What is the recommended treatment for influenza A virus?

Treatment of Influenza A Virus

Oseltamivir 75 mg orally twice daily for 5 days is the recommended first-line treatment for influenza A in adults and adolescents ≥13 years, with treatment initiated as soon as possible and ideally within 48 hours of symptom onset. 1, 2

Primary Antiviral Agent: Oseltamivir

Oseltamivir is the preferred neuraminidase inhibitor because it is active against both influenza A and B viruses, can be administered orally, and has demonstrated efficacy in reducing illness duration by approximately 24 hours when started early. 1, 3

Dosing by Age Group

• Adults and adolescents ≥13 years: 75 mg orally twice daily for 5 days 4, 1
• Children ≥12 months: Weight-based dosing from 30 mg twice daily (≤15 kg) to 75 mg twice daily (>40 kg) 1
• Infants <12 months: 3.5 mg/kg per dose twice daily for 9-11 months; 3 mg/kg per dose twice daily for 0-8 months 1
• Preterm infants: Dosing ranges from 1.0 mg/kg twice daily (<38 weeks postmenstrual age) to 3.0 mg/kg twice daily (>40 weeks postmenstrual age) 1

Renal Dose Adjustment

For patients with creatinine clearance <30 mL/min, reduce the oseltamivir dose by 50% to 75 mg once daily. 1 Oseltamivir is not recommended for end-stage renal disease patients not undergoing dialysis. 2

Alternative Antiviral: Zanamivir

Zanamivir (inhaled) 10 mg (two 5-mg inhalations) twice daily for 5 days is an alternative for patients ≥7 years who cannot tolerate oseltamivir or prefer inhaled therapy. 4, 1, 5

Critical Contraindication

Zanamivir is contraindicated in patients with underlying airways disease (asthma or chronic obstructive pulmonary disease) due to risk of serious bronchospasm. 5 This is a major advantage of oseltamivir over zanamivir in clinical practice.

Adamantanes: Not Recommended

Amantadine and rimantadine are NOT recommended for treatment of currently circulating influenza A strains due to widespread resistance. 4, 1 Circulating 2009 H1N1 and influenza A (H3N2) viruses demonstrate high resistance rates to adamantanes. 4

Timing of Treatment Initiation

Within 48 Hours of Symptom Onset

Greatest benefit occurs when antiviral treatment is started within 48 hours of illness onset. 4, 2 Treatment initiated within 36 hours provides even faster symptom resolution. 3

Beyond 48 Hours

Treatment may still be beneficial >48 hours from onset in the following situations: 4

• Hospitalized patients with confirmed or suspected influenza
• Patients with severe, progressive, or complicated illness (pneumonia, respiratory failure)
• Patients requiring hospitalization regardless of symptom duration

Early initiation of treatment (<2 days from onset) in severely ill patients was associated with reduced morbidity and mortality in observational studies, with greater benefit from earlier treatment. 4

Patient Groups Requiring Immediate Treatment

Antiviral treatment should be started immediately (regardless of symptom duration or vaccination status) for: 4, 1

• All hospitalized patients with confirmed or suspected influenza
• Patients with severe, complicated, or progressive illness
• High-risk patients:
  • Children <2 years (highest risk <6 months)
  • Adults ≥65 years
  • Pregnant and postpartum women (within 2 weeks after delivery)
  • Immunocompromised patients
  • Patients with chronic conditions (pulmonary including asthma, cardiovascular except hypertension alone, renal, hepatic, hematological, metabolic including diabetes, neurologic and neurodevelopmental conditions)

Extended Treatment Duration

Standard treatment duration is 5 days. 4, 1 Longer treatment courses may be considered for: 1

• Patients with persistent fever after 6 days
• Immunocompromised patients
• Critically ill patients who remain severely ill after 5 days of treatment 4

Managing Side Effects

Nausea and vomiting occur in 10-15% of patients taking oseltamivir. 1 Taking oseltamivir with food significantly reduces gastrointestinal side effects. 1, 3 Diarrhea may occur in infants <1 year. 1

Common Pitfalls to Avoid

• Do not delay treatment waiting for laboratory confirmation in high-risk or severely ill patients—clinical judgment and epidemiologic context are sufficient to initiate therapy 4
• Do not withhold treatment beyond 48 hours in hospitalized or severely ill patients, as benefit may still occur 4
• Do not use adamantanes (amantadine/rimantadine) due to resistance 4, 1
• Do not prescribe zanamivir for patients with underlying airways disease 5
• Do not consider antivirals a substitute for annual influenza vaccination 2, 5

Prophylaxis Considerations

Post-exposure prophylaxis: Oseltamivir 75 mg once daily for 10 days, initiated within 48 hours of exposure 1

Seasonal prophylaxis: Oseltamivir 75 mg once daily for up to 6 weeks during community outbreaks 1