Top 5 Takeaways

  1. High Effectiveness of Nirsevimab: Nirsevimab demonstrated a 90% effectiveness in preventing RSV-associated hospitalization among infants in their first RSV season.
  2. Median Time to Symptom Onset: The median time from receipt of Nirsevimab to the onset of RSV symptoms was 45 days, with an interquartile range of 19–76 days.
  3. Public Health Implications: To reduce RSV-associated hospitalization risks, infants should receive protection through maternal RSV vaccination or Nirsevimab administration.
  4. Study Methodology: The study used data from the New Vaccine Surveillance Network, evaluating Nirsevimab effectiveness among hospitalized infants with acute respiratory illness from October 1, 2023, to February 29, 2024.
  5. Support for Current Recommendations: This early effectiveness estimate reinforces the CDC’s recommendation for Nirsevimab to prevent severe RSV disease in infants during their first RSV season.

Original Article Author and Citation

Corresponding Author

Heidi L. Moline, MD,

Suggested Citation

Moline HL, Tannis A, Toepfer AP, et al. Early Estimate of Nirsevimab Effectiveness for Prevention of Respiratory Syncytial Virus–Associated Hospitalization Among Infants Entering Their First Respiratory Syncytial Virus Season — New Vaccine Surveillance Network, October 2023–February 2024. MMWR Morb Mortal Wkly Rep 2024;73:209–214. DOI:


This MMWR report provides an early estimate of Nirsevimab’s effectiveness in preventing RSV-associated hospitalizations among infants entering their first RSV season. The study, conducted by the New Vaccine Surveillance Network between October 2023 and February 2024, found a 90% effectiveness rate. This supports the CDC’s recommendation for Nirsevimab use in infants to mitigate the risk of severe RSV disease.


The analysis included 699 infants hospitalized with acute respiratory illness (ARI), with data collected through parent interviews, medical records, and state immunization systems. Nirsevimab recipients were identified if they received the drug ≥7 days before symptom onset.


The study’s findings underscore Nirsevimab’s potential to significantly reduce RSV-associated hospitalization among infants. However, the effectiveness is expected to decrease over time due to antibody decay, highlighting the importance of continued monitoring and research.


The early effectiveness of Nirsevimab in preventing RSV-associated hospitalization among infants supports the current public health recommendations. Continued efforts to promote Nirsevimab administration, alongside maternal RSV vaccination, are crucial to protect this vulnerable population.

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