Diagnosis and Management of Acute Influenza A Infection
Immediate Clinical Diagnosis
In otherwise healthy adults and children, diagnose influenza A clinically during flu season when patients present with fever plus respiratory symptoms—do not delay treatment waiting for laboratory confirmation. 1
Key Diagnostic Features
- Classic presentation: Abrupt onset of fever (≥38°C), cough, plus at least one systemic symptom (headache, myalgias, chills, fatigue) 1, 2
- Children 3+ years: Cough, headache, and pharyngitis (triad has 80% sensitivity, 78% specificity during epidemics) 1
- Young children (<3 years): May present atypically with fever, gastrointestinal symptoms (diarrhea, vomiting), or febrile seizures 1
- Infants: Often present with fever alone, increased irritability, or feeding difficulties 1
When Laboratory Testing Is Useful
- Rapid molecular assays (RT-PCR, NAAT) are preferred when testing is needed—they have >90% sensitivity/specificity and provide results in <20 minutes 1, 3
- Do NOT use rapid antigen tests to rule out influenza—sensitivity is only 10-70%, and negative results should not guide treatment decisions 1, 4
- Test hospitalized patients and those where confirmation changes management, but never delay treatment for results 1
Antiviral Treatment: Who and When
Treat Immediately Without Waiting for Testing
Start oseltamivir immediately in these groups, regardless of vaccination status or time since symptom onset: 1, 5
- All hospitalized patients with suspected influenza 1
- All children <2 years old (exceptionally high complication risk) 1, 5
- Any patient with severe, complicated, or progressive illness 1
- High-risk patients of any age including:
Consider Treatment in Otherwise Healthy Patients
For healthy adults and children >2 years: Treatment reduces illness duration by ~36 hours (26% reduction) and decreases complications when started within 48 hours 5, 2
- Greatest benefit: Treatment within 24 hours of symptom onset 1, 2
- Still beneficial: Treatment between 24-48 hours 1
- Consider even after 48 hours if illness is progressive or patient remains symptomatic 1
Oseltamivir Dosing (Drug of Choice)
Children (Treatment Course: 5 Days, Twice Daily) 5
| Age/Weight | Dose |
|---|---|
| 0-8 months | 3 mg/kg per dose |
| 9-11 months | 3.5 mg/kg per dose |
| ≥12 months, ≤15 kg | 30 mg |
| >15-23 kg | 45 mg |
| >23-40 kg | 60 mg |
| >40 kg | 75 mg |
Adults
- Standard dose: 75 mg twice daily for 5 days 1
Formulation Details
- Oral suspension: 6 mg/mL concentration (preferred for children) 1, 5
- Can give with food to reduce nausea/vomiting 5
- If commercial suspension unavailable, pharmacies can compound from capsules 1, 5
Alternative Antivirals (When Oseltamivir Cannot Be Used)
- Zanamivir (inhaled): Acceptable alternative for patients ≥7 years without chronic respiratory disease 1, 5
- Peramivir (IV): Approved for children ≥2 years with acute uncomplicated influenza, symptomatic ≤2 days 1
- Baloxavir: Single-dose option for patients ≥12 years, ≥40 kg; may be less effective against influenza B 7, 3
Do NOT Use
- Amantadine/rimantadine: High resistance rates, ineffective against influenza B 1
Supportive Care and Monitoring
Outpatient Management
- Antipyretics: Acetaminophen or ibuprofen (never aspirin in children—Reye's syndrome risk) 1, 8
- Hydration: Encourage oral fluids 1
- Symptom monitoring: Educate on warning signs requiring immediate evaluation 5
Warning Signs Requiring Immediate Medical Attention 1, 5
Children:
- Respiratory distress (increased rate, grunting, retractions, breathlessness)
- Cyanosis or oxygen saturation ≤92%
- Severe dehydration
- Altered consciousness or extreme irritability
- Prolonged/complicated seizures
- Vomiting >24 hours
Adults:
- Difficulty breathing or shortness of breath
- Persistent chest pain or pressure
- Confusion or altered mental status
- Severe weakness or inability to stand
Antibiotic Considerations
Do NOT routinely prescribe antibiotics—only add if bacterial co-infection is suspected 1
Indications for Antibiotics
- Clinical pneumonia with lobar consolidation on chest X-ray 1
- Persistent fever beyond 4-5 days or fever that returns after improvement 1
- Severe illness requiring hospitalization 1
Antibiotic Coverage (When Indicated)
Target S. pneumoniae, S. aureus (including MRSA), and H. influenzae: 1, 6
Children <12 years:
- First-line: Co-amoxiclav (amoxicillin-clavulanate) 1
- Penicillin allergy: Clarithromycin or cefuroxime 1
Children >12 years and adults:
- Doxycycline is an alternative 1
Severe pneumonia: Add second agent (clarithromycin or cefuroxime) and give IV 1
Hospital Admission Criteria
Children 1
- Signs of respiratory distress (see warning signs above)
- Oxygen saturation ≤92%
- Severe dehydration requiring IV fluids
- Altered consciousness
- Signs of sepsis (extreme pallor, hypotension, floppy infant)
Adults 1
- CRB65 score ≥2 (Confusion, Respiratory rate ≥30, Blood pressure systolic <90 or diastolic ≤60, age ≥65) 1
- Oxygen saturation ≤92%
- Severe dehydration or inability to maintain oral intake
Critical Pitfalls to Avoid
- Do not wait for test results to start treatment in high-risk patients or those with severe illness 1, 5
- Do not rely on negative rapid antigen tests to rule out influenza—sensitivity is poor 1, 4
- Do not withhold treatment beyond 48 hours in high-risk patients—they still benefit 1, 5
- Do not use aspirin in children with influenza (Reye's syndrome risk) 8
- Do not prescribe antibiotics routinely—reserve for documented/suspected bacterial co-infection 1
- Do not use amantadine/rimantadine—widespread resistance renders them ineffective 1
Special Populations
Infants <6 Months
- Highest risk group for complications and death 5, 6
- Treat immediately with oseltamivir 3 mg/kg twice daily, even if >48 hours from symptom onset 5
- Low threshold for hospitalization 1
Pregnant Women
- High-risk group—treat immediately with oseltamivir 6
- Benefits of treatment outweigh theoretical risks 6