Effectiveness of Nirsevimab Against RSV and RSV-Related Events in Infants

BACKGROUND AND OBJECTIVES Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract disease (LRTD) in infants and young children. In 2023, the Advisory Committee on Immunization Practices recommended nirsevimab for US infants to prevent RSV-associated LRTD. We assessed nirsevimab effectiveness against polymerase chain reaction (PCR)-confirmed RSV LRTD and RSV-associated health care utilization at Kaiser Permanente Northern California during the 2023–2024 RSV season. METHODS All nirsevimab-eligible, healthy term infants born April 2023 or later were included. Infants of RSV-vaccinated mothers or with high-risk conditions were excluded. We assessed nirsevimab effectiveness against RSV LRTD by Cox regression on a calendar timeline conditioned on birth month, adjusting for sex and race and/or ethnicity. Nirsevimab effectiveness was calculated as (1 − adjusted hazard ratio) × 100%. We estimated nirsevimab impact on the number of medical encounters per RSV LRTD by linear regression and odds of hospitalization by logistic regression. RESULTS The study included 31 900 infants; 15 647 (49.1%) received nirsevimab. There were 35 RSV LRTD episodes (6.10/1000 person-years) among nirsevimab-immunized infants vs 462 (58.51/1000 person-years) among nonimmunized infants. Nirsevimab effectiveness was 87.2% (CI, 81.7%–91.1%) against RSV LRTD, 98.0% (CI, 85.1%–99.7%) against hospitalized RSV LRTD, and 71.0% (CI, 65.3%–75.8%) against PCR-confirmed RSV. Nirsevimab-immunized infants with RSV LRTD had fewer encounters (adjusted means difference −0.86; P = .001) and lower odds of hospitalization (odds ratio, 0.11; CI, 0.01–0.85) than nonimmunized infants. CONCLUSIONS Nirsevimab was highly effective in protecting infants against RSV-associated LRTD as well as against milder RSV infection. Nirsevimab-immunized infants with RSV LRTD had significantly fewer medical encounters and were less likely to be hospitalized than were nonimmunized infants.