What is the efficacy of the RSV monoclonal antibody nirsevimab in preventing medically‑attended lower respiratory tract infection and hospitalization in infants?

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Last updated: November 3, 2026 • View editorial policy

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Efficacy of Nirsevimab for RSV Prevention in Infants

Nirsevimab demonstrates approximately 70-80% effectiveness against RSV-associated hospitalizations in real-world settings, with clinical trial data showing 74.5% efficacy against medically-attended RSV lower respiratory tract infections and 90.9% efficacy against RSV-related hospitalizations in healthy term and late preterm infants.

Clinical Trial Efficacy Data

The FDA-approved nirsevimab (Beyfortus) has been rigorously evaluated in multiple randomized controlled trials 1:

Primary Efficacy Outcomes

For healthy term and late preterm infants (≥35 weeks gestational age):

  • 74.9% efficacy (95% CI: 50.6-87.3%) against medically-attended RSV lower respiratory tract infection through 150 days

    • Occurred in 1.2% (12/994) of nirsevimab recipients vs 5.0% (25/496) of placebo recipients 1

  • 60.2% efficacy (95% CI: -14.6 to 86.2%) against RSV-related hospitalization, though this did not reach statistical significance (p=0.09) 1

For preterm infants (<35 weeks) and high-risk infants:

  • Efficacy was established through pharmacokinetic extrapolation from the term infant trials
  • In the safety trial (Trial 05), incidence of medically-attended RSV LRTI was 0.6% (4/616) in nirsevimab recipients vs 1.0% (3/309) in palivizumab recipients 1

Extended Duration Efficacy

A pooled analysis of phase 2b and MELODY trials demonstrated sustained protection 2:

  • 79.5% relative risk reduction (95% CI: 65.9-87.7%) for medically-attended RSV LRTI
  • 77.3% reduction (95% CI: 50.3-89.7%) for RSV hospitalization
  • 86.0% reduction (95% CI: 62.5-94.8%) for very severe RSV requiring oxygen or IV fluids

The HARMONIE trial showed 82.7% efficacy (95% CI: 67.8-91.5%) against RSV hospitalization through 180 days, demonstrating protection extending beyond the typical 5-month RSV season 3.

Real-World Effectiveness

Real-world data consistently shows effectiveness meeting or exceeding clinical trial results 4:

  • 70-80% protection against RSV-associated hospitalizations in infants based on multiple effectiveness studies 4

  • During the 2024-2025 RSV season (first full implementation season), RSV hospitalization rates in infants aged 0-7 months were 43% and 28% lower compared to pre-COVID pandemic seasons, according to two national surveillance networks 4

  • A Kaiser Permanente study showed 87.2% effectiveness (95% CI: 81.7-91.1%) against RSV LRTI and 98.0% effectiveness (95% CI: 85.1-99.7%) against hospitalized RSV LRTI 5

  • Meta-analysis of real-world evidence confirmed 84.5% reduction in RSV-LRTI hospitalization risk and 85.9% reduction in ICU admissions 6

Additional Clinical Benefits

Beyond the primary outcomes, nirsevimab recipients demonstrated 2:

  • 43.8% reduction in any-cause respiratory illness hospitalizations
  • 35.4% reduction in any-cause medically-attended LRTI
  • 41.9% reduction in LRTI outpatient visits
  • 23.6% reduction in antibiotic prescriptions

Importantly, infants who received nirsevimab but still developed RSV LRTI had significantly milder disease, with fewer medical encounters (adjusted mean difference -0.86; p=0.001) and lower odds of hospitalization (OR 0.11; 95% CI: 0.01-0.85) compared to non-immunized infants 5.

Current ACIP Recommendations

As of June 2025, ACIP recommends that all infants be protected against RSV through one of three options 4:

  1. Maternal RSV vaccination during pregnancy (32-36 weeks gestation)
  2. Infant receipt of nirsevimab
  3. Infant receipt of clesrovimab (newly approved alternative)

No single product is preferred—selection should be based on parent preference, product availability, and timing relative to RSV season 4.

Safety Profile

Nirsevimab has demonstrated a favorable safety profile across all trials 4:

  • Serious adverse events occurred at similar rates in nirsevimab (11.5%) and placebo (12.4%) groups (RR 0.93; 95% CI: 0.77-1.12)
  • Most adverse events were mild to moderate, with irritability and somnolence being most common
  • Antidrug antibodies developed in 6.1% of recipients but did not impact efficacy 7

Clinical Implications

The evidence unequivocally supports nirsevimab as highly effective for preventing RSV disease in infants, with protection extending at least 6 months and covering the entire RSV season. The consistency between clinical trial efficacy and real-world effectiveness, combined with the excellent safety profile, establishes nirsevimab as a transformative intervention for reducing infant RSV morbidity and mortality 4.

References

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