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Infectious Diseases
Influenza in children: diagnosis, treatment, and chemoprophylaxis
Clinical Overview and When to Suspect Influenza
Influenza is caused by RNA orthomyxoviruses (types A and B; type C causes mild illness). It is the most important vaccine-preventable respiratory infection causing annual winter epidemics in children.
Peak season in temperate climates: October–March (Northern Hemisphere)
Highest hospitalization rates: children <2 years (especially <6 months) and those with chronic conditions
Suspect influenza when a child presents during flu season with abrupt onset of fever ≥39°C, myalgia, headache, cough, and sore throat — the classic "flu-like illness" (ILI)
Unlike common cold: influenza produces prominent systemic symptoms (malaise, myalgia) in addition to respiratory symptoms
Board pearl: In infants and young children, influenza often presents atypically — isolated high fever without localizing signs, febrile seizure, croup, bronchiolitis, or sepsis-like picture — making clinical diagnosis unreliable without testing
Influenza A causes more severe pandemics (antigenic shift via reassortment); influenza B causes epidemics but not pandemics
Incubation period: 1–4 days (average 2 days); viral shedding begins ~1 day before symptom onset and continues 5–10 days in children (longer than adults)
History — Key Elements and Symptom Patterns
A thorough history in suspected influenza should capture:
Abrupt onset high fever, chills/rigors, headache, myalgia, malaise, anorexia
Respiratory: dry cough (often prominent and persistent), sore throat, nasal congestion, rhinorrhea
GI symptoms — nausea, vomiting, diarrhea — more common in children than adults
Exposure history: sick contacts at school/daycare, household members with ILI, known community influenza activity
Vaccination status: did the child receive influenza vaccine this season? First-time vaccinees aged 6 months–8 years require TWO doses ≥4 weeks apart; a child who received only one dose is incompletely protected
Underlying high-risk conditions: asthma, chronic lung disease, congenital heart disease, immunodeficiency, neuromuscular/neurodevelopmental disorders, obesity (BMI ≥40), diabetes, chronic renal/hepatic disease, SCD, age <2 years, pregnancy (adolescents)
Clinical tip: Ask about aspirin/salicylate use — Reye syndrome risk with influenza; aspirin is contraindicated in children with influenza
Duration of symptoms matters for antiviral treatment decisions — oseltamivir is most effective when started within 48 hours of symptom onset
Physical Exam — Findings and Red Flags
Red flags requiring urgent evaluation:
General: toxic or ill-appearing child with high fever, flushed face, listlessness
HEENT: pharyngeal erythema (usually without exudates — exudates suggest bacterial superinfection or GAS), conjunctival injection, clear rhinorrhea
Respiratory: tachypnea, diffuse rhonchi or wheezes; crackles suggest lower tract involvement or secondary bacterial pneumonia
Musculoskeletal: myalgia with tenderness to palpation of large muscle groups; in severe cases → calf tenderness suggests benign acute childhood myositis (usually influenza B)
Neurologic: altered mental status, meningismus → consider influenza-associated encephalopathy
Respiratory distress (grunting, retractions, SpO₂ <92%)
Dehydration (↓ urine output, dry mucous membranes, prolonged capillary refill)
Altered consciousness, seizures, focal neurologic deficits
Inability to tolerate oral fluids
Board pearl: Benign acute childhood myositis — typically follows influenza B in school-age boys; sudden bilateral calf pain with refusal to walk, ↑ CK; self-limited over days; distinguish from myocarditis, rhabdomyolysis, and pyomyositis
Diagnostic Workup — Testing Modalities
Diagnostic testing guides treatment and infection control decisions. Options differ in sensitivity and turnaround time:
Rapid influenza diagnostic tests (RIDTs): Antigen detection via immunoassay; results in 15 minutes; specificity ~95–99% but sensitivity only 50–70% → a NEGATIVE RIDT does NOT rule out influenza
