Effectiveness of nirsevimab among infants in their first RSV season in the United States, October 2023-March 2024: a test-negative design analysis.
case_control · Level III
Where this comes from
- Record sourced from PubMed, PMID 41040536.
- Also identified by DOI 10.1016/j.lana.2025.101196 and PMC identifier 12485534.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
In August 2023, the Centers for Disease Control and Prevention recommended nirsevimab, a long-acting monoclonal antibody, for all U.S. infants aged <8 months entering or born during their first respiratory syncytial virus (RSV) season. Our aim was to estimate nirsevimab effectiveness against RSV-associated emergency department (ED) encounters and hospitalisation among U.S. infants during the 2023-2024 RSV season. We conducted a test-negative analysis using electronic health record (EHR) data from 6 healthcare systems, including ED encounters and hospitalizations with a diagnosis of RSV-like illness (RLI) during October 8, 2023-March 31, 2024, among infants aged <8 months as of October 1, 2023, or born during the study period. Nirsevimab effectiveness was estimated by comparing children who received nirsevimab with those who did not among RSV-positive and RSV-negative encounters, adjusting for age, race and ethnicity, sex, calendar day, and geographic region and excluding infants whose mother received RSV vaccination during pregnancy. Among 5039 ED encounters with RLI among infants in their first RSV season, 2045 (41%) were RSV-positive and 446 (9%) received nirsevimab, with a median time since dose of 52 days (interquartile range [IQR]: 27-84 days). Among 1025 hospitalizations with RLI among infants in their first RSV season, 605 (59%) were RSV-positive and 95 (9%) received nirsevimab, with a median time since dose of 48 days (IQR: 24-82 days). Nirsevimab effectiveness was 77% (95% CI: 69%-83%) against RSV-associated ED encounters and 98% (95% CI: 95%-99%) against RSV-associated hospitalisation. Nirsevimab was effective in preventing RSV-associated ED encounters and hospitalisation among infants in their first RSV season, with greatest protection against hospitalisation. However, these estimates reflect a short interval from nirsevimab administration to RLI onset. Since nirsevimab is a passive immunization and concentration is expected to wane over time, it is important to continue monitoring effectiveness to assess effectiveness with increased time since dose. This work was supported by the Centers for Disease Control and Prevention (contracts 75D30121D12779 to Westat and 75D30123C18039 to Kaiser Foundation Hospitals).