Treatment of Influenza
For influenza, start oral oseltamivir immediately for all hospitalized patients, those with severe/progressive illness, and high-risk individuals (including children <2 years, adults ≥65 years, pregnant women, and those with chronic conditions like diabetes, heart disease, or lung disease), regardless of vaccination status or time since symptom onset. 1, 2
Who Requires Immediate Antiviral Treatment
Mandatory treatment groups include 3, 1, 2:
- Any hospitalized child or adult with suspected or confirmed influenza
- Patients with severe, complicated, or progressive illness attributable to influenza
- Children younger than 2 years of age (exceptionally high complication risk)
- Adults 65 years and older
- Pregnant and postpartum women
- Patients with chronic medical conditions:
- Pulmonary diseases (asthma, COPD)
- Cardiac disease (hemodynamically significant)
- Metabolic diseases (diabetes mellitus)
- Renal or hepatic disorders
- Immunosuppression
- Neurologic and neurodevelopmental disorders
- Hemoglobinopathies (sickle cell disease)
Treatment should also be considered for 3, 1:
- Otherwise healthy outpatients with confirmed or suspected influenza, especially if treatment can be initiated within 48 hours of symptom onset
- Children whose siblings or household contacts are younger than 6 months or have underlying medical conditions predisposing them to complications
Recommended Antiviral Medications
Oral oseltamivir is the antiviral drug of choice for managing influenza infections in all age groups 3, 1. It is FDA-approved for children as young as 2 weeks of age 3, 1.
Alternative options include 3, 4:
- Inhaled zanamivir (for patients ≥7 years without chronic respiratory disease, though more difficult to administer)
- Intravenous peramivir (for patients ≥2 years with acute uncomplicated influenza who have been symptomatic ≤2 days)
- Baloxavir (FDA-approved for treatment and prophylaxis in patients ≥12 years)
Do not use amantadine or rimantadine due to high levels of resistance among currently circulating influenza A viruses 3, 2.
Dosing Guidelines for Oseltamivir
Treatment dosing (5 days) 3, 1:
| Age/Weight | Dosage |
|---|---|
| Adults and children ≥40 kg | 75 mg twice daily |
| Children ≥12 months, >23-40 kg | 60 mg twice daily |
| Children ≥12 months, >15-23 kg | 45 mg twice daily |
| Children ≥12 months, ≤15 kg | 30 mg twice daily |
| Infants 9-11 months | 3.5 mg/kg per dose twice daily |
| Infants 0-8 months | 3 mg/kg per dose twice daily |
For preterm infants 1:
- <38 weeks postmenstrual age: 1.0 mg/kg per dose twice daily
- 38-40 weeks: 1.5 mg/kg per dose twice daily
40 weeks: 3.0 mg/kg per dose twice daily
Prophylaxis dosing (10 days for household exposure, 28 days for community outbreak): Use once-daily dosing at the same total daily dose as treatment 3, 5.
Critical Timing Considerations
Treatment should begin as soon as possible, ideally within 48 hours of symptom onset 3, 1, 2. The greatest effect on outcomes occurs with early initiation 3, 1.
However, do not withhold treatment in severely ill, hospitalized, or high-risk patients even if >48 hours have passed since symptom onset 3, 2. Studies show benefit even when treatment is initiated up to 5 days after symptom onset in critically ill patients 3, 6.
Do not delay treatment while awaiting confirmatory influenza test results—clinical judgment based on symptoms and local influenza activity should guide immediate treatment decisions 3, 1.
Expected Clinical Benefits
Oseltamivir treatment reduces 1, 7, 8, 9:
- Duration of illness by approximately 36 hours (26% reduction) when started within 48 hours
- Duration of fever and individual symptoms (fatigue by 29%, myalgia by 26%)
- Risk of complications including hospitalization, otitis media (34% lower in children), and death
- Viral shedding duration
Efficacy is similar for influenza A and B infections 3, 8, though some data suggest slightly less pronounced effects against influenza B 1.
Common Side Effects and Safety
Vomiting is the most common side effect, occurring in approximately 5-15% of patients, but is generally mild and transient 3, 1. Administration with food may reduce gastrointestinal side effects 1, 7.
Despite historical concerns, controlled clinical trials and ongoing surveillance have failed to establish a link between oseltamivir and neurologic or psychiatric events 3, 1.
Diarrhea may occur in infants under 1 year of age 1.
Special Considerations for Zanamivir
Zanamivir is contraindicated in patients with underlying airways disease (asthma, COPD) due to risk of serious, sometimes fatal bronchospasm 4. It should be discontinued immediately if bronchospasm or decline in respiratory function develops 4.
Zanamivir dosing for treatment: 10 mg (2 inhalations) twice daily for 5 days in patients ≥7 years 4.
Patients using inhaled bronchodilators should use their bronchodilator before taking zanamivir 4.
Postexposure Chemoprophylaxis
Reserve postexposure prophylaxis for specific high-risk situations 5:
- Asymptomatic individuals at very high risk of complications (severely immunocompromised)
- Unvaccinated household contacts of very high-risk individuals
- Must be initiated within 48 hours of exposure
Prophylaxis dosing: Oseltamivir once daily for 7 days after most recent exposure (or 10 days for household setting) 5, 4. Baloxavir as single dose within 48 hours for patients ≥12 years 5.
If symptoms develop while on prophylaxis, immediately switch to full treatment dosing 5.
Critical Pitfalls to Avoid
- Do not use chemoprophylaxis as a substitute for vaccination—influenza vaccine should always be offered when not contraindicated 3, 5
- Do not delay treatment in high-risk or severely ill patients based on time since symptom onset 3, 2
- Do not rely on negative rapid antigen tests to rule out influenza—these have low sensitivity and should not guide treatment decisions in high-risk patients 3, 1
- Do not use corticosteroids as adjunctive therapy for seasonal influenza 2
- Consider empiric bacterial coinfection treatment if patients present with initial severe disease, clinical deterioration after initial improvement, or failure to improve after 3-5 days of antiviral treatment 2
Antiviral Resistance Monitoring
During recent influenza seasons, >99% of circulating influenza viruses have been susceptible to oseltamivir, zanamivir, and peramivir 3, 1. Resistance to oseltamivir during treatment is rare overall (<1%) 9.
Clinicians should monitor CDC surveillance data for local antiviral resistance patterns, as these can change over time 3.