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. 2025 Mar 3;8(3):e250380.
doi: 10.1001/jamanetworkopen.2025.0380.

Estimated Effectiveness of Nirsevimab Against Respiratory Syncytial Virus

Hanmeng Xu  1 , Camila Aparicio  2 , Aanchal Wats  2 , Barbara L Araujo  2 , Virginia E Pitzer  1 , Joshua L Warren  3 , Eugene D Shapiro  1   2 , Linda M Niccolai  1 , Daniel M Weinberger  1 , Carlos R Oliveira  2   4   5
Affiliations

Estimated Effectiveness of Nirsevimab Against Respiratory Syncytial Virus

Hanmeng Xu et al. JAMA Netw Open. .

Erratum in

  • Errors in Results, Figures, and Supplement 1.
    [No authors listed] [No authors listed] JAMA Netw Open. 2025 Apr 1;8(4):e2512578. doi: 10.1001/jamanetworkopen.2025.12578. JAMA Netw Open. 2025. PMID: 40266624 Free PMC article. No abstract available.

Abstract

Importance: Nirsevimab, a long-acting monoclonal antibody, demonstrated efficacy against respiratory syncytial virus (RSV)-associated lower respiratory tract infections (LRTI) in clinical trials. Postlicensure monitoring is essential to confirm these benefits in clinical settings.

Objective: To estimate the effectiveness of nirsevimab against medically attended RSV infections in infants and to assess how effectiveness varies by disease severity, dosage, and time since immunization.

Design, setting, and participants: This test-negative case-control study utilized inpatient, outpatient, and emergency department data from the Yale New Haven Health System. Nirsevimab-eligible infants who were tested for RSV using polymerase chain reaction between October 1, 2023, and May 9, 2024, were included. Infants with RSV-positive results were cases and infants with RSV-negative results were controls.

Exposure: Nirsevimab immunization, verified through state immunization registries.

Main outcomes and measures: Effectiveness was estimated using multivariable logistic regression, adjusting for age, calendar month, and potential confounders. Separate models examined estimated effectiveness by clinical setting, dosage, time since immunization, and severity (defined as needing high-flow oxygen or intensive care unit admission). Broader outcomes were also analyzed, including all-cause LRTI and all-cause LRTI-associated hospitalization.

Results: The analytic sample included 3090 infants (1722 male [57.3%]; median [IQR] age at testing, 6.7 [3.6-9.7] months), with 680 (22.0%) RSV-positive cases and 2410 (78.0%) RSV-negative controls. Nirsevimab uptake was 10.7% (330 patients), with 21 RSV-positive cases and 309 RSV-negative controls immunized. Adjusted effectiveness was 68.4% (95% CI, 50.3%-80.8%) against medically attended RSV infection, 61.6% (95% CI, 35.6%-78.6%) against outpatient visits, and 80.5% (95% CI, 52.0%-93.5%) against hospitalizations. The highest estimated effectiveness (84.6%; 95% CI, 58.7%-95.6%) was observed against severe RSV disease. Although estimated effectiveness against RSV infections declined from 79.3% (95% CI, 63.4%-90.6%) at 2 weeks postimmunization to 54.8% (95% CI, 16.3%-74.7%) at 14 weeks postimmunization, it remained significant. Estimated effectiveness did not vary substantially by dosage. During peak RSV season, nirsevimab appeared effective against all-cause LRTI (49.4%; 95% CI, 10.7%-72.9%) and all-cause LRTI-associated hospitalizations (79.1%; 95% CI, 27.6%-94.9%). From February to May 2024, when most LRTIs were caused by other viruses, its estimated effectiveness against these broader outcomes was negligible.

Conclusions and relevance: In this case-control study, nirsevimab provided substantial protection against RSV-associated outcomes. These findings support its continued use and provide evidence that may help boost public confidence in the immunization program.

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Conflict of interest statement

Conflict of Interest Disclosures: Dr Pitzer reported receiving grants from the National of Institutes of Health outside the submitted work. Dr Warren reported receiving consulting fees from Pfizer outside the submitted work. Dr Niccolai reported receiving personal fees from Merck and GSK outside the submitted work. Dr Weinberger reported receiving grants from the National Institutes of Health, Pfizer, Merck, and GSK, and personal fees from Pfizer, Merck, and GSK outside the submitted work. No other disclosures were reported.

Figures

Figure 1.
Figure 1.. Selection Process of Respiratory Syncytial Virus (RSV) Test Records
ARI indicates acute respiratory infection. aIndividuals aged older than 8 months on October 1, 2023, (when nirsevimab became available) but did not have risk factors for severe RSV disease.
Figure 2.
Figure 2.. Estimated Effectiveness of Nirsevimab Against Medically Attended Respiratory Syncytial Virus (RSV) by Clinical Setting and Severity
Adjusted models controlled for age, calendar month, and other potential confounders. Only hospitalizations and intensive care unit admissions with an admission date within 14 days of RSV testing were included in the analysis.
Figure 3.
Figure 3.. Estimated Effectiveness of Nirsevimab by Time Since Immunization, Estimated From a Bayesian Framework
The boxes represent the median estimates of the effectiveness of nirsevimab in preventing various clinical outcomes (A-D), and the error bars indicate the 95% credible intervals (95% CrIs) of the estimates. The numbers in the second and third columns indicate the number of participants immunized a certain period of time before being tested for RSV.
Figure 4.
Figure 4.. Nirsevimab Estimated Effectiveness Against All-Cause Lower Resipiratory Tract Infection (LRTI) and All-Cause LRTI Hospitalization, Stratified by Time
For comparison with current study estimates, estimates from 5 previous studies,,,, are also included. Estimates were stratified by time (full season,,, peak months,, and off-peak months). Only the estimate for the age group 3 to 12 months was shown for Levy et al. Estimates were adjusted for age, calendar time, presence of at least 1 risk factor for severe respiratory syncytial virus disease.
See this image and copyright information in PMC

References

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