What is the standard of care for influenza (flu)?

Standard of Care for Influenza

For patients with confirmed or suspected influenza presenting within 48 hours of symptom onset, initiate oseltamivir 75 mg twice daily for 5 days if they are at high risk for complications; otherwise healthy patients with mild illness do not require antiviral treatment. 1, 2

Initial Assessment and Risk Stratification

Assess severity using clinical parameters rather than routine laboratory testing in outpatient settings:

• Monitor vital signs including temperature, respiratory rate, pulse, blood pressure, mental status, and oxygen saturation 3, 1
• Consider hospital admission if two or more unstable clinical factors are present: 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, or abnormal mental status 3, 1
• For children, additional red flags include respiratory distress, cyanosis, severe dehydration, altered consciousness, or prolonged seizures 1

Laboratory testing is generally unnecessary for outpatient diagnosis but should be considered for hospitalized patients or when confirmation would change management decisions 4. When testing is indicated, nucleic acid amplification tests (NAAT) or rapid molecular assays are preferred over rapid antigen tests due to superior accuracy 4, 5.

Antiviral Treatment Decision Algorithm

Antiviral therapy should ONLY be considered when ALL three criteria are met: 3, 1

1. Acute influenza-like illness with fever >38°C
2. Symptomatic for ≤48 hours (ideally within 24 hours for maximum benefit)
3. Patient is at high risk for complications OR has severe illness requiring hospitalization

High-Risk Populations Who Benefit from Antivirals:

• Adults ≥65 years of age 3, 6
• Immunocompromised patients 3
• Patients with chronic cardiac or respiratory disease 7
• Residents of long-term care facilities during outbreaks 6
• Pregnant women 7

Antiviral Drug Selection:

First-line: Oseltamivir 75 mg orally twice daily for 5 days 3, 1

• Reduce dose to 75 mg once daily if creatinine clearance <30 mL/min 3
• For children: dose based on weight (see pediatric dosing guidelines) 3
• Expected benefit: reduces illness duration by approximately 24 hours and may reduce hospitalization rates 7, 2
• Moderate certainty evidence shows oseltamivir probably has no important effect on symptom duration in low-risk patients 2

Alternative: Baloxavir (single dose based on weight) 8, 2

• 40 mg for patients 40-80 kg; 80 mg for patients >80 kg 8
• Baloxavir probably reduces hospital admission risk in high-risk patients (risk difference -1.6%) and may reduce symptom duration by approximately 1 day without increasing adverse events 2
• High certainty evidence shows fewer treatment-related adverse events compared to oseltamivir 2

Important caveat: Patients unable to mount adequate febrile response (immunocompromised, very elderly) may still be eligible for treatment despite lack of documented fever 3

Antibiotic Management - Critical Pitfall to Avoid

Do NOT routinely prescribe antibiotics for previously healthy adults with acute bronchitis complicating influenza in the absence of pneumonia 1, 9. This is a common error in practice.

Consider antibiotics ONLY when: 3, 1

• Previously well adults develop worsening symptoms (recrudescent fever, increasing dyspnea)
• High-risk patients develop lower respiratory tract features
• Confirmed or suspected bacterial pneumonia (positive sputum culture, infiltrate on chest X-ray)
• Severe pneumonia with CURB-65 score ≥3 or bilateral chest X-ray changes 3

Supportive Care Management

Oxygen therapy: 3, 1

• Initiate for hypoxic patients with goal to maintain PaO₂ >8 kPa and SaO₂ ≥92%
• High concentrations can be safely given in uncomplicated pneumonia
• For COPD patients with ventilatory failure, guide therapy by repeated arterial blood gas measurements

Non-invasive ventilation: 3

• May be helpful in COPD patients with ventilatory failure
• Can serve as bridge to invasive ventilation when ICU beds are limited
• Should only be used in experienced respiratory/critical care units with proper infection control

Additional supportive measures: 3

• Assess for cardiac complications and volume depletion
• Provide nutritional support in severe or prolonged illness
• Ensure adequate hydration

Monitoring and Discharge Criteria

Inpatient monitoring: 3, 1

• Record vital signs at least twice daily (more frequently in severe cases)
• Use Early Warning Score system for systematic monitoring

Discharge readiness - patient should NOT have ≥2 of the following: 3, 1

• 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

Follow-up: Arrange clinical review for patients with significant complications or worsening of underlying disease 3, 1

Post-Exposure Prophylaxis

Consider prophylaxis with oseltamivir 75 mg once daily for 10 days OR baloxavir (single dose) for: 8, 6, 5

• Unvaccinated high-risk individuals exposed within 48 hours to confirmed influenza case
• Residents of long-term care facilities during outbreaks
• Healthcare workers with high-risk exposure who are unvaccinated

Prophylaxis should be combined with other measures: immediate vaccination, case isolation, face masks, and frequent hand washing 6

Key Evidence Considerations

The most recent high-quality evidence from a 2025 network meta-analysis demonstrates that baloxavir has superior efficacy for reducing hospital admissions in high-risk patients and symptom duration, with fewer adverse events compared to oseltamivir 2. However, oseltamivir remains widely used due to established safety profile, availability, and cost considerations. The 2017 AAP guidelines and 2025 WHO guidelines support both agents as appropriate first-line options 3, 5.

Critical limitation: No antiviral drug has demonstrated mortality benefit in randomized trials, though this outcome is difficult to power studies for given the relatively low mortality rate in most influenza seasons 6, 2.