RSV Immunization (Nirsevimab)
Content reviewed against published CDC, AAP, WHO guidelines
Last reviewed:
Key takeaways
- Prevents: Respiratory Syncytial Virus (RSV) lower respiratory tract infection, RSV bronchiolitis
- Schedule: 2 doses starting at One dose for all infants under 8 months entering their first RSV season (typically October-March)
- Effectiveness: In clinical trials, nirsevimab reduced RSV-associated lower respiratory tract infections requiring medical attention by approximately 79% and RSV hospitalizations by approximately 77-83% through the RSV season.
- Most common side effect: Injection site reactions
How it works
Nirsevimab (Beyfortus) is a long-acting monoclonal antibody, not a traditional vaccine. It provides passive immunization by directly supplying pre-formed antibodies that target the RSV F protein in its prefusion form. Unlike vaccines that stimulate the baby's immune system to produce its own antibodies, nirsevimab gives the baby ready-made protection that lasts approximately 5 months — covering one RSV season.
Recommended schedule
| Dose | Age |
|---|---|
| Dose 1 | One dose for all infants under 8 months entering their first RSV season (typically October-March) |
| Dose 2 | One dose for high-risk children 8-19 months entering their second RSV season |
Side effects
Common (mild, resolve in 1-2 days)
- Injection site reactions (redness, swelling, or tenderness) — 0.3%
- Rash (0.9%)
- Mild fever (rare)
- Note: Side effects are less common than with traditional vaccines because monoclonal antibodies do not activate the immune system
Serious (very rare)
- Severe allergic reaction (anaphylaxis) — extremely rare
- Thrombocytopenia — reported very rarely in clinical trials
- Clinical trials (over 3,000 infants) showed no significant difference in serious adverse events between nirsevimab and placebo groups
Who should NOT get this vaccine
- Known serious hypersensitivity to nirsevimab or any component
- Note: Prematurity, congenital heart disease, chronic lung disease, and immunodeficiency are NOT contraindications — these are actually indications for nirsevimab
- Note: Can be given alongside routine childhood vaccines
Effectiveness
In clinical trials, nirsevimab reduced RSV-associated lower respiratory tract infections requiring medical attention by approximately 79% and RSV hospitalizations by approximately 77-83% through the RSV season. Protection lasts approximately 5 months from the single injection.
Available brands
Beyfortus
Sanofi and AstraZeneca
Myths vs. facts
Myth: “Nirsevimab is a vaccine that could overwhelm my baby's immune system”
Fact: Nirsevimab is NOT a vaccine — it is a monoclonal antibody (passive immunization). It does not stimulate the baby's immune system at all. It simply provides pre-made antibodies that neutralize RSV directly, similar to antibodies a mother passes to her baby during pregnancy.
Myth: “Only premature babies need RSV protection”
Fact: RSV hospitalizes approximately 58,000-80,000 US children under 5 each year. Most hospitalized infants were born full-term with no underlying conditions. The CDC recommends nirsevimab for ALL infants under 8 months entering their first RSV season.
Myth: “Nirsevimab replaces the need for other preventive measures”
Fact: While nirsevimab significantly reduces RSV risk, families should still practice hand hygiene, avoid exposing young infants to sick contacts, and keep infants away from cigarette smoke — all of which reduce respiratory infection risk.
Frequently asked questions
What is the difference between nirsevimab and palivizumab (Synagis)?
When should my baby receive nirsevimab?
Can my baby get nirsevimab along with other vaccines?
Does my baby still need nirsevimab if I received the RSV vaccine during pregnancy?
Sources
All content follows our editorial policy and is reviewed against published clinical guidelines.
2,705 evidence-based guides6 authoritative medical sources
Bottom line
The RSV Immunization (Nirsevimab) (RSV-mAb) is safe, well-studied, and recommended by the CDC, AAP, and WHO. Talk to your pediatrician if you have questions about timing or your child's specific health needs.
Choosing to vaccinate your child is one of the most important things you can do to protect their health. If you have concerns, your pediatrician is always happy to talk through them with you.