When is Tamiflu (oseltamivir) appropriate to treat influenza in Canada?

When is Tamiflu (Oseltamivir) Appropriate to Treat Influenza in Canada

Oseltamivir should be started immediately for all hospitalized patients with suspected influenza, all patients at high risk for complications (including children under 2 years, adults over 65 years, pregnant women, and those with chronic medical conditions), and any patient with severe or progressive illness, regardless of time since symptom onset. 1, 2

Mandatory Treatment Groups (Start Immediately)

Offer treatment as soon as possible to:

• All hospitalized patients with presumed influenza, regardless of symptom duration 1
• Children under 2 years of age (highest risk group, with infants under 6 months at greatest risk) 1, 3
• Adults 65 years and older 1, 2
• Patients with chronic conditions including:
  • Chronic pulmonary disease (including asthma) 1
  • Cardiovascular disease (except hypertension alone) 1
  • Chronic renal, hepatic, or metabolic disorders (including diabetes) 1
  • Neurologic and neurodevelopmental conditions 1
  • Immunosuppression (including medication-induced) 1, 2
  • Hematological disorders (including sickle cell disease) 1
• Pregnant women 2
• Patients with severe, complicated, or progressive illness at any stage 1, 2

Strongly Consider Treatment For

• Any healthy child with presumed influenza, particularly if they live with siblings under 6 months or household contacts with high-risk conditions 1, 3
• Previously healthy outpatients presenting within 48 hours of symptom onset during confirmed influenza activity 1, 4

Critical Timing Considerations

The 48-hour window is NOT absolute for high-risk patients. While the FDA label indicates treatment for patients "symptomatic for no more than 48 hours" 4, multiple guidelines emphasize that treatment beyond 48 hours still provides significant benefit in high-risk populations:

• Treatment after 48 hours in hospitalized patients reduces mortality (OR 0.21) 2
• Treatment initiated up to 96 hours after symptom onset shows mortality benefit in hospitalized adults 2
• For moderate-to-severe or progressive disease, treatment after 48 hours "has been shown to provide some benefit and should be strongly considered" 1

For otherwise healthy outpatients beyond 48 hours: Treatment benefit is modest and generally not recommended unless illness is worsening 1

Dosing Recommendations

Adults and adolescents (≥13 years): 75 mg twice daily for 5 days 1, 4

Pediatric dosing (weight-based): 1, 3, 4

• 0-8 months: 3 mg/kg twice daily
• 9-11 months: 3.5 mg/kg twice daily
• ≥12 months, ≤15 kg: 30 mg twice daily
• >15-23 kg: 45 mg twice daily
• >23-40 kg: 60 mg twice daily
• >40 kg: 75 mg twice daily

Treatment duration is typically 5 days, though longer courses (up to 10 days) may be considered in immunocompromised patients 1

Important Clinical Caveats

Do not delay treatment waiting for laboratory confirmation. Start empirically based on clinical suspicion during influenza season in high-risk patients 1, 2, 3

Negative rapid antigen tests should not rule out influenza due to suboptimal sensitivity; clinical judgment should guide treatment decisions 1, 3

Take with food to reduce nausea, the most common side effect (occurs in ~15% of treated children vs 9% on placebo) 1

Adjust dose for renal impairment: Reduce by 50% if creatinine clearance <30 mL/min; not recommended for end-stage renal disease without dialysis 4

Patients with hereditary fructose intolerance: One 75 mg dose contains 2 grams of sorbitol, which exceeds daily limits and may cause gastrointestinal upset 4

Expected Benefits in High-Risk Patients

• Mortality reduction in hospitalized patients (OR 0.21 when started within 48 hours, OR 0.2 even after 48 hours) 2
• 50% reduction in pneumonia risk 1, 2
• 34% reduction in otitis media in children 1, 3
• Shortened illness duration by approximately 1 day (range 0.99-1.5 days) 1, 5, 6, 7
• Reduced hospitalizations in outpatients 2
• Decreased viral shedding 2, 8

Common Pitfalls to Avoid

Do not withhold treatment from high-risk patients presenting after 48 hours - the mortality benefit persists even with delayed initiation 1, 2

Do not use double-dose oseltamivir - no benefit over standard dosing has been demonstrated 1

Do not administer live attenuated influenza vaccine (LAIV) within 2 weeks before or 48 hours after oseltamivir - the antiviral may reduce vaccine efficacy 4

Do not rely solely on rapid diagnostic tests - their low sensitivity means negative results cannot exclude influenza in high-risk patients during active community transmission 1, 3