What are the recommended steps after heavy exposure to influenza A?

Post-Exposure Prophylaxis After Heavy Influenza A Exposure

After heavy exposure to influenza A, initiate antiviral chemoprophylaxis as soon as possible and ideally within 48 hours of exposure, using oseltamivir 75 mg once daily for 7 days in adults, or consider baloxavir as a single-dose alternative if the exposed person is at high risk of severe disease. 1, 2

Critical Timing Window

• Chemoprophylaxis must be started within 48 hours of exposure to be effective 1, 2
• If more than 48 hours has elapsed since exposure, do not give prophylactic dosing; instead, educate the patient to start full treatment doses (oseltamivir 75 mg twice daily) immediately if symptoms develop 1, 2
• The sooner prophylaxis is initiated after exposure, the more effective it will be 1

Risk Stratification: Who Should Receive Prophylaxis

High-priority candidates (strong recommendation for prophylaxis): 1, 2

• Severely immunocompromised persons (e.g., hematopoietic stem cell transplant recipients) for whom vaccination is contraindicated, unavailable, or expected to have low effectiveness 1
• Unvaccinated household contacts of persons at very high risk of complications 1
• Individuals at high risk during the first 2 weeks after influenza vaccination, before optimal immunity develops 1, 2
• Pregnant women in high-risk exposure groups 2

Lower-priority candidates (may consider prophylaxis): 1

• Asymptomatic adults and children ≥3 months who are at very high risk after household exposure 1
• Healthcare workers with unprotected exposure to confirmed influenza patients 1

Not recommended for routine prophylaxis: 2, 3

• Individuals at low risk of severe disease after exposure to seasonal influenza (moderate certainty evidence) 3

Antiviral Agent Selection and Dosing

First-line option - Oseltamivir (oral): 1, 2, 4

• Adults and adolescents ≥13 years: 75 mg once daily for 7 days after most recent exposure 1, 2, 4
• Children 1-12 years: Weight-based dosing once daily for 10 days 2, 4
  • ≤15 kg: 30 mg once daily
  • 15.1-23 kg: 45 mg once daily
  • 23.1-40 kg: 60 mg once daily

  • 40 kg: 75 mg once daily

• Infants 3-11 months: 3 mg/kg once daily (3-8 months) or 3.5 mg/kg once daily (9-11 months) 2, 4

Alternative option - Zanamivir (inhaled): 1, 2

• 10 mg (two 5-mg inhalations) once daily for 7-10 days 1, 2
• Approved for ages ≥5 years 2
• May be preferred in settings with oseltamivir resistance concerns 3

Newer option - Baloxavir (oral, single dose): 1, 2, 5, 3

• Single-dose regimen for patients ≥12 years 1, 5
  • 20-<80 kg: 40 mg single dose
  • ≥80 kg: 80 mg single dose
• Reduces household transmission by 87% (1% vs 13% infection rate compared to placebo) 1, 2
• Particularly useful for adherence concerns with multi-day regimens 1, 5
• Probably achieves important reductions in symptomatic influenza in high-risk individuals (moderate certainty) 3

Not recommended - Adamantanes: 1

• Amantadine and rimantadine should not be used due to high resistance rates among current influenza A strains 1

Special Considerations and Monitoring

If symptoms develop during prophylaxis: 1, 2

• Test for influenza immediately 1
• Switch to full treatment dosing: oseltamivir 75 mg twice daily for 5 days 1, 2
• Consider using an antiviral with a different resistance profile if not contraindicated 1

Renal dose adjustment: 4

• For creatinine clearance <30 mL/min, reduce oseltamivir prophylaxis dose to 75 mg every other day 4

Concurrent vaccination: 1, 2

• Oseltamivir prophylaxis does not interfere with inactivated influenza vaccine (IIV) antibody response 1, 2
• Can administer both simultaneously in high-risk patients 2
• Important caveat: Oseltamivir will decrease effectiveness of live attenuated influenza vaccine (LAIV); avoid LAIV during and for 2 weeks after oseltamivir use 1, 2

Common Pitfalls to Avoid

Timing errors: 1

• Do not start once-daily prophylactic dosing if >48 hours has elapsed since exposure 1
• Do not use prophylactic doses for symptomatic patients—use full treatment doses 1

Misunderstanding prophylaxis limitations: 1, 2

• Prophylaxis is not a substitute for annual influenza vaccination 1, 2, 4
• Risk of influenza is lowered but not eliminated while taking medication 1
• Susceptibility returns when medication is discontinued 1

Alternative strategy: 1, 2

• Instead of prophylaxis, clinicians can educate patients and provide a prescription for early empiric treatment to start immediately if symptoms develop 1, 2
• This "watchful waiting with treatment ready" approach may be appropriate for lower-risk exposures 1

Institutional Outbreak Settings

Different duration for healthcare facilities: 1

• In long-term care facilities or hospital outbreaks, continue prophylaxis for 14 days and at least 7 days after onset of symptoms in the last identified case 1
• Implement active surveillance for additional cases when healthcare-associated influenza is identified 1