Nirsevimab RSV protection fades by second year

Clinical takeaway: Nirsevimab substantially reduces RSV-related hospitalization during an infant’s first RSV season, but clinicians should not expect a single dose to provide protection into the following season or prevent other infections or later asthma.

Nirsevimab substantially reduces RSV hospitalization during an infant’s first RSV season, but how long that protection lasts has been less clear. A large, real-world study now shows that protection from a single dose does not extend into the following RSV season.

Researchers analyzed two matched cohorts totaling more than 250,000 infants during the 2023-2024 and 2024-2025 RSV seasons in France. During the first year after nirsevimab administration, RSV lower respiratory tract infection–related hospitalization occurred in 0.8% of immunized versus 2.4% of unimmunized infants in the 2023 cohort and 0.5% versus 1.9% in the 2024 cohort. These differences corresponded to estimated effectiveness of 66% and 72%, respectively.

The consistency across seasons is notable because RSV A predominated in 2023-2024, whereas RSV B predominated the following season, when resistance-associated RSV B variants had also emerged. The findings suggest nirsevimab retained substantial real-world effectiveness despite that shift.

Protection did not carry into a second RSV season. Among infants immunized in 2023, RSV-related hospitalization during the second year was essentially identical between previously immunized and unimmunized groups (0.2% in each), consistent with nirsevimab’s approximately five months of passive protection.

Nirsevimab also did not reduce hospitalization for non-RSV infections or asthma. Higher rates of hospitalization for bacterial ear, nose, and throat infections, primarily acute otitis media, were observed after immunization, as was a small increase in asthma-related hospitalization during the second year. The authors cautioned that these findings involved relatively few events, may reflect residual confounding, and require further study—particularly because asthma diagnoses before age 2 can be difficult to interpret.

The findings help clarify the duration and scope of nirsevimab’s benefits and underscore the need for longer-term studies to determine whether preventing RSV in infancy has effects on respiratory health beyond the initial RSV season.

Source: Gabet A, et al. (2026 Aug 10) JAMA Pediatr. First- and second-year outcomes after nirsevimab immunization