What is the first line treatment for influenza (flu)?

First-Line Treatment for Influenza

For patients with confirmed or suspected influenza, prescribe oseltamivir 75 mg orally twice daily for 5 days, ideally within 48 hours of symptom onset, for those at high risk of complications or with severe disease. 1, 2, 3

Who Should Receive Antiviral Treatment

High-Priority Patients (Treat Regardless of Timing)

• All hospitalized patients with confirmed or suspected influenza should receive antivirals even if presenting >48 hours after symptom onset, as treatment may still reduce mortality and complications 4, 1
• Children under 2 years of age are at increased risk of hospitalization and complications and should be treated 4
• Patients with serious, complicated, or progressive disease should receive treatment irrespective of vaccination status or illness duration 4
• Immunocompromised or elderly patients may benefit from treatment even without documented fever, as they may not mount adequate febrile responses 4, 1

Outpatients Meeting All Three Criteria

• Acute influenza-like illness present 4, 2
• Fever >38°C (though this may be absent in immunocompromised/elderly) 4, 2
• Symptomatic for ≤48 hours 4, 1, 2

Treatment Benefits

• Starting treatment within 24 hours provides maximum benefit, reducing illness duration by approximately 3.5 days compared to treatment at 48 hours 5
• Treatment within 48 hours reduces symptom duration by approximately 1.5 days in healthy adults and 2.5 days in high-risk patients 6, 7
• Antivirals decrease risk of complications including pneumonia, bronchitis, otitis media, hospitalization, and death 5, 8

Antiviral Medication Options

First-Line: Oseltamivir (Preferred)

• Dosing: 75 mg orally twice daily for 5 days 1, 2, 3
• Dose adjustment: Reduce to 75 mg once daily if creatinine clearance <30 mL/min 4, 3
• FDA-approved for treatment in patients ≥2 weeks of age and prophylaxis in patients ≥1 year 3
• Advantages: Oral administration, proven reduction in complications and hospitalizations 5, 8
• Common adverse effect: Vomiting in ~15% of children (vs 9% with placebo); transient GI symptoms in ~10% of adults 4, 5

Alternative: Zanamivir

• Dosing: 10 mg (two inhalations) twice daily for 5 days 9
• FDA-approved for treatment in patients ≥7 years and prophylaxis in patients ≥5 years 9
• Critical contraindication: NOT recommended for patients with underlying airways disease (asthma, COPD) due to risk of life-threatening bronchospasm 9, 10
• Advantage: May be considered when oseltamivir compliance is a concern 4

Emerging Option: Baloxavir

• Single-dose oral medication that may be considered as alternative when compliance is a concern 4
• Similar efficacy to oseltamivir, with some studies showing more rapid fever resolution 4
• Limitation: Oral suspension formulation availability in the US has been inconsistent 4

When Antibiotics Are Indicated

Do NOT Routinely Prescribe Antibiotics For:

• Previously healthy adults with acute bronchitis complicating influenza without pneumonia 4, 1, 2
• Uncomplicated influenza-like illness without evidence of bacterial superinfection 1

Consider Antibiotics When:

• Worsening symptoms develop (recrudescent fever, increasing dyspnea) 4, 1
• High-risk patients with lower respiratory tract features 4, 1
• Confirmed or suspected influenza-related pneumonia 4, 1
• Secondary bacterial pneumonia (typically develops 4-5 days after initial influenza symptoms) 1

Antibiotic Selection for Influenza-Related Pneumonia

Non-Severe Pneumonia (Oral Therapy):

• First-line: Co-amoxiclav OR tetracycline 4, 1, 2
• Alternative: Macrolide (clarithromycin/erythromycin) or respiratory fluoroquinolone (levofloxacin, moxifloxacin) for penicillin-intolerant patients 4, 1

Severe Pneumonia (IV Therapy):

• Preferred: IV co-amoxiclav or 2nd/3rd generation cephalosporin (cefuroxime/cefotaxime) PLUS macrolide (clarithromycin/erythromycin) 4, 1
• Alternative: Respiratory fluoroquinolone plus broad-spectrum beta-lactamase stable antibiotic 4
• Target pathogens: Streptococcus pneumoniae, Staphylococcus aureus (including MRSA), Streptococcus pyogenes 4, 1

Antibiotic Duration

• Non-severe, uncomplicated pneumonia: 7 days 4, 2
• Severe, microbiologically undefined pneumonia: 10 days 4, 2
• Confirmed/suspected S. aureus or Gram-negative pneumonia: 14-21 days 4, 2
• Switch to oral: When clinically improved, afebrile for 24 hours, and no contraindication to oral route 4, 2

Critical Pitfalls to Avoid

• Do not withhold antivirals from hospitalized patients presenting >48 hours after symptom onset—they may still benefit 4, 1
• Avoid zanamivir in patients with asthma or COPD due to bronchospasm risk 9, 10
• Do not prescribe antibiotics for uncomplicated influenza without evidence of bacterial superinfection 1, 2
• Monitor for secondary bacterial pneumonia, particularly S. aureus, which is more common during influenza outbreaks than in routine community-acquired pneumonia 1
• Antivirals are not a substitute for annual influenza vaccination, which remains the primary preventive measure 3, 9
• Consider local resistance patterns when selecting antivirals, though neuraminidase inhibitor resistance remains uncommon 4, 8