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Effectiveness and Impact of Maternal RSV Immunization and Nirsevimab on Medically Attended RSV in US Children
Heidi L Moline1,2, Ayzsa Tannis1, Leah Goldstein1
1Coronavirus and Other Respiratory Viruses Division, National Center for Immunization and Respiratory Diseases, US Centers for Disease Control and Prevention, Atlanta, Georgia.
Insights
Nirsevimab and maternal respiratory syncytial virus (RSV) vaccination significantly reduced infant hospitalizations during the 2024-2025 season. These interventions proved effective in preventing severe RSV illness in young children.
Area of Science:
- Pediatric Infectious Diseases
- Vaccinology
- Public Health Surveillance
Background:
- The 2024-2025 respiratory syncytial virus (RSV) season saw the introduction of nirsevimab and maternal RSV vaccination for infant protection.
- Real-world data on the effectiveness and impact of these interventions are crucial for informing public health policy.
Purpose of the Study:
- To assess the real-world effectiveness of nirsevimab and maternal RSV vaccination against medically attended RSV-associated acute respiratory illness (ARI).
- To estimate the impact of these products on RSV-associated hospitalizations during the 2024-2025 season.
Main Methods:
- Population-based surveillance of medically attended ARI in children under 2 years old with molecular RSV testing.
- A test-negative case-control design was employed to evaluate maternal RSV vaccine and nirsevimab effectiveness.
- Effectiveness was calculated as (1 - adjusted odds ratio) × 100%; population impact was assessed by comparing hospitalization rates to pre-intervention seasons.
Main Results:
- Maternal RSV vaccine effectiveness was 64% against medically attended RSV-associated ARI and 70% against hospitalization in infants under 6 months.
- Nirsevimab demonstrated 81% effectiveness against RSV-associated hospitalization, maintaining 77% effectiveness up to 210 days post-receipt.
- RSV-associated hospitalizations decreased by 41%-51% in infants aged 0-11 months, with the greatest reduction (56%-63%) in those aged 0-2 months.
Conclusions:
- Both nirsevimab and maternal RSV vaccination were effective in protecting infants against RSV-associated hospitalizations during their first RSV season.
- Introduction of these products led to substantial reductions in RSV-associated hospitalization rates among young infants, halving rates compared to previous seasons.
Importance:
During the 2024-2025 respiratory syncytial virus (RSV) season in the US, nirsevimab and maternal RSV vaccination became widely available to prevent severe RSV disease in infants. Assessments of the real-world effectiveness and impact of both products are needed to inform RSV prevention policy.
Objectives:
To estimate nirsevimab and maternal RSV vaccine effectiveness against medically attended RSV-associated acute respiratory illness (ARI) and to estimate the impact of these products on RSV-associated hospitalizations during 2024-2025.
Design, Setting, And Participants:
Population-based surveillance for medically attended ARI was conducted among children younger than 2 years with systematic molecular testing for RSV. Children were enrolled at 7 US pediatric medical centers from October 1, 2024, through April 30, 2025. A test-negative case-control design was used to estimate maternal RSV vaccine and nirsevimab effectiveness.
Exposures:
To estimate maternal RSV vaccine effectiveness, the exposure was maternal RSV vaccination among newborns and infants younger than 6 months at medical encounters; to estimate nirsevimab effectiveness, the exposure was nirsevimab receipt among newborns and infants younger than 8 months as of October 1, 2024, or born after that date.
Main Outcomes And Measures:
The primary outcome was medically attended RSV-associated ARI and RSV-associated hospitalization. Immunization effectiveness was calculated as (1 - adjusted odds ratio) × 100%. To estimate population-level impact of RSV prevention products, relative rate reductions were estimated by comparing observed RSV-associated hospitalization rates during 2024-2025 to (1) observed rates in 2017-2020 or (2) counterfactual 2024-2025 rates estimated by a difference-in-differences approach; estimates from both approaches are presented as ranges.
Results:
Overall, 5029 children younger than 2 years with medically attended ARI were enrolled during October 2024 to April 2025. Median (IQR) age was 10 months (5-16 months), and 2176 children (43.3%) were female. Among newborns and infants younger than 6 months, maternal RSV vaccine effectiveness was 64% (95% CI, 37%-79%) against any medically attended RSV-associated ARI and 70% (95% CI, 37%-86%) against RSV-associated hospitalization. Nirsevimab effectiveness was 81% (95% CI, 71%-87%) against RSV-associated hospitalization, and nirsevimab remained 77% (95% CI, 42%-92%) effective against RSV-associated hospitalization at 130 to 210 days after receipt. RSV-associated hospitalizations were reduced by 41% to 51% among newborns and infants aged 0 to 11 months, with the highest reduction of 56% to 63% in those aged 0 to 2 months.
Conclusions And Relevance:
According to the results of this population-based surveillance study, during 2024-2025, both maternal RSV vaccine and nirsevimab were estimated to be effective at protecting infants from RSV-associated hospitalizations in their first RSV season, and RSV-associated hospitalization rates in newborns and infants aged 0 to 11 months were reduced by up to half compared to seasons before these products were introduced.
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