Respiratory Syncytial Virus Disease Burden and Nirsevimab Effectiveness in Young Children From 2023-2024
Authors: Moline Heidi L. , Toepfer Ariana P. , Tannis Ayzsa , Weinberg Geoffrey A. , Staat Mary A. , Halasa Natasha B. , Boom Julie A. , Klein Eileen J. , Williams John V. , Schuster Jennifer E. , Goldstein Leah , McKeever Erin R. , Kalman Casey , Paden Clinton , Atherton Lydia , Aggarwal Megha , Roychoudhury Pavitra , Piedra Pedro A. , Sahni Leila C. , Stewart Laura S. , Selvarangan Rangaraj , Michaels Marian G. , Schlaudecker Elizabeth P. , Szilagyi Peter G. , Englund Janet A. , Clopper Benjamin R. , Thornburg Natalie J. , Derado Gordana , McMorrow Meredith L. , Dawood Fatimah S. ,   New Vaccine Surveillance Network Collaborators, Albertin Christina , Amarin Justin , Arth Heidi , Avadhanula Vasanthi , Banerjee Dithi , Bartlett Carla A , Betters Kristina , Castro Juan , Caudill Eva , Chappell Jim , Cook Peter , Correa Ximenia A , Doddapaneni Harshavardhan , Dosdos Dinah , Fregoe Wende , Gauthier Emma C , Greninger Alexander , Grioni Hanna , Guevara Claudia , Hamdan Olla , Hayek Haya , Howard Miranda , Hughes Caymden , Cregeen Sara Joan J , Johnson Monika , Lin Teresa , Loftis Laura , Masten Alyssa , Mohammad Nida , Moffat Mary , Munoz Flor , Musa Samar , Muzny Donna M , Ostrow Amy , Payne Amanda , Quigley Christina , Ragsdale Collin , Rice Marylin , Rohlfs Chelsea , Sasidharan Anjana , Spieker Andrew , Strelitz Bonnie , Surath Anil , Stopczynski Tess , Vega Ever , Wang Lijuan , Weddle Gina , Weltmer Krirsten , Williams Tricia , Zerr Danielle 
Published: December 2024

ImportanceDuring the 2023-2024 respiratory syncytial virus (RSV) season in the United States, 2 new RSV prevention products were recommended to protect infants in their first RSV season: nirsevimab and Pfizer’s maternal RSV vaccine. Postlicensure studies are needed to assess prevention product impact and effectiveness.ObjectiveTo compare the epidemiology and disease burden of medically attended RSV-associated acute respiratory illness (ARI) among children younger than 5 years during the 2023-2024 RSV season with 3 prepandemic RSV seasons (2017-2020), estimate nirsevimab effectiveness against medically attended RSV-associated ARI, and compare nirsevimab binding site mutations among circulating RSV in infants with and without nirsevimab receipt.Design, Setting, and ParticipantsThis study included prospective population-based surveillance for medically attended ARI with systematic molecular testing for RSV and whole-genome sequencing of RSV positive samples, as well as a test-negative case-control design to estimate nirsevimab effectiveness. The study was conducted in 7 academic pediatric medical centers in the United States with data from RSV seasons (September 1 through April 30) in 2017 through 2020. Participants were children younger than 5 years with medically attended ARI.ExposureFor the nirsevimab effectiveness analyses, nirsevimab receipt among infants younger than 8 months as of or born after October 1, 2023.Main Outcome and MeasureMedically attended RSV-associated ARI.ResultsOverall, 28 689 children younger than 5 years with medically attended ARI were enrolled, including 9536 during September 1, 2023, through April 30, 2024, and 19 153 during the same calendar period of 2017-2020. Of these children, 16 196 (57%) were male, and 12 444 (43.4) were female; the median (IQR) age was 15 (6-29) months. During 2023-2024, the proportion of children with RSV was 23% (2199/9490) among all medically attended episodes, similar to 2017-2020. RSV-associated hospitalization rates in 2023-2024 were similar to average 2017-2020 seasonal rates with 5.0 (95% CI, 4.6-5.3) per 1000 among children younger than 5 years; the highest rates were among children aged 0 to 2 months (26.6; 95% CI, 23.0-30.2). Low maternal RSV vaccine uptake precluded assessment of effectiveness. Overall, 10 of 765 case patients (1%) who were RSV positive and 126 of 851 control patients (15%) who were RSV negative received nirsevimab. Nirsevimab effectiveness was 89% (95% CI, 79%-94%) against medically attended RSV-associated ARI and 93% (95% CI, 82%-97%) against RSV-associated hospitalization. Among 229 sequenced specimens, there were no differences in nirsevimab binding site mutations by infant nirsevimab receipt status.Conclusions and RelevanceThis analysis documented the continued high burden of medically attended RSV-associated ARI among young children in the US. There is a potential for substantial public health impact with increased and equitable prevention product coverage in future seasons.