What is the efficacy of the respiratory syncytial virus (RSV) vaccine in infants?

RSV Protection in Infants: Maternal Vaccination vs. Infant Monoclonal Antibody

Infants should be protected against RSV through either maternal RSVpreF vaccination during pregnancy (32-36 weeks' gestation) or nirsevimab monoclonal antibody administration after birth, but both products are not needed for most infants. 1

Direct Answer on Efficacy

The maternal RSVpreF vaccine demonstrates significant efficacy when administered at 32-36 weeks' gestation during September-January in most of the continental United States. 1 The FDA approved this vaccine specifically to prevent RSV-associated lower respiratory tract disease and severe lower respiratory tract disease in infants aged <6 months. 1

Key efficacy consideration: At least 14 days are required after maternal vaccination for adequate antibody development and transplacental transfer to protect the infant. 2, 3 This means the earliest an infant can be considered protected is if born at 34 weeks' gestation or later when the vaccine is given at the earliest recommended time of 32 weeks. 2

Comparative Protection Strategies

Maternal RSVpreF Vaccination

• Provides protection immediately after birth 1
• Produces a polyclonal immune response that is more resistant to potential RSV F protein mutations compared to monoclonal antibodies 1
• Protection likely wanes after 3 months, similar to what has been observed with maternal influenza and COVID-19 vaccines 1

Nirsevimab (Infant Monoclonal Antibody)

• Recommended for infants <8 months born during or entering their first RSV season 1, 2
• Protection may wane more slowly than maternal vaccine-derived antibodies 1, 2
• Assures direct receipt of antibodies rather than relying on transplacental transfer 1, 2
• No risk for adverse pregnancy outcomes 1, 2

Critical Safety Considerations

The FDA labeled a potential risk for preterm birth as a warning with maternal RSVpreF vaccination. 1 Clinical trials showed more preterm births (<37 weeks' gestation) among vaccine recipients than placebo recipients, though differences were not statistically significant. 1 The vaccine was approved for use at 32-36 weeks' gestation specifically to avoid potential preterm birth risk at <32 weeks' gestation, which is associated with increased morbidity and mortality. 1

More hypertensive disorders of pregnancy were observed among vaccine recipients compared with placebo recipients, though not statistically significant. 1 The FDA determined that when administered at 32-36 weeks' gestation, the benefits of preventing RSV-associated lower respiratory tract infections in infants outweigh these risks. 1

Clinical Decision Algorithm

Step 1: Determine if maternal vaccination window is available

• If pregnant person is between 32 weeks 0 days and 36 weeks 6 days during September-January → offer maternal RSVpreF vaccination 1
• If beyond 36 weeks 6 days or outside seasonal window → plan for infant nirsevimab after delivery 2

Step 2: Special populations requiring nirsevimab regardless

• Infants born at <34 weeks' gestation 2
• Infants whose mothers are immunocompromised 2
• Infants whose mothers did not receive vaccine or vaccination status unknown 2, 4
• Infants whose mothers received vaccine <14 days before birth 2, 3

Step 3: High-risk children requiring second-season nirsevimab

• Children aged 8-19 months with chronic lung disease of prematurity requiring medical support 4
• Children with severe immunocompromise 4
• Children with cystic fibrosis with severe lung disease or poor growth 4
• American Indian or Alaska Native children 4

Common Pitfalls to Avoid

Do NOT administer both maternal RSV vaccine and infant nirsevimab — both products are not needed for most infants. 1, 2 This is the single most important pitfall to avoid.

Do NOT give maternal vaccine outside the 32-36 week gestational window — the FDA specifically restricted approval to this window due to preterm birth concerns. 1

Do NOT assume protection if vaccine given <14 days before birth — inadequate time for antibody development and transfer means the infant should receive nirsevimab instead. 2, 3

Do NOT forget seasonal timing — maternal vaccination should only occur September-January in most of the continental United States (Alaska, southern Florida, Guam, Hawaii, Puerto Rico, U.S.-affiliated Pacific Islands, and U.S. Virgin Islands have different timing). 1

Practical Implementation Considerations

Maternal vaccination can be co-administered with other recommended vaccines including Tdap, influenza, and COVID-19 vaccines at different anatomic sites on the same day. 1, 3

No data currently exist on efficacy of first lifetime dose to protect infants born after subsequent pregnancies or safety of additional doses during subsequent pregnancies. 1 ACIP will update recommendations as data become available. 1

Nirsevimab availability should be confirmed as there were supply concerns during the 2023-24 RSV season. 2 Unlike palivizumab, nirsevimab requires only a single dose for the entire RSV season rather than monthly injections. 4

Contraindications

Both products are contraindicated in persons with history of severe allergic reaction (anaphylaxis) to any vaccine component or previous dose. 2, 4 Vaccination should be delayed for moderate or severe acute illness with or without fever. 2