Rapid molecular assays (RT-PCR-based, e.g., Xpert Flu): Results in 15–30 minutes; sensitivity and specificity >95% → preferred point-of-care test when available
RT-PCR (lab-based): Gold standard; sensitivity >99%; turnaround 1–6 hours depending on lab
Viral culture: Sensitivity high but results take 3–7 days; useful for surveillance/resistance testing, NOT for clinical decision-making
Direct fluorescent antibody (DFA): Moderate sensitivity (~70–80%), rapid; less used now that molecular assays are available
Board pearl: When clinical suspicion for influenza is high during peak season and an RIDT is negative, do NOT withhold antivirals — obtain a confirmatory molecular test (RT-PCR) and initiate empiric oseltamivir while awaiting results
Nasopharyngeal swab/aspirate preferred over throat swab for optimal specimen quality
Interpreting Results and Additional Labs
— CBC: may show leukopenia, lymphopenia; leukocytosis with left shift raises concern for bacterial superinfection
— CRP/procalcitonin: can help differentiate viral from bacterial co-infection; procalcitonin <0.25 ng/mL makes bacterial superinfection less likely
— Blood cultures: obtain if bacterial sepsis or pneumonia suspected
— CXR: not routinely needed for uncomplicated influenza; obtain if hypoxia, respiratory distress, or clinical concern for pneumonia; may show bilateral diffuse infiltrates (primary viral) or lobar consolidation (bacterial superinfection)
— CK and urinalysis: if myositis suspected (↑ CK, myoglobinuria → risk of rhabdomyolysis in severe cases)
— CSF analysis: if encephalitis/encephalopathy suspected
— BMP: assess electrolytes, renal function in dehydrated or critically ill children
Positive influenza test in a well-appearing child with uncomplicated ILI → no additional labs needed
Additional workup is indicated for hospitalized, severely ill, or high-risk children:
Key distinction: Primary influenza pneumonia → diffuse bilateral interstitial/alveolar infiltrates; secondary bacterial pneumonia → new lobar consolidation after initial improvement ("double sickening")
Management — Antiviral Treatment Overview
Antivirals reduce symptom duration by ~1 day and ↓ risk of complications, hospitalization, and death — most effective when initiated within 48 hours of symptom onset, but can still benefit high-risk patients even if started later.
— First-line antiviral for ALL ages in pediatrics
— Treatment dose by weight: <15 kg → 3 mg/kg/dose BID; 15–23 kg → 45 mg BID; 23–40 kg → 60 mg BID; >40 kg → 75 mg BID
— Duration: 5 days for treatment
Oseltamivir (Tamiflu): Neuraminidase inhibitor; ORAL; FDA-approved for treatment in children ≥2 weeks old
Zanamivir (Relenza): Inhaled neuraminidase inhibitor; approved for treatment ≥7 years; avoid in children with asthma/reactive airway disease (may trigger bronchospasm)
Baloxavir marboxil (Xofluza): Cap-dependent endonuclease inhibitor; single oral dose; approved for treatment ≥5 years and ≥20 kg; avoid in immunocompromised (resistance concerns)
Peramivir: IV neuraminidase inhibitor; approved ≥6 months for treatment; useful when oral/inhaled route is not feasible
Board pearl: Oseltamivir is the ONLY antiviral approved for infants <1 year — it can be given to neonates as young as 2 weeks for treatment
Management — When to Treat and Empiric Decision-Making
Treat ALL of the following with antivirals regardless of symptom duration:
Treat otherwise healthy children ≥2 years if:
Clinical decision logic:
Hospitalized patients
Severe or progressive illness
Children <2 years (high-risk by age alone)
Children of any age with high-risk conditions (asthma, heart disease, immunosuppression, neurodevelopmental disorders, SCD, obesity, chronic renal/hepatic disease)
Symptom onset ≤48 hours — greatest benefit
May also consider treatment for household contacts of high-risk individuals
During influenza season, do NOT wait for test results in high-risk children — start oseltamivir empirically and adjust based on results
After 48 hours of symptoms in a low-risk, well-appearing older child → antiviral benefit is marginal; supportive care is appropriate
Clinical tip: Antivirals reduce otitis media complications in young children with influenza — another reason to treat early in <5 year olds
Antipyretics (acetaminophen, ibuprofen) for fever/myalgia; ensure adequate hydration
NEVER give aspirin or aspirin-containing products → risk of Reye syndrome
Management — Bacterial Superinfection and Referral
Secondary bacterial infection is the most important complication to recognize:
When to refer/escalate:
Classic pattern: child improving from influenza → new/recurrent fever + productive cough + focal lung findings at days 4–7 ("double sickening")
Most common organisms: Staphylococcus aureus (including MRSA), Streptococcus pneumoniae, Streptococcus pyogenes (GAS)
If bacterial pneumonia suspected → CXR + blood cultures → add appropriate antibiotics (e.g., ampicillin-sulbactam or ceftriaxone ± vancomycin if MRSA concern) ON TOP of continuing antivirals
Board pearl: S. aureus (especially MRSA/PVL-producing strains) co-infection with influenza causes rapidly progressive necrotizing pneumonia with high mortality — suspect when hemorrhagic/cavitary lung disease develops
ICU: respiratory failure, ARDS, septic shock, encephalopathy
Pulmonology: empyema, necrotizing pneumonia, refractory respiratory failure
Neurology: encephalitis, status epilepticus
Infectious disease: immunocompromised host, suspected oseltamivir resistance
Supportive care: supplemental O₂, IV fluids, mechanical ventilation as needed
Age-Specific Considerations — Neonates and Infants
— Oseltamivir dosing for neonates: 3 mg/kg/dose BID (FDA-approved ≥2 weeks for treatment)
— Premature infants may need dose adjustment due to immature renal clearance
Neonates (<28 days): Influenza is uncommon but can be devastating; presents with fever, apnea, lethargy, poor feeding, sepsis-like picture; often indistinguishable from bacterial sepsis → full sepsis workup AND oseltamivir if influenza suspected during season
Infants <6 months: Cannot receive influenza vaccine → rely on cocooning strategy (vaccinate all household contacts and caregivers) and maternal vaccination during pregnancy (transplacental antibodies provide partial protection for ~6 months)
Infants 6 months–2 years: High-risk group by age; lower threshold to test and treat; atypical presentations common (isolated fever, febrile seizure, croup-like illness, bronchiolitis-like illness)
Board pearl: Infants <6 months who are too young for vaccination represent the highest hospitalization rate age group — maternal immunization during pregnancy is the MOST effective strategy to protect this vulnerable group
Use weight-based oseltamivir dosing in all children <1 year
Age-Specific Considerations — Older Children and Adolescents
— Benign acute childhood myositis is most common in this age group (influenza B > A); bilateral calf pain, refusal to walk, ↑ CK; resolves in days
— Zanamivir (inhaled) may be used for treatment ≥7 years IF no reactive airway disease
— Baloxavir (single oral dose) approved ≥5 years and ≥20 kg — convenient but avoid in immunocompromised patients
— Pregnant adolescents: influenza vaccine is recommended during any trimester; oseltamivir is the preferred antiviral (pregnancy category C but benefits clearly outweigh risks; extensive safety data)
— Remind about aspirin avoidance; some teens use OTC products containing salicylates
— Adolescents with neuromuscular disease (e.g., muscular dystrophy) are at extreme risk for respiratory failure
School-age children (5–12 years): Classic ILI presentation is most reliable in this age group; myalgia, headache, dry cough, high fever; school absenteeism is a major morbidity
Adolescents: Presentation similar to adults; assess for high-risk conditions (obesity with BMI ≥40, pregnancy, chronic disease)
Key distinction: First-time vaccinees aged 6 months through 8 years need 2 doses ≥4 weeks apart in their first season; ≥9 years need only 1 dose regardless of prior vaccination history
Complications — Pulmonary and Extrapulmonary
Pulmonary complications:
Extrapulmonary complications:
Primary viral pneumonia: diffuse bilateral infiltrates, hypoxemia, can progress to ARDS
Secondary bacterial pneumonia: S. aureus, S. pneumoniae, GAS — "double sickening" pattern
Exacerbation of underlying asthma or chronic lung disease
Croup (laryngotracheobronchitis) — influenza is a common viral cause of croup after parainfluenza
Myositis/rhabdomyolysis: ↑ CK; monitor for myoglobinuria, acute kidney injury; IV hydration if CK markedly elevated
Myocarditis/pericarditis: Rare; chest pain, new murmur, ↓ function on echo, ↑ troponin
Encephalopathy/encephalitis: Altered mental status, seizures; influenza-associated encephalopathy more common in East Asian populations; CT/MRI may show bilateral thalamic necrosis (acute necrotizing encephalopathy — ANE)
Febrile seizures: Very common in children 6 months–5 years with influenza
Reye syndrome: Hepatic mitochondrial dysfunction + encephalopathy triggered by aspirin use during influenza/varicella → now rare due to aspirin avoidance
Board pearl: Acute necrotizing encephalopathy (ANE) of childhood — classically associated with influenza; bilateral symmetric thalamic lesions on MRI; high mortality
When to Hospitalize and Escalate Care
Hospitalization criteria:
ICU-level care:
Hypoxemia (SpO₂ <92% on room air)
Respiratory distress: tachypnea beyond age norms, retractions, grunting, inability to maintain oral intake
Signs of dehydration unresponsive to oral rehydration
Altered mental status, encephalopathy, or new seizures
Infants <3 months with fever and suspected influenza (need to rule out concurrent bacterial infection)
Severely immunocompromised patients
Clinical deterioration after initial improvement ("double sickening" → suspect bacterial superinfection)
ARDS requiring high-flow nasal cannula, CPAP/BiPAP, or mechanical ventilation
Septic shock with hemodynamic instability
Rhabdomyolysis with acute kidney injury requiring aggressive fluid resuscitation
Status epilepticus or rapidly progressive encephalopathy
Clinical tip: In hospitalized patients, continue oseltamivir even if started >48 hours after symptom onset — there is still mortality benefit in severe/hospitalized influenza
Droplet + contact precautions for hospitalized patients; if aerosol-generating procedures → airborne precautions
Duration of isolation: until afebrile and improving OR per institutional policy (typically ≥7 days from symptom onset)
Key Differentials — Viral
Many respiratory viruses produce influenza-like illness; clinical differentiation is unreliable without testing:
Key distinction: During influenza season, abrupt onset of high fever + myalgia + cough in a school-age child has reasonable positive predictive value for influenza, but testing is still recommended to guide antiviral use (antibiotic stewardship) — clinical diagnosis alone leads to overdiagnosis and unnecessary antibiotic avoidance errors
RSV: Peak in infants <12 months; bronchiolitis (wheezing, crackles, hypoxia); less prominent myalgia/systemic symptoms; rapid antigen or PCR distinguishes
SARS-CoV-2 (COVID-19): Overlapping symptoms; anosmia/ageusia more specific for COVID; coinfection with influenza possible → test for both during season
Parainfluenza: Croup (#1 cause), bronchiolitis; less systemic toxicity
Adenovirus: Pharyngoconjunctival fever; may mimic flu with high persistent fever; can cause severe pneumonia in immunocompromised
Human metapneumovirus (hMPV): Similar to RSV; bronchiolitis/pneumonia in young children
Enterovirus/Rhinovirus: Common cold; less systemic symptoms; enterovirus D68 can cause severe wheezing and acute flaccid myelitis
Multiplex respiratory PCR panels can simultaneously identify influenza and other viruses
Key Differentials — Bacterial and Other
GAS pharyngitis: Sore throat, tonsillar exudates, anterior cervical LAD, absence of cough → rapid strep test; influenza has cough, myalgia, more systemic symptoms
Bacterial pneumonia (no preceding virus): Lobar consolidation, high WBC with left shift, elevated procalcitonin; can coexist with influenza
Bacterial sepsis (in infants): Influenza in neonates/young infants can be indistinguishable → full sepsis workup + blood culture + empiric antibiotics AND oseltamivir until results known
Mycoplasma pneumoniae: School-age children; gradual onset, prominent cough, walking pneumonia; cold agglutinins; treat with macrolides
Pertussis: Paroxysmal cough, post-tussive emesis, inspiratory whoop (older children), apnea (infants); lymphocytosis; PCR from nasopharyngeal swab
Kawasaki disease: Prolonged fever ≥5 days with mucocutaneous findings; can overlap timing with influenza season; do NOT attribute persistent fever solely to influenza without considering KD
Board pearl: Influenza co-infection with S. aureus should be suspected in any child with influenza who develops rapidly progressive pneumonia, hemoptysis, or shock — mortality is high without aggressive anti-staphylococcal therapy
Prevention — Influenza Vaccination
Annual influenza vaccination is the cornerstone of prevention:
— IIV (inactivated influenza vaccine): IM; approved ≥6 months; safe in egg allergy (even severe — ACIP 2023+ no longer requires special precautions for egg allergy)
— LAIV (live attenuated, FluMist): intranasal; approved ≥2 years; avoid in immunocompromised, children on aspirin, asthma in children 2–4 years, pregnancy
— Cell-culture and recombinant vaccines: available but less commonly used in pediatrics
— First-time vaccinees aged 6 months through 8 years → 2 doses ≥4 weeks apart in the FIRST season; 1 dose each subsequent season
— ≥9 years → 1 dose each season regardless of history
Who: ALL children ≥6 months of age, annually, every season
Formulations:
Dosing schedule:
Timing: Ideally before end of October; acceptable through late season if circulating
Board pearl: Children aged 6 months through 8 years who received only 1 dose in their first-ever season are considered incompletely vaccinated and need 2 doses the following season
Egg allergy (any severity) is NO LONGER a contraindication to any influenza vaccine
Prevention — Chemoprophylaxis and Infection Control
Antiviral chemoprophylaxis with oseltamivir:
— Cannot be vaccinated (e.g., <6 months old)
— Were vaccinated <2 weeks ago (insufficient time for immune response)
— Are immunocompromised and may not mount adequate vaccine response
— Have high-risk conditions with known close exposure
Institutional outbreak control:
Infection control:
Indicated for HIGH-RISK children who are exposed to influenza AND:
Dosing: Same mg/kg as treatment but given ONCE daily × 7–10 days (vs. BID for treatment)
Chemoprophylaxis is NOT a substitute for vaccination
Post-exposure prophylaxis should be started within 48 hours of exposure for maximum efficacy
In long-term care or closed settings with confirmed influenza outbreak → chemoprophylaxis for all exposed residents/patients regardless of vaccination status
Droplet precautions (surgical mask within 3–6 feet) for confirmed/suspected influenza
Contact precautions in pediatrics due to young children's secretion management
Hand hygiene, respiratory etiquette
Board pearl: The classic board scenario — infant <6 months with immunocompromised sibling at home; household contact has confirmed influenza → give oseltamivir prophylaxis to the infant (too young for vaccine) AND treat the ill contact
Family Counseling and Education
Vaccine counseling: Influenza vaccine does NOT cause influenza; IIV contains inactivated virus, and LAIV (though live) is cold-adapted and cannot replicate in the lower respiratory tract; mild injection-site soreness and low-grade fever for 1–2 days are normal
Return-to-school guidance: Children should stay home until afebrile for ≥24 hours without antipyretics and clinically improving (typically 5–7 days from symptom onset)
Medication adherence: Complete the full 5-day course of oseltamivir even if feeling better; common side effects include nausea/vomiting — administer with food to reduce GI upset
When to return for care: Persistent/worsening fever after 3–5 days, new difficulty breathing, chest pain, inability to keep down fluids, confusion or decreased responsiveness, severe muscle pain with dark urine (rhabdomyolysis)
Cocooning for infants <6 months: ALL household members and caregivers should be vaccinated every season to protect the infant who is too young for vaccination
Aspirin warning: No aspirin-containing products during any febrile viral illness in children — risk of Reye syndrome
Psychosocial: Reassure families that most healthy children recover fully in 5–7 days; validate concern for high-risk children and emphasize the importance of early antiviral treatment
High-Yield Associations and Rapid-Fire Facts
Influenza A → antigenic SHIFT (reassortment of genome segments between species) → pandemics; antigenic DRIFT (point mutations in HA/NA) → seasonal epidemics
Influenza B → DRIFT only → epidemics, NO pandemics; more commonly causes myositis in children
Hemagglutinin (HA) → viral attachment; Neuraminidase (NA) → viral release → target of oseltamivir/zanamivir/peramivir
Oseltamivir resistance: most commonly due to H275Y mutation in NA; rare but monitor; if resistant → zanamivir or peramivir
Reye syndrome = influenza or varicella + aspirin → hepatic microvesicular fatty infiltration + encephalopathy → ↑ ammonia, ↑ LFTs, normal bilirubin
Acute necrotizing encephalopathy (ANE): bilateral symmetric thalamic necrosis on MRI; associated with influenza; high mortality
Most common cause of death from influenza in children: secondary bacterial pneumonia (S. aureus, S. pneumoniae)
Children shed influenza virus LONGER than adults (up to 10 days vs. 5–7 days)
Board pearl: Neuraminidase inhibitors work against both influenza A and B; baloxavir also covers A and B; amantadine/rimantadine cover ONLY influenza A but are NOT recommended due to widespread resistance
One-Line Recap
Influenza in children — caused by orthomyxovirus types A (pandemics via antigenic shift) and B (epidemics, more myositis) — presents as abrupt-onset high fever, myalgia, and cough in school-age children but atypically in infants (isolated fever, sepsis-like, croup, bronchiolitis); diagnose with rapid molecular assay (preferred) or RT-PCR (gold standard), NOT relying on negative RIDTs in high-risk patients; treat with oseltamivir (first-line, approved ≥2 weeks old, weight-based BID × 5 days) started within 48 hours for maximum benefit but given regardless of timing in hospitalized/high-risk children including all those <2 years; prevent with annual influenza vaccine for ALL children ≥6 months (2 doses first season for ages 6 months–8 years; egg allergy is NOT a contraindication; LAIV avoided in asthmatics 2–4 years and immunocompromised); use oseltamivir once-daily chemoprophylaxis for high-risk exposed children who cannot be vaccinated or were recently vaccinated; watch for secondary bacterial pneumonia (S. aureus, S. pneumoniae — "double sickening"), benign acute childhood myositis (influenza B, ↑ CK, self-limited), and acute necrotizing encephalopathy (bilateral thalamic necrosis); and NEVER give aspirin to children with influenza due to Reye syndrome risk.
Board Question Stem Patterns
4-year-old, January, abrupt fever 40°C, myalgia, cough, ill contacts at daycare → suspect influenza → test with rapid molecular assay → treat with oseltamivir within 48 hr
8-month-old, high fever, no source, influenza season → test for influenza (atypical presentation in infants) → positive → oseltamivir 3 mg/kg/dose BID × 5 days
3-year-old with asthma, RIDT negative for influenza, clinical suspicion high → next step: send RT-PCR AND start empiric oseltamivir (do NOT rely on negative RIDT)
Child with influenza improving on day 4, then new fever + productive cough + lobar consolidation → secondary bacterial pneumonia → add antibiotics covering S. aureus and S. pneumoniae
6-year-old, influenza B, sudden bilateral calf pain, refusal to walk, ↑ CK → benign acute childhood myositis → supportive care, monitor for rhabdomyolysis
3-month-old, household contact has confirmed influenza → infant too young for vaccine → oseltamivir chemoprophylaxis once daily × 7–10 days
7-year-old receiving first-ever influenza vaccine in October → needs 2 doses ≥4 weeks apart this season (age 6 mo–8 yr, first season)
Child with influenza and severe egg allergy → give IIV (egg allergy is NOT a contraindication)
10-year-old with influenza, parent asks about aspirin for fever → NEVER give aspirin → Reye syndrome risk → use acetaminophen or ibuprofen
Immunocompromised child exposed to confirmed influenza, vaccinated 1 week ago → give oseltamivir prophylaxis (vaccine given <2 weeks ago = insufficient protection)
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