What is the appropriate dose of baloxavir marboxil for post‑exposure prophylaxis of influenza?

Baloxavir Dose for Post‑Exposure Prophylaxis (PEP)

Use a single, weight-based oral dose: 40 mg if 40–80 kg; 80 mg if ≥80 kg, for persons ≥12 years old (and ≥40 kg). 1, 2

• Evidence and scope

  • Guideline (AAP 2021–22) lists baloxavir for chemoprophylaxis with the same single-dose, weight-based regimen used for treatment in persons ≥12 years and ≥40 kg: 40 mg (40–80 kg) or 80 mg (≥80 kg) orally once 1, 2.
  • FDA labeling supports weight-based dosing and includes PEP effectiveness and pediatric data extension; avoid coadministration with polyvalent cations (e.g., antacids, iron, calcium, magnesium, dairy) to prevent reduced exposure and failure 3.
  • Highest-quality RCT evidence shows single-dose baloxavir significantly reduces household PEP clinical influenza vs placebo (1.9% vs 13.6%) 4.

• Practical algorithm

  • Confirm exposure within 48 hours and candidate age/weight:
    • ≥12 years and ≥40 kg: give baloxavir now.
      • 40–80 kg: 40 mg PO once.
      • ≥80 kg: 80 mg PO once. 1, 2
  • Pediatric notes:
    • Current AAP table specifies ≥12 years; emerging PK/PEP data in younger children exist, but follow local labeling/guidelines for <12 years until formally endorsed 1, 5.
  • Administration cautions (to protect efficacy, morbidity, and QOL):
    • Do NOT take with dairy, calcium/iron/magnesium/zinc, antacids, or cation-containing laxatives/supplements around the dose; separate by at least several hours 3.
    • Do not coadminister with intranasal live attenuated influenza vaccine (LAIV) at the same time; antivirals may reduce LAIV effectiveness 3.

• Common pitfalls to avoid

  • Under-dosing heavier patients (forgetting 80 mg if ≥80 kg) 1.
  • Giving multiple daily doses—PEP is a single dose, not a course 1.
  • Administering beyond 48 hours post-exposure—efficacy wanes; use promptly to impact outcomes 4.
  • Cation interactions reducing drug levels and protection 3.