Causal Agent
Influenza is caused by RNA viruses of the family Orthomyxoviridae. There are three main types that affect humans: A, B and C.
Influenza A
- It causes annual epidemics and pandemics.
- It is subclassified according to its surface glycoproteins: Hemagglutinin (H) and Neuraminidase (N).
- High mutation rates (antigenic drift and shift).
Influenza B & C
- Type B: Causes epidemics (generally milder than A, but severe in children). Two main lineages: Victoria and Yamagata.
- Type C: Mild or asymptomatic respiratory infections. It does not cause epidemics.
Incubation period: 1 to 4 days (average 2 days).
Transmission: Respiratory droplets, aerosols and contact with contaminated surfaces (fomites).
Symptoms and Signs
The onset is usually abrupt. In infants and preschoolers, the symptoms may be nonspecific or simulate sepsis.
Classics (Older Children)
- High fever (39-40°C), chills
- Intense myalgia and headache
- Dry cough (can last for weeks)
- Odynophagia and rhinitis
Pediatric Peculiarities
- GI symptoms: Nausea, vomiting, abdominal pain, and diarrhea (more common in children than adults).
- Infants: Food refusal, irritability, apnea, croup, bronchiolitis.
- Acute myositis (especially due to Influenza B).
Red Flags (Complications)
Primary viral pneumonia, bacterial superinfection (S. pneumoniae, S. aureus [MRSA], S. pyogenes), AOM, encephalopathy/encephalitis, myocarditis, Reye's syndrome (avoid salicylates).
Laboratory Findings
Virological Tests
High sensitivity and specificity. Detects and types A and B. Ideal for hospitalized patients.
Results in 10-15 min. High specificity, but variable sensitivity (50-70%). A negative result does not rule out infection.
General Analytics (Non-routine)
- Hemogram: Leukopenia (early), relative lymphocytosis. Leukocytosis suggests bacterial superinfection.
- Acute phase reactants: CRP and PCT may be normal or slightly elevated. Marked increases suggest coinfection.
- Biochemistry: Increased CPK if there is associated myositis. Transient elevation of transaminases.
Oseltamivir Calculator
Calculate the treatment dose (5 days) according to weight and age (oral route).
Recommended dosage:
Pharmacological Management (Oseltamivir) Active vs Influenza A and B
Oseltamivir (neuraminidase inhibitor) is the antiviral of choice and is fully effective for the treatment of infections due to both Influenza A as for Influenza B.
Precise Treatment Indications
- Hospitalized: All children with suspicion or confirmation (start empirical).
- Serious/complicated illness: Pneumonia, encephalopathy, myocarditis.
- High risk of complications:
- Children under 2 years (especially < 6 months).
- Asthma, illness neurological/neurodevelopment, congenital heart disease, immunosuppression.
- Diabetes, chronic kidney/liver disease, extreme obesity.
- Note: Greater effectiveness if started in the first 48 hours from the onset of symptoms, but in severe or hospitalized patients it is administered regardless of the time of evolution.
| Age/Weight | Tto dose (BID x 5 days) | Prophylaxis (QD x 10d)* |
|---|---|---|
| Under 1 year old (FDA approved) | ||
| 0 to 8 months | 3 mg/kg/dose (every 12h) | Not recommended |
| 9 to 11 months | 3 mg/kg/dose (every 12h) | Not recommended |
| 1 year or older | ||
| ≤15kg | 30 mg (every 12 hours) | 30 mg (every 24 hours) |
| > 15 to 23 kg | 45 mg (every 12h) | 45 mg (every 24 hours) |
| > 23 to 40 kg | 60 mg (every 12 hours) | 60 mg (every 24 hours) |
| > 40kg | 75 mg (every 12h) | 75 mg (every 24 hours) |
* Post-exposure prophylaxis approved in > 1 year. Indicated in high-risk unvaccinated people with close exposure (< 48h).
Alternatives: Zanamivir (inhaled, >7 years treatment, >5 years prophylaxis). Baloxavir marboxil (single oral dose, >5 years without comorbidities - consult recent guidelines).
Literature
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American Academy of Pediatrics (AAP). Red Book: 2021-2024 Report of the Committee on Infectious Diseases. Influenza.
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Centers for Disease Control and Prevention (CDC). Influenza Antiviral Medications: Summary for Clinicians (Updated 2023-2024 Season).
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Spanish Association of Pediatrics (AEP). Advisory Committee on Vaccines and Pediatric Infectious Diseases Protocols (SEIP). Recommendations on prevention and management of influenza in pediatrics. Spain.
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Uptodate Clinical Guide. Seasonal influenza in children: Management (Revised 2024).