Maternal Abrysvo (RSVpreF) Vaccine Efficacy Against RSV Hospitalization in Infants
Maternal RSVpreF vaccination during pregnancy is highly effective at preventing RSV-related hospitalizations in infants, with vaccine efficacy of approximately 57-82% depending on the specific outcome and timing measured.
Primary Efficacy Data
The FDA-approved Abrysvo vaccine demonstrates robust protection against severe RSV disease in infants when administered to pregnant persons at 32-36 weeks' gestation 1:
Severe RSV Lower Respiratory Tract Disease (LRTD)
- 69.4% efficacy (97.58% CI: 44.3-84.1) at preventing severe RSV-LRTD through 180 days of infant age 1
- 81.8% efficacy (99.5% CI: 40.6-96.3) through 90 days of infant age 1
- When vaccination occurred specifically at 32-36 weeks (the FDA-approved window), efficacy reached 76.5% (95% CI: 41.3-92.1) at 180 days 1
RSV-Related Hospitalizations
- 56.8% efficacy (99.17% CI: 10.1-80.7) at preventing RSV hospitalizations through 180 days 1
- This translates to preventing approximately 11 hospitalizations per 1000 infants born to vaccinated mothers 2
General RSV LRTD (Less Severe Disease)
- 51.3% efficacy (97.58% CI: 29.4-66.8) at preventing any medically attended RSV-LRTD through 180 days 1
- Note: This endpoint did not meet the prespecified statistical success criterion at 90 days, though clinically meaningful protection emerged after 90 days 3
Real-World Effectiveness Data
Recent population-based studies confirm the clinical trial findings and demonstrate even higher effectiveness in real-world settings:
- Scotland (2024-2025): 82.2% effectiveness (95% CI: 75.1-87.3) against RSV-associated LRTI hospitalizations in infants ≤90 days, with 219 hospitalizations averted during the study period 4
- Argentina (2024): 78.7% effectiveness (95% CI: 51.4-90.7) against RSV-associated hospitalizations in infants <6 months 5
Both real-world studies showed effectiveness remained high in preterm infants, with Scotland reporting 89.9% effectiveness (95% CI: 55.3-97.7) in infants born <37 weeks' gestation 4.
Clinical Context and Recommendations
The CDC/ACIP recommends maternal RSVpreF vaccination as a one-time dose at 32-36 weeks' gestation using seasonal administration (September through January in most of the continental U.S.) to prevent RSV-associated LRTI in infants aged <6 months 2.
Key Implementation Points:
- Timing matters: Vaccination should occur >14 days before delivery for optimal protection. Vaccination ≤14 days before delivery provides suboptimal immunization 4
- Either/or strategy: Either maternal RSVpreF vaccination OR infant nirsevimab is recommended—both are not needed for most infants 2
- Seasonal administration: Vaccination should align with RSV season timing in your geographic region 2
Important Safety Considerations:
The FDA labeled a warning for potential preterm birth risk based on clinical trial observations of more preterm births (<37 weeks) in vaccine recipients versus placebo, though differences were not statistically significant 2. This is why the FDA restricted approval to 32-36 weeks' gestation rather than the broader 24-36 weeks studied in trials—to avoid potential preterm birth at <32 weeks, which carries substantially higher morbidity and mortality 2.
The Cochrane review found maternal RSV vaccination may increase preterm birth risk (RR 1.16,95% CI: 0.99-1.36), though the evidence is very uncertain and the confidence interval includes no effect 6. This safety signal warrants ongoing surveillance but should not prevent vaccination at the approved gestational window, as the FDA determined benefits outweigh risks when administered at 32-36 weeks 2.
Comparative Effectiveness:
One recent French study suggests nirsevimab may provide superior protection compared to maternal RSVpreF vaccination (adjusted HR 0.74,95% CI: 0.61-0.88 favoring nirsevimab) 7. However, this represents only one RSV season in one country, and the choice between strategies should consider local availability, programmatic factors, and individual patient circumstances.
Clinical Impact
RSV is the leading cause of hospitalization among U.S. infants, with approximately 58,000-80,000 RSV-associated hospitalizations and 100-300 deaths annually in children <5 years 2. Hospitalization rates peak at 1 month of age, and 79% of hospitalized children <2 years have no underlying medical conditions 2. Maternal vaccination provides substantial protection during this highest-risk period, with efficacy strongest in the first 90 days of life when infants are most vulnerable.