Impact of nirsevimab implementation on the mid-term burden of bronchiolitis in the outpatient setting: results from the prospective OURSYN study

Context Nirsevimab was implemented in Europe and the US in September 2023. While its effectiveness in preventing RSV-bronchiolitis has been demonstrated by real-life studies, its impact on mid-term child health, quality of life, and parental absenteeism remains unstudied. Methods Infants under 1-year with a first bronchiolitis episode were recruited through the OURSYN study involving 37 primary care pediatricians in France between 2021 and 2025 (pre-nirsevimab period: 02/2021-03/2023 and nirsevimab period: 09/2023-03/2025). RSV status was determined by antigenic nasopharyngeal testing. At day-15, parents reported symptoms, quality of life (PedsQL Infant), and work absenteeism. Their associations with (i) RSV status, (ii) period of inclusion, and (iii) nirsevimab immunization status among RSV-positive cases were assessed using multivariable logistic regression adjusted for confounders. Results Among 1870 bronchiolitis cases included, 637 (34.0%) occurred during the nirsevimab period and 812 (43.4%) were RSV-related. Over the study period, at day-15, RSV-cases had more feeding difficulties (aOR = 1.41 [1.06–1.88]), longer daycare absence (4.7 vs 3.2 days), and more parental absenteeism (aOR = 1.61 [0.99–2.62]) compared to RSV-negative. The proportion of RSV-cases decreased during the nirsevimab period compared with the pre-nirsevimab period (47.1% vs 35.7% p < 0.001). Bronchiolitis cases recruited during this period had shorter daycare absences (3.2 vs 4.3 days). Among RSV-cases, immunized infants had lower risk of persistent fever (aOR = 0.14 [0.01–0.79]). Conclusion This large prospective study provides the first evidence that the mid-term burden of RSV-bronchiolitis in infants may be reduced following the implementation of nirsevimab, with potential benefits for infant health, quality of life and parental absenteeism.