Research Summary

Nirsevimab or Maternal RSV Vaccination Are Highly Effective in US Infants

Edited by:
Ashton L. Stahl

Key Highlights

  • Nirsevimab was 62% effective against RSV-associated emergency department (ED) encounters and 77% effective against hospitalization.
  • Maternal RSV vaccination was 49% effective against RSV-associated ED encounters and 82% effective against hospitalization.
  • Both strategies were also protective against RSV-associated critical illness during infants’ first RSV season.

Nirsevimab and maternal respiratory syncytial virus (RSV) vaccination were effective against RSV-associated emergency department (ED) encounters and hospitalization among US infants during the 2024-2025 RSV season, according to a study published in Pediatrics. The study evaluated infants during their first RSV season, when uptake of both prevention strategies was higher than during the previous season.

The researchers conducted separate test-negative analyses using electronic health record data from 5 US healthcare systems participating in the Virtual SARS-CoV-2, Influenza, and Other Respiratory Viruses Network (VISION). The network included 108 EDs and 104 hospitals.

The analyses included ED encounters and hospitalizations for RSV-like illness from October 2024 through March 2025. Investigators compared RSV-positive and RSV-negative encounters according to immunization status using logistic regression models adjusted for age, race and ethnicity, sex, calendar day, and geographic region.

Study Findings

The nirsevimab analysis included 3531 ED encounters and 470 hospitalizations. Of these, 1078 ED encounters and 235 hospitalizations were RSV-positive. Approximately one-third of ED encounters and 29% of hospitalizations involved infants who had received nirsevimab.

Nirsevimab was 62% effective against RSV-associated ED encounters (95% CI, 54%-68%) and 77% effective against hospitalization (95% CI, 61%-87%). Its estimated effectiveness against RSV-associated critical illness, defined as intensive care unit admission or in-hospital death, was 79% (95% CI, 47%-92%).

The maternal RSV vaccine analysis included 810 ED encounters and 187 hospitalizations, of which 273 and 105, respectively, were RSV-positive. Maternal RSV vaccination was 49% effective against RSV-associated ED encounters (95% CI, 26%-65%) and 82% effective against hospitalization (95% CI, 54%-93%). Estimated effectiveness against critical illness was 86% (95% CI, 15%-98%).

Clinical Implications

According to the study authors, the findings highlight the potential of infant RSV immunization to reduce medically attended illness, including RSV-associated critical illness. They emphasized the importance of continuing to monitor the effectiveness of RSV prevention products during future seasons to inform recommendations and better understand the duration of protection.

The authors noted that possible misclassification of immunization or RSV status could have biased the estimates. Excluding untested encounters may have limited generalizability, and residual confounding from unmeasured factors remained possible.

Expert Commentary

“Our analyses of ED and inpatient encounters among infants in their first RSV season during the 2024 to 2025 RSV season in the United States indicate infant RSV immunization, through either postnatal nirsevimab administration or maternal vaccination, was effective against RSV-associated ED encounters and hospitalization,” the researchers concluded.

Reference

Payne AB, Battan-Wraith S, Reese SE, et al. Effectiveness of RSV prevention strategies in US infants: 2024-2025. Pediatrics. 2026;158(3):e2026076089. doi:10.1542/peds.2026-076089