What is the recommended course of treatment for influenza (flu)?

Influenza Course of Illness and Treatment

Natural Course and Duration

Influenza typically causes acute illness lasting 3-7 days in otherwise healthy adults, with antiviral treatment reducing this duration by approximately 1-1.5 days when started within 48 hours of symptom onset. 1, 2

The hallmark presentation includes:

• Abrupt onset of fever (>38°C), cough, chills or sweats, myalgias, and malaise 3
• Peak symptoms occur in the first 2-3 days
• Fever duration typically 3-5 days without treatment, reduced by approximately 1 day with oseltamivir 2
• Return to normal activities occurs approximately 1.5 days earlier with antiviral treatment 2

Antiviral Treatment Recommendations

When to Initiate Treatment

Start oseltamivir 75 mg twice daily for 5 days immediately in high-risk patients with suspected influenza during flu season, without waiting for laboratory confirmation. 1, 4

High-risk patients who benefit from treatment include: 5, 4

• Children <2 years (especially infants <6 months)
• Adults ≥65 years
• Pregnant women
• Immunocompromised patients (including those on chronic corticosteroids)
• Chronic pulmonary disease (including asthma)
• Chronic cardiovascular disease (except hypertension alone)
• Chronic renal, hepatic, hematologic, or metabolic disorders (including diabetes)
• Neurologic and neurodevelopmental conditions
• All hospitalized patients with suspected influenza

Treatment Timing

Maximum benefit occurs when treatment starts within 48 hours of symptom onset, but hospitalized and severely ill patients benefit even when treatment is initiated after 48 hours. 1, 4

• Within 24 hours: Greatest symptom reduction (up to 1.5 days shorter illness) 2, 3
• Within 48 hours: Standard recommendation for otherwise healthy patients 5, 1
• After 48 hours: Still provides significant mortality benefit in high-risk and hospitalized patients (OR 0.21 for death within 15 days) 4
• Up to 96 hours: Associated with lower risk for severe outcomes in hospitalized patients 4

Dosing

Adults and adolescents ≥13 years: 75 mg twice daily for 5 days 5, 6

Pediatric dosing (weight-based): 6

• ≤15 kg: 30 mg twice daily
• 15.1-23 kg: 45 mg twice daily
• 23.1-40 kg: 60 mg twice daily

• 40 kg: 75 mg twice daily

Renal impairment: Reduce dose by 50% if creatinine clearance <30 mL/min (75 mg once daily) 5, 6

Antibiotic Management

When Antibiotics Are NOT Needed

Previously healthy adults with acute bronchitis complicating influenza, in the absence of pneumonia, do not routinely require antibiotics. 5, 1

When to Add Antibiotics

Consider antibiotics if: 5, 1

• Worsening symptoms develop (recrudescent fever or increasing dyspnea)
• High-risk patients develop lower respiratory tract features
• New consolidation on chest imaging
• Purulent sputum production with clinical deterioration
• Elevated inflammatory markers suggesting bacterial superinfection

Antibiotic Selection

For non-severe influenza-related pneumonia: 5, 1

• First-line oral: Co-amoxiclav or tetracycline
• Alternative: Macrolide (clarithromycin/erythromycin) or respiratory fluoroquinolone

For severe influenza-related pneumonia: 1

• IV combination therapy: Co-amoxiclav or 2nd/3rd generation cephalosporin (cefuroxime, cefotaxime) PLUS macrolide (clarithromycin or erythromycin)
• Must be administered within 4 hours of admission 1

Antibiotic Duration

Switch from IV to oral when: 1

• Clinical improvement occurs
• Temperature normal for 24 hours
• No contraindication to oral route

Duration: 1

• 7 days for non-severe, uncomplicated pneumonia
• 10 days for severe, microbiologically undefined pneumonia
• 14-21 days for confirmed/suspected S. aureus or Gram-negative pneumonia

Supportive Care and Monitoring

All patients should receive: 1

• Antipyretics for fever control (avoid aspirin in children due to Reye's syndrome risk)
• Adequate hydration
• Rest

Hospitalized patients require monitoring of: 5

• Temperature, respiratory rate, pulse, blood pressure
• Mental status
• Oxygen saturation and inspired oxygen concentration
• At least twice daily initially, more frequently if severe illness

Discharge Criteria

Patients should remain hospitalized if they have ≥2 of the following unstable factors: 5

• Temperature >37.8°C
• Heart rate >100/min
• Respiratory rate >24/min
• Systolic blood pressure <90 mmHg
• Oxygen saturation <90%
• Inability to maintain oral intake
• Abnormal mental status

Common Pitfalls to Avoid

Critical errors include: 4

• Delaying oseltamivir while awaiting laboratory confirmation in high-risk patients—rapid tests have poor sensitivity and negative results should not exclude treatment 4
• Withholding treatment after 48 hours in hospitalized or high-risk patients—mortality benefit persists even with late initiation 4
• Reflexively adding antibiotics for viral influenza symptoms alone without evidence of bacterial superinfection—this contributes to antibiotic resistance 1
• Assuming absence of fever excludes influenza in immunocompromised or very elderly patients who may not mount adequate febrile responses 5, 4

Expected Outcomes with Treatment

Oseltamivir treatment provides: 4, 2

• Symptom duration reduction: 17.6-29.9 hours in otherwise healthy patients
• Pneumonia risk reduction: 50% lower risk
• Otitis media reduction in children: 34% lower risk
• Mortality reduction in hospitalized patients: OR 0.21 for death within 15 days
• Faster return to normal activities and reduced antibiotic use

Alternative Antiviral: Zanamivir

Zanamivir (inhaled) 10 mg twice daily for 5 days is an alternative for patients unable to take oseltamivir. 1, 7

Important contraindication: Not recommended for patients with underlying respiratory disease (asthma, COPD) due to risk of life-threatening bronchospasm 7, 8