New RSV drug for babies is over 90% effective at preventing hospitalization

A new RSV drug designed to protect young children was 93% effective at preventing hospitalization for the viral disease, a new study reports. Furthermore, the drug was 89% effective at preventing all types of doctor visits for RSV, which is short for "respiratory syncytial virus."

The new research, published Monday (Dec. 9) in the journal JAMA Pediatrics, focused on nirsevimab (Beyfortus), a drug approved in 2023. The medicine, which is given as an injection, uses lab-made antibodies to block RSV from getting into cells. Unlike a vaccine, nirsevimab doesn't teach the body to make its own antibodies; rather, it provides a ready-made supply.

The new study's results suggest that nirsevimab is very effective at protecting young children from needing hospitalization for RSV, as well as other lesser degrees of medical care, such as outpatient visits. However, "only a small fraction" of infants in the study who were eligible for the drug actually got nirsevimab, the study authors noted.

Ultimately, the findings suggest that nirsevimab could have a "substantial public health impact" in future RSV seasons if the drug were used more widely, they concluded.

Related: Who should get the new RSV vaccines? Here's everything you need to know

Before nirsevimab's approval in 2023, there were no widespread strategies for preventing RSV in infants, for whom the virus is the leading cause of hospitalization.

Each year in the United States, 2 to 3 out of every 100 infants under 6 months old are hospitalized for RSV, according to the Centers for Disease Control and Prevention (CDC). These cases start off mild, causing a runny nose and cough, but then progress to trigger inflammation and infection in the lungs. Children hospitalized for RSV often need supplemental oxygen and IV fluids, as well as breathing support from a ventilator.

To see how well nirsevimab is working in the real world, the study's authors compared three RSV seasons before the drug's approval to the 2023-2024 season, after its approval. The three pre-approval seasons spanned 2017 to 2020, before the COVID-19 pandemic, which disrupted typical patterns of RSV spread.

The drug is currently recommended for all babies younger than 8 months old whose mothers haven't gotten the maternal vaccine against RSV. (If a person gets the vaccine during pregnancy, the resulting antibodies pass to the fetus before birth.)

The CDC recommends that eligible babies get nirsevimab just before the start of their first RSV season — around October — or within a week of birth, if they're born between October and March. Select older children are also recommended to get the drug before their second RSV season.

In all, the new study included data from nearly 28,700 children under age 5 who required medical care for a respiratory infection during RSV season. The children had been treated at seven academic pediatric medical centers, and they needed different levels of care, ranging from outpatient doctor visits to hospitalization.

Of the children, roughly 7,500 were treated for RSV, and 4,500 of those kids were hospitalized for the infection. The remaining children, who had tested negative for RSV, served as a point of comparison for the study's analyses.

The study found that RSV accounted for a similar proportion of respiratory-infection-related medical visits before and after nirsevimab's approval. Looking at the 2023-2024 season, the researchers found that the number of infants who got nirsevimab was small: 402 got the new drug, while 16 got an older drug called palivizumab (Synagis) that's recommended only for certain kids.

"Only a small fraction of infants in their first RSV season had received nirsevimab," the authors wrote.

An additional 70 infants were born to mothers who'd gotten the maternal RSV vaccine. The researchers had planned to study the vaccine's real-world effectiveness as well, but they said they would have needed more data to do so.

There are a number of reasons that uptake of both nirsevimab and the vaccine might have been low in 2023, the authors noted. There were supply issues with nirsevimab during the 2023-2024 season, for example. In addition, RSV season kicked off unusually early in 2023, and the maternal vaccine didn't become available until around the same time.

Despite the study's limitations, the research and other work collectively suggest that nirsevimab has the potential to substantially reduce infant RSV hospitalizations — if and when it's used more widely.

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