What are the indication, weight‑based dosing, administration route, and contraindications for Beyfortus (nirsevimab) as prophylaxis against respiratory syncytial virus in infants ≤12 months of age?

Beyfortus (Nirsevimab) for RSV Prophylaxis

Beyfortus is indicated for all infants <8 months born during or entering their first RSV season, and for high-risk children aged 8-19 months entering their second RSV season, administered as a single intramuscular injection with weight-based dosing (50 mg for <5 kg, 100 mg for ≥5 kg in first season; 200 mg for second season), and is contraindicated only in those with prior severe hypersensitivity reactions to nirsevimab or its components. 1, 2, 3

Indications

First RSV Season (Universal Recommendation)

• All infants aged <8 months who are born during or entering their first RSV season should receive nirsevimab, regardless of gestational age or underlying medical conditions. 4, 2, 3
• This represents a major paradigm shift from palivizumab, which was restricted to high-risk infants comprising <5% of the population. 2, 4
• The universal recommendation is based on the fact that 79% of infants hospitalized with RSV have no underlying medical conditions. 2

Second RSV Season (High-Risk Only)

• Children aged 8-19 months who remain at increased risk for severe RSV disease and are entering their second RSV season should receive nirsevimab. 2, 4
• High-risk populations include: 3, 4
  • Chronic lung disease of prematurity requiring medical support
  • Hemodynamically significant congenital heart disease
  • Severe immunocompromise
  • Cystic fibrosis with severe lung disease or poor growth
  • American Indian or Alaska Native children

Weight-Based Dosing

First RSV Season

• 50 mg (0.5 mL) for infants weighing <5 kg 1
• 100 mg (1 mL) for infants weighing ≥5 kg 1

Second RSV Season

• 200 mg administered as two separate 100 mg injections 1

Administration Route and Timing

Route

• Administer as a single intramuscular injection 1, 2, 3

Timing

• Give shortly before or during the RSV season, typically October through the end of March in most of the continental United States. 4, 2, 3
• Do not delay administration waiting for "peak" RSV season—protection is optimal when given before or at the start of the season. 4
• American Indian/Alaska Native children in Alaska should consult state or local guidance due to less predictable and often longer RSV seasonality. 4
• Protection extends through 150 days post-injection, covering an entire RSV season with a single dose. 4, 5

Contraindications

• History of serious hypersensitivity reactions, including anaphylaxis, to nirsevimab-alip or any excipients 1, 3
• This is the only absolute contraindication listed in the FDA-approved labeling. 1

Warnings and Precautions

Hypersensitivity Reactions

• Serious hypersensitivity reactions including urticaria, dyspnea, cyanosis, and/or hypotonia have been reported following administration. 1
• Anaphylaxis has been observed with human IgG1 monoclonal antibodies. 1
• If signs or symptoms of anaphylaxis or other clinically significant hypersensitivity reactions occur, initiate appropriate treatment immediately. 1

Bleeding Disorders

• Theoretical risk of bleeding complications in patients with bleeding disorders; special precautions are recommended. 3

Common Adverse Reactions

• Rash (0.9%) and injection site reactions (0.3%) are the most common adverse events. 1
• Treatment-related adverse events occurred in 2.1% of infants receiving nirsevimab. 6
• Serious adverse events were reported in 6.8% of infants receiving nirsevimab versus 7.3% receiving placebo, indicating no increased risk. 5

Clinical Efficacy

• Nirsevimab demonstrated 83.2% efficacy in preventing hospitalization for RSV-associated lower respiratory tract infection in real-world conditions. 6
• 74.5% efficacy in preventing medically attended RSV-associated lower respiratory tract infection in healthy late-preterm and term infants. 5
• 75.7% efficacy against very severe RSV-associated lower respiratory tract infection requiring supplemental oxygen. 6

Critical Implementation Points

• Do not restrict first-season nirsevimab to only high-risk infants—the recommendation is universal for all infants <8 months. 4, 2
• Do not give second-season doses to healthy children without specific risk factors—this is only for high-risk populations. 4
• Unlike palivizumab, nirsevimab requires only a single dose for the entire RSV season rather than monthly injections. 3, 4
• Adverse events should be reported to FDA MedWatch or to VAERS if co-administered with vaccines. 3

Age Limitations

• The safety and effectiveness of nirsevimab in children older than 24 months have not been established. 1
• No safety or efficacy data supports use in adult populations. 3