Efficacy of Infant RSV Prevention Strategies
Direct Answer
Maternal RSVpreF vaccine administered at 32–36 weeks' gestation demonstrates 81.8% efficacy in preventing medically attended severe RSV-associated lower respiratory tract infection (LRTI) at 90 days and 69.4% efficacy at 180 days, while nirsevimab monoclonal antibody shows 79.0% efficacy in preventing medically attended RSV-associated LRTI and 80.6% efficacy in preventing RSV-associated LRTI requiring hospitalization. 1, 2
Maternal RSVpreF Vaccine Efficacy
Primary Efficacy Outcomes
Severe RSV-associated LRTI: The vaccine achieves 81.8% efficacy (99.5% CI: 40.6–96.3) at 90 days after birth and 69.4% efficacy (97.58% CI: 44.3–84.1) at 180 days in preventing medically attended severe lower respiratory tract disease 2
Overall medically attended RSV-associated LRTI: The vaccine demonstrates 57.3% efficacy when administered at 32–36 weeks' gestation 1
RSV hospitalization prevention: Efficacy reaches 48.2% in preventing RSV-related hospitalizations 1
Severe RSV-associated LRTI (FDA-approved indication): The FDA determined efficacy of 76.5% for preventing severe disease when the vaccine is given during the approved 32–36 week window 1
Mechanism and Timing Considerations
Transplacental antibody transfer begins within 14 days of maternal vaccination, meaning infants born at ≥34 weeks' gestation can be considered protected if the vaccine was administered at 32 weeks 3, 1
Protection from maternal antibodies wanes after approximately 3 months, similar to patterns observed with maternal influenza and COVID-19 vaccines 3
The polyclonal antibody response generated by maternal vaccination offers theoretical advantages in resilience against potential RSV F-protein mutations compared to monoclonal antibodies 3
Nirsevimab Monoclonal Antibody Efficacy
Primary Efficacy Outcomes
Medically attended RSV-associated LRTI: Nirsevimab achieves 79.0% efficacy in preventing medically attended RSV-associated lower respiratory tract infection 1
RSV-associated LRTI with hospitalization: The monoclonal antibody demonstrates 80.6% efficacy in preventing RSV-associated LRTI requiring hospitalization 1
RSV-associated LRTI with ICU admission: Efficacy reaches 90.0% in preventing RSV-associated LRTI requiring intensive care unit admission 1
Duration of protection: A single dose provides protection for approximately 150 days (5–6 months), covering the entire RSV season 1, 4
Extended efficacy data: Nirsevimab shows 76.8% efficacy (95% CI: 49.4–89.4) in preventing lower respiratory infection 150 days after administration, with a 90.1% reduction (95% CI: 16.4–98.8) in risk of ICU admission 4
Advantages of Nirsevimab
Protection wanes more slowly than maternal vaccine-derived antibodies, extending coverage throughout the RSV season 3, 1
Direct antibody administration eliminates reliance on transplacental transfer, ensuring consistent serum levels 3
No risk of adverse pregnancy outcomes, unlike the potential preterm birth warning associated with maternal vaccination 3
Comparative Context and Clinical Decision Algorithm
When to Choose Maternal Vaccination
Optimal timing: Administer RSVpreF vaccine between 32 weeks 0 days and 36 weeks 6 days' gestation during September through January in most continental U.S. regions 5, 1, 6
Minimum lead time: At least 14 days are required before delivery for adequate antibody development and transplacental transfer 3, 1
Efficacy advantage: Maternal vaccination provides slightly higher efficacy against severe disease (81.8% at 90 days) compared to nirsevimab's overall LRTI prevention 2
When to Choose Nirsevimab
Missed vaccination window: If currently beyond 36 weeks 6 days' gestation, plan for infant nirsevimab administration after delivery 3
Insufficient lead time: If delivery will occur within 14 days of potential vaccination, nirsevimab is the appropriate choice 3
Preterm infants: All infants born at <34 weeks' gestation should receive nirsevimab regardless of maternal vaccination status 1, 6
Immunocompromised mothers: Infants whose mothers are immunocompromised require nirsevimab regardless of maternal vaccination status 3
High-risk conditions: Consider nirsevimab for infants with hemodynamically significant congenital heart disease, chronic lung disease requiring medical intervention, or those who underwent cardiopulmonary bypass or extracorporeal membrane oxygenation 1
Critical Safety Considerations
Maternal Vaccination Risks
The FDA labeled a warning for potential preterm birth risk, with 5.7% preterm births in the vaccine group versus 4.7% in placebo during the 24–36 week trial dosing interval, though differences were not statistically significant 6
In the approved 32–36 week interval, preterm births occurred in 4.2% of vaccine recipients versus 3.7% of placebo recipients 6
Most preterm births in the approved interval occurred at 36 weeks' gestation, accounting for 72% of vaccine group preterm births 6
Hypertensive disorders of pregnancy were observed more frequently in vaccine recipients, though not statistically significant 5, 6
The FDA determined that benefits outweigh risks when administered at 32–36 weeks' gestation 5, 1, 6
Critical Pitfalls to Avoid
Do NOT administer both maternal RSVpreF vaccine and infant nirsevimab to the same infant—both products are not needed for most infants, and dual administration provides no additional benefit 5, 3, 1
Do NOT give maternal vaccine outside the 32–36 week gestational window due to the labeled preterm birth risk 3, 6
Do NOT assume adequate infant protection if vaccine is administered less than 14 days before delivery—in such cases, nirsevimab should be used instead 3
Do NOT vaccinate pregnant persons with history of severe allergic reaction (anaphylaxis) to any vaccine component 1
Seasonal timing is critical: maternal vaccination should be limited to September–January in most continental U.S. regions, with different timing for Alaska, southern Florida, Guam, Hawaii, Puerto Rico, U.S.-affiliated Pacific Islands, and the U.S. Virgin Islands 3
Real-World Implementation Data
In Wisconsin during the 2023–24 RSV season, only 18.2% of infants received nirsevimab and 17.2% of pregnant persons received maternal RSVpreF vaccine, with overall 36.2% of infants protected against RSV 7
These low uptake rates highlight the need for improved access and education to ensure adequate infant protection 7
Maternal RSVpreF vaccination can be co-administered on the same day with other pregnancy vaccines (Tdap, influenza, COVID-19) at separate anatomic sites without compromising safety or efficacy 3, 6