P2.E099: Effectiveness of Nirsevimab in Infants: A Population-based Cohort Study

Background: Respiratory syncytial virus (RSV) is the leading cause of acute respiratory infections (ARI) in infants. Infants with severe RSV infections frequently have wheezing and respiratory distress that sometimes requires hospitalization. In 2023, the FDA approved, nirsevimab, a single-dose, long-acting RSV monoclonal antibody for infants age < 8 months, regardless of risk status. While clinical trials have demonstrated the efficacy of nirsevimab, there are no nirsevimab post authorization efficacy studies.

Methods: We conducted a population-based cohort study from September 2023 to November 2023 by analyzing electronic medical records from TriNetX, a large real-world research network of deidentified patient data from 89 health care organizations globally. We compared infants (i.e., age < 1 year) who received 1 dose of nirsevimab (the nirsevimab group) to those who received other childhood immunizations, but not nirsevimab (the unexposed group). We used 1:1 propensity-score matching based on age, sex, race, congenital comorbidities (e.g., congenital cardiac and pulmonary anomalies), perinatal conditions (e.g. preterm birth), and immunodeficient status (e.g., human immunodeficiency virus disease). The index date was the first administration of the monoclonal antibody or the first encounter for immunization. Outcomes occurring within 180 days after the index date were assessed, including all-cause mortality, symptoms and diseases (wheezing, bronchiolitis, pneumonia, and acute upper respiratory infections), and acute care services (emergency department visits and neonatal/pediatric intensive care). Risk ratios (RRs) with 95% confidence intervals (CIs) were calculated to compare the risk of outcomes between two groups.

Results: There were 1,487 infants in the nirsevimab group and 110,144 infants in the unexposed group. After 1:1 matching, each group had 1,482 infants. Compared to the unexposed infants, nirsevimab recipients over the subsequent 180 days exhibited a lower risk of several diagnoses: wheezing (RR = 0.62; 95% CI = 0.41 – 0.94), bronchiolitis (RR = 0.47; 95% CI = 0.35 – 0.63), and acute upper respiratory infections (RR = 0.83; 95% CI = 0.72 – 0.97). The risk of emergency department visits, intensive care use, and all-cause mortality did not differ between the two groups. (Table 1).

Conclusion: In this first real-world post authorization study of nirsevimab, we found that compared with infants who are immunized but unexposed to nirsevimab, those receiving nirsevimab were less likely to develop subsequent wheezing, bronchiolitis, and acute upper respiratory infections in 180 days following administration. Although hospitalization outcomes did not differ between groups, these results still support the American Academy of Pediatrics and Advisory Committee on Immunization Practices recommendation to provide nirsevimab to infants entering their first RSV season. Long-term real-world studies are warranted to evaluate the sustainability of these effects, and additionally if nirsevimab may be utilized for the primary prevention of recurrent wheezing and/or asthma.