The new RSV antibody treatment for babies has been highly effective in its first season, according to a first look at data from four children’s hospitals.

Babies who received the new preventive treatment for RSV shortly after birth were 90% less likely to be severely sickened with the potentially deadly respiratory illness, according to the new estimate published by the CDC. The data is the first real-world evaluation of Beyfortus (the generic name is nirsevimab), which was approved by the FDA last July.

RSV is a seasonal illness that affects more people — particularly infants and the elderly — in the fall and winter. Symptoms are usually mild in healthy adults, but infants are particularly at risk of getting bronchiolitis, which results in exhausting wheezing and coughing in babies due to swelling in their airways and lungs. Babies who are hospitalized may need fluids and medical devices to help them breathe.

RSV peaked this season from November to January, with more than 10,000 monthly diagnoses reported to the CDC. 

The new CDC analysis was conducted among about 700 babies hospitalized for severe respiratory problems from October to the end of February. Among the babies in the study, 407 were diagnosed with RSV and 292 tested negative. The researchers found that 1% of babies in the study who were diagnosed with RSV had received Beyfortus, while the remaining babies who were positive for the virus had not. 

Among the babies hospitalized for other severe respiratory problems, 18% had received Beyfortus. Overall, just 59 babies among the nearly 700 in the study received Beyfortus, perhaps reflecting the short supply of the medicine the first season it was available. The report authors noted that babies in the study who did receive Beyfortus also tended to have high-risk medical conditions.

The number of babies nationwide who received Beyfortus during this first season of availability is unclear, but a January CDC survey showed that 4 in 10 parents said their babies under 8 months old had received the treatment. The Wall Street Journal reported recently that a shortage last fall resulted from underestimated demand and from production plans that were set before the CDC decided to recommend that all infants under 8 months old receive Beyfortus if their mothers did not get a maternal vaccine that can protect infants from RSV.

Both the antibody treatment for infants and the maternal vaccine were shown in clinical trials to be about 80% effective at preventing severe illness stemming from RSV.

The authors of the latest CDC report concluded that “…this early estimate supports the current nirsevimab recommendation for the prevention of severe RSV disease in infants. Infants should be protected by maternal RSV vaccination or infant receipt of nirsevimab.”

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