What is the treatment for H3N2 influenza?

Treatment of H3N2 Influenza

Neuraminidase inhibitors (oseltamivir or zanamivir) are the first-line treatment for H3N2 influenza, with treatment ideally initiated within 48 hours of symptom onset to maximize effectiveness. 1

First-Line Treatment Options

Oseltamivir (Preferred Option)

• Dosing for Adults and Children by Weight:

  • Adults and children >40 kg: 75 mg twice daily for 5 days
  • Children 23-40 kg: 60 mg twice daily for 5 days
  • Children 15-23 kg: 45 mg twice daily for 5 days
  • Children ≤15 kg: 30 mg twice daily for 5 days 2

• Dosing for Infants:

  • 9-11 months: 3.5 mg/kg twice daily for 5 days
  • 0-8 months: 3 mg/kg twice daily for 5 days 2

• Renal Dose Adjustment:

  • For CrCl 10-30 mL/min: 75 mg once daily for 5 days 2

Zanamivir (Alternative Option)

• 10 mg (2 inhalations) twice daily for 5 days for patients ≥7 years 1
• Not recommended for patients with underlying respiratory disease due to risk of bronchospasm 2

Treatment Priority Groups

Treatment should be prioritized for:

1. Patients with severe, complicated, or progressive illness requiring hospitalization 1
2. High-risk patients with suspected or confirmed influenza, including:
   • Adults ≥65 years
   • Children <2 years
   • Pregnant or postpartum women (within 2 weeks after delivery)
   • Persons with chronic medical conditions (cardiac, pulmonary, renal, hepatic, neurological, hematologic, or metabolic disorders)
   • Immunocompromised individuals
   • Morbidly obese individuals (BMI ≥40)
   • American Indians/Alaska Natives
   • Residents of nursing homes and chronic-care facilities 1

Treatment Timing and Efficacy

• Optimal timing: Treatment should begin within 48 hours of symptom onset for maximum effectiveness 1, 2
• Clinical benefit: Reduces duration of symptoms by approximately 1 day in uncomplicated cases 1
• Additional benefits: May reduce risk of complications such as pneumonia by approximately 50% and decrease hospitalizations, particularly in high-risk patients 2
• Late treatment: Treatment may still be beneficial for hospitalized patients or those with severe illness even if started >48 hours after symptom onset 1

Alternative Antiviral Option

Baloxavir

• Single-dose treatment option (weight-based: 40-80 mg once)
• Effective against influenza A and B
• May reduce symptom duration by approximately 1 day
• Fewer adverse events compared to oseltamivir 2, 3
• Consider for patients where compliance is a concern 2

Important Considerations

Resistance Patterns

• Current H3N2 strains are resistant to adamantanes (amantadine and rimantadine), so these drugs should not be used 1
• Neuraminidase inhibitor resistance should be monitored through local surveillance data 1

Combination Therapy

• For patients with severe influenza-related pneumonia, consider combination therapy with antibiotics (broad-spectrum β-lactamase stable antibiotic plus a macrolide) 2
• Antibacterial therapy should target likely pathogens such as S. pneumoniae, S. pyogenes, and S. aureus (including MRSA) 1

Special Populations

• Immunocompromised patients: May benefit from treatment even if started >48 hours after symptom onset and may require longer treatment duration 2
• Pregnant women: Benefits of treatment generally outweigh risks in cases of confirmed or suspected influenza 2

Common Treatment Pitfalls to Avoid

1. Delayed initiation of treatment - Don't wait for laboratory confirmation in high-risk patients during influenza season
2. Using adamantanes for H3N2 - Current H3N2 strains are resistant to these medications
3. Stopping treatment too early - Complete the full 5-day course even if symptoms improve
4. Not considering antiviral treatment for outpatients - Even those without risk factors may benefit if treated early

By following these evidence-based guidelines, clinicians can effectively manage H3N2 influenza infections and reduce the risk of complications, hospitalizations, and mortality.