Generic Name

Jynarque

Mechanism

• RSV‑specific human monoclonal antibody: *nirsevimab* binds the fusion (F) glycoprotein of the Respiratory Syncytial Virus (RSV).
• Neutralization: Prevents viral attachment and fusion to respiratory epithelial cells, thereby blocking infection.
• Long half‑life: Fc engineering extends serum persistence, providing protection over a full RSV season with a single dose.

Pharmacokinetics

Indications

• Prophylaxis of RSV infection in infants and young children:
• Term infants born ≤ 8 days old (first RSV season).
• Preterm infants born ≤ 32 weeks GA or with chronic lung disease/hypotension requiring supplemental oxygen.
• Infants with severe congenital or acquired heart disease.
• Infants with congenital or acquired immunodeficiency.
• Infants with Down syndrome.

Regulatory status: Approved by the FDA (2023) and EMA (2024) for use during a single RSV season.

Contraindications

• Contraindications:
• Confirmed hypersensitivity to *nirsevimab* or any excipients.
• Warnings:
• *Serious hypersensitivity reactions* (anaphylaxis) may occur; observe for ≥30 min post‑dose.
• *Potential for reduced efficacy* if infants receive other RSV‑specific monoclonal antibodies concurrently.
• Precautions:
• Use in neonates with hemodynamic instability requires careful monitoring.
• No data on use with live viral vaccines.

Dosing

• Dose: 1 mg/kg (≤ 15 kg) or 50 mg (≥ 15 kg).
• Route: Intramuscular injection (deltoid or gluteal muscle).
• Timing: Administered within the first 8 days of life for term infants, or at the start of RSV season for other high‑risk groups.
• Injections:
• Single injection per RSV season; no retreatment needed the same year.

Adverse Effects

• Common (≤ 2 %):
• Injection site pain, erythema, or induration.
• Transient low‑grade fever.
• Mild upper respiratory tract symptoms.
• Serious (≤ 0.1 %):
• Anaphylaxis or severe hypersensitivity reactions.
• Infusion‑related reactions (rare, typically mild).
• Rare hypersensitivity: urticaria, angioedema.
• Other rare events:
• Hypotension (rare, associated with acute allergic reaction).
• Severe gastrointestinal symptoms (diarrhea, vomiting, but not dose‑related).

Monitoring

• Clinical:
• Observe for signs of hypersensitivity post‑dose (pruritus, rash, wheeze, hypotension).
• Monitor respiratory status (apnea, hypoxia) especially in preterm or high‑risk infants.
• Laboratory:
• No routine labs required.
• Consider CBC and serum creatinine prior to dose if comorbid conditions present.
• Follow‑up:
• Clinical exam at day 7 and then at the end of RSV season if symptomatic.

Clinical Pearls

• Single‑dose sufficiency: Because of the long half‑life, a *single IM injection* covers both the first and following RSV seasons for most infants—great for adherence in high‑risk populations.
• Preterm infants benefit irrespective of birth weight: Dosing is weight‑based only to 15 kg; infants over this weight receive the fixed 50 mg dose, simplifying calculations.
• Avoid concurrent anti‑RSV mAbs: Co‑administration with palivizumab or newer anti‑RSV antibodies is not recommended due to overlapping mechanisms and no proven benefit.
• Safety profile in infants with Down syndrome: Real‑world data show no increase in adverse events, providing confidence in prophylaxis for this group.
• Injection site site selection: For infants  6 months, the deltoid is acceptable and reduces muscular injury risk.
• Documentation: Recording the exact dose, date, and infant weight in the medical record is essential for ensuring coverage across RSV seasons, particularly in multi‑hospital settings.

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• References: FDA Drug Approval (2023), EMA Summary of Product Characteristics (2024), American Academy of Pediatrics RSV Prophylaxis Guidelines (2022).