Sustained clinical and epidemiological impact of Respiratory Syncytial Virus (RSV) in young infants exposed to universal immunization with Nirsevimab at birth: An Italian multicenter, retrospective, cohort study 2024/25

We designed a multicenter, retrospective, test-negative, case-control study including a total of 17 hospitals in various Italian Regions. The study was performed during the first RSV season benefiting from Nirsevimab delivery in eligible children (i.e. age < 5 months by November, 1st, 2024). Our study included all infants aged 7 months or younger who were hospitalized with a diagnosis of LRTD (ICD-10 J21.-) between 01/11/ 2024 and 30/04/2025. All eligible cases were tested for RSV using PCR. All infants with any positive RSV test were considered RSV- positive cases, while controls were all subjects with a PCR-confirmed negative status for RSV. Diagnoses for the underlying pathogens are provided in Supplementary Tables 1and 2. Nirsevimab immunization effectiveness (IE) was measured by preliminary calculation of the crude odds ratio (cOR) for being immunized with Nirsevimab among cases vs. the odds of being immunized among controls. OR were then calculated as crude and adjusted estimates (aOR, binary logistic regression analysis) controlled for gender, age, calendar month of hospitalization, time since delivery of Nirsevimab. IE was then estimated as IE = 100 x (1-OR), where the OR refers either to the cOR or aOR.