Nirsevimab, a long-acting monoclonal antibody designed to protect infants from respiratory syncytial virus (RSV), was associated with a substantial reduction in hospitalizations caused by RSV-related lower respiratory tract infections during the first year after administration, according to a study published in JAMA Pediatrics. The findings were observed across both the 2023–2024 and 2024–2025 RSV seasons, providing early real-world evidence that the preventive treatment can reduce severe disease requiring hospital care in young children.
RSV is one of the most common causes of lower respiratory tract infection in infants and young children. Although infection often produces cold-like symptoms, the virus can spread into the small airways and lungs, causing bronchiolitis or pneumonia. Infants, particularly those born prematurely or with underlying medical conditions, are at increased risk of respiratory distress, dehydration, and hospitalization. Because the virus circulates seasonally and can infect children repeatedly throughout life, reducing the severity of a first infection is a major public health objective.
Nirsevimab works by providing passive immunity rather than stimulating the infant’s immune system to produce its own antibodies. The drug is a laboratory-engineered antibody that binds to the fusion protein on the surface of RSV. This protein is essential for the virus to enter human cells and spread between them. By attaching to a vulnerable region of the fusion protein, nirsevimab can neutralize RSV particles before they establish infection or limit the progression of infection after exposure. Its extended duration of action allows a single administration to provide protection throughout much of an RSV season.
The study evaluated outcomes during two consecutive RSV seasons and compared hospitalization patterns among children who received nirsevimab with those who did not. Its principal focus was hospitalization associated with RSV lower respiratory tract infection, a clinically important endpoint because it reflects severe disease rather than infection detected only through outpatient testing. The investigators also examined whether nirsevimab was associated with changes in hospitalizations from non-RSV infections and asthma, conditions that could help indicate whether the observed effect was specifically linked to RSV prevention.
Across the 2023–2024 and 2024–2025 seasons, children who received nirsevimab experienced substantially fewer hospitalizations related to RSV lower respiratory tract infections during the first year of follow-up. The consistency of the association across both seasons is notable because the timing and intensity of RSV circulation can vary from year to year. Seasonal differences in viral transmission, population immunity, healthcare-seeking behavior, and the prevalence of other respiratory pathogens can all influence hospitalization rates. Observing a similar protective pattern in two successive seasons strengthens the evidence that the reduction was related to nirsevimab rather than to a single unusual epidemic.
The researchers did not find evidence that nirsevimab reduced hospitalizations caused by non-RSV infections. This distinction is biologically important. Nirsevimab targets RSV specifically and does not function as a broad antiviral or general immune stimulant. A selective reduction in RSV-related admissions, without a comparable decline in hospitalizations from unrelated infections, supports the interpretation that the treatment’s benefit is pathogen-specific. It also suggests that the results were not simply the consequence of broader differences in healthcare access, general illness prevention, or the likelihood that treated children were hospitalized.
The analysis also found no association between nirsevimab and reduced asthma-related hospitalizations. Asthma is influenced by multiple factors, including genetic susceptibility, airway inflammation, environmental exposures, and viral triggers. Severe RSV infection early in life has been linked in some research with later respiratory problems, but a potential long-term relationship cannot be established by observing short-term hospitalization patterns alone. The absence of a detected reduction in asthma admissions indicates that the immediate preventive effect of nirsevimab should not be interpreted as evidence that the antibody prevents asthma or modifies all forms of respiratory disease.
The apparent benefit was limited to the first year of follow-up. During the second year, the study did not identify a corresponding reduction in RSV-related hospitalizations. Several explanations are possible. The antibody’s protection is designed to last through a defined period rather than indefinitely, and children may encounter RSV after the period of effective antibody coverage has ended. In addition, older children generally face a different risk profile than infants, with more mature airways and immune systems reducing the likelihood of severe disease. Changes in exposure, vaccination or prophylaxis practices, and the underlying circulation of RSV may also contribute to differences between the first and second years.
Because the study was observational, the findings demonstrate an association rather than definitive proof of causation. Treatment decisions, underlying health, birth history, healthcare access, and other characteristics may differ between children who receive nirsevimab and those who do not. Researchers use statistical methods to account for measurable differences, but unmeasured factors can remain. Even so, the results complement clinical trial evidence by showing how nirsevimab performed under routine conditions and across more than one RSV season. The findings suggest that targeted antibody protection can meaningfully reduce severe RSV disease in infants during the period when they are most vulnerable, while underscoring that it does not prevent unrelated infections or guarantee protection beyond the first year.
Subject of Research: Nirsevimab and its association with RSV lower respiratory tract infection hospitalizations in children.
Web References: https://doi.org/10.1001/jamapediatrics.2026.3384
References: Gabet A, et al. Study published in JAMA Pediatrics. DOI: 10.1001/jamapediatrics.2026.3384.
Keywords: Nirsevimab, respiratory syncytial virus, RSV, lower respiratory tract infection, bronchiolitis, infant hospitalization, monoclonal antibody, passive immunity, asthma, pediatric infectious disease, viral prevention, immunization
