Influenza
Also known as: Flu
Acute respiratory illness caused by influenza viruses (Orthomyxoviridae family): types A (most clinically significant, subtyped by hemagglutinin/neuraminidase, e.g., H1N1, H3N2; capable of antigenic shift causing pandemics) and B (antigenic drift only, seasonal).
Transmitted via respiratory droplets/aerosols and fomites; highly contagious, seasonal epidemics in temperate climates.
Classified by:
Uncomplicated influenza: self-limiting febrile respiratory illness
Complicated/severe influenza: pneumonia (primary viral or secondary bacterial), exacerbation of underlying chronic disease, extrapulmonary complications (myocarditis, encephalitis, myositis, Guillain-Barré syndrome), requiring hospitalization
Abrupt onset (distinguishes from gradual onset of common cold): high fever, chills, myalgia, headache, malaise/fatigue (often profound, "hit by a truck" sensation)
Respiratory symptoms: dry cough, sore throat, nasal congestion/rhinorrhea: may be less prominent initially than systemic symptoms
Symptom duration typically 5-7 days for uncomplicated illness, cough/fatigue may persist longer (1-2 weeks)
High-risk groups for complications: age <5 (especially <2) or >65, pregnancy, chronic lung/cardiac/renal/hepatic/neurological disease, immunosuppression, diabetes, morbid obesity, residents of long-term care facilities
Warning signs of complications: dyspnea/tachypnea, chest pain, persistent/recurrent fever after initial improvement (suggests secondary bacterial infection/pneumonia), altered mental status, severe dehydration, worsening of underlying chronic conditions
Secondary bacterial pneumonia: classically S. aureus (including MRSA), S. pneumoniae, H. influenzae ; presents as recurrence of fever after initial defervescence, new focal chest signs
Primarily clinical during known seasonal circulation, especially with typical abrupt-onset presentation.
Confirmatory testing (useful for high-risk patients, hospitalized patients, outbreak settings, or when result changes management; antiviral eligibility, isolation, cohorting):
Rapid influenza diagnostic tests (RIDTs): rapid but lower sensitivity (~50-70%), high specificity; negative result does not exclude infection in high clinical suspicion
RT-PCR (nasopharyngeal swab): gold standard, high sensitivity/specificity, distinguishes influenza A/B and subtype
CXR if pneumonia suspected: viral pneumonia; bilateral interstitial infiltrates; secondary bacterial; focal consolidation
Differentials: COVID-19 (clinically similar, requires specific testing to distinguish), other respiratory viruses (RSV, parainfluenza, adenovirus, rhinovirus), streptococcal pharyngitis, atypical pneumonia (Mycoplasma), early bacterial pneumonia.
Uncomplicated influenza (outpatient, otherwise healthy):
Supportive care: rest, hydration, antipyretics/analgesics (paracetamol, NSAIDs ; avoid aspirin in children/adolescents due to Reye's syndrome risk)
Antivirals generally not required in low-risk patients with mild illness presenting late, but may be considered if within 48 hours of onset
Antiviral therapy; indicated for:
High-risk patients (age extremes, pregnancy, chronic disease, immunosuppression) regardless of illness duration
All hospitalized patients with suspected/confirmed influenza
Severe/progressive illness
Ideally started within 48 hours of symptom onset for maximal benefit (reduces duration by ~1 day, may reduce complications), though treatment for hospitalized/high-risk patients still recommended beyond 48 hours
Antiviral options:
Oseltamivir (neuraminidase inhibitor, oral): 75mg bd for 5 days (adults); dose-adjusted for renal impairment and weight-based in children
Zanamivir (inhaled): alternative, avoid in underlying respiratory disease (bronchospasm risk)
Baloxavir marboxil (cap-dependent endonuclease inhibitor): single oral dose, alternative option, weight-based dosing
Peramivir (IV): for patients unable to tolerate oral/inhaled route
Complicated influenza (hospitalized, pneