwards said supply started to catch up with demand in her health care system last season, and the company pledged to produce more.

The other option to protect an infant is a vaccine that a person can get during pregnancy.

Together, Beyfortus and the vaccine have made a difference. A CDC study published in March found that RSV-associated hospitalization rates among infants up to 7 months during 2024-25 season were lower than in seasons when those things weren’t available.

Edwards just hopes people will get protection for their infants.

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

Correction: A previous version of this story incorrectly described the newly approved monoclonal antibody as a therapy.

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