Study: Beyfortus Reduces RSV Hospitalizations Over Two Seasons Amid FDA Warning

A universal respiratory syncytial virus (RSV) immunization program with Sanofi and AstraZeneca’s monoclonal antibody Beyfortus (nirsevimab) was associated with a significant reduction in RSV-related hospitalizations, with benefits extending into a second RSV season, according to a prospective, real-world study in Galicia, Spain. The findings, published in The Lancet Infectious Diseases and presented at the RSVVW ’26 conference in Rome, provide the first population-level evidence evaluating a universal Beyfortus program across two consecutive seasons.

The NIRSE-GAL study achieved a 94.4% coverage rate among eligible infants. During the first RSV season, the program was linked to an 85.9% reduction in RSV-related lower respiratory tract infection (LRTI) hospitalizations. Protection persisted into the second season, with 55.3% fewer RSV-related hospitalizations among infants who had received Beyfortus during infancy compared with expected rates. In addition, primary care consultations declined by 30.8% for acute bronchitis or bronchiolitis, 33.4% for lower respiratory tract infections, and 27.7% for wheezing or asthma. Among infants previously hospitalized for RSV, rehospitalizations dropped by 78.2% for RSV-related conditions and 62.4% for LRTI in the second season. “This universal RSV immunization program with Beyfortus showed decreased RSV-related hospitalizations and outpatient illness burden during the first season, with persistent impact seen on RSV hospitalizations through the second season,” the head of Pediatrics at Santiago University Hospital and principal investigator of the study said in a statement.

The RSV prevention market is expanding, with increased focus on protecting high-risk children entering their second RSV season. RSV infects more than 90% of children before age two, and the market was valued at $3.51 billion in 2024, projected to reach $28.39 billion by 2030 at a compound annual growth rate of 42.17%. Sanofi’s Beyfortus received EMA authorization in 2022 and FDA approval in July 2023; as of December 2025, more than 11 million infants globally had received the antibody. It is the only preventive antibody approved for high-risk children under two years of age entering their second RSV season, including those with chronic lung disease, congenital heart disease, or immunocompromise. MSD’s Enflonsia (clesrovimab) is also approved for first-season use but not for the second season. Previously, Synagis (palivizumab) required five monthly injections, while Beyfortus provides season-long protection with a single dose.

In South Korea, where Beyfortus was launched in February 2024, sentinel surveillance data showed a reduction in infant RSV hospitalizations from a peak of 191 in the 2024–2025 season to 79 in the 2025–2026 season after introduction. “Since it is impossible to predict in advance which infants will progress to severe disease, we should consider not only universal prevention for infants entering their first RSV season but also additional preventive management through the second season for high-risk children,” said a professor of pediatrics at Korea University Anam Hospital.

Separately, the U.S. Food and Drug Administration (FDA) has accused Sanofi of making false or misleading promotional claims about Beyfortus. In an Untitled Letter, the agency said the company’s promotional emails suggested the antibody prevents RSV disease generally, rather than the FDA-approved indication of RSV lower respiratory tract disease. The letter states that such claims “misbrand Beyfortus” and creates a misleading impression. The FDA requested that Sanofi immediately stop disseminating the communications and respond within 15 working days. An Untitled Letter requests voluntary compliance and does not carry the same legal weight as a Warning Letter. Last year, the FDA launched a safety review of RSV shots including Beyfortus and Enflonsia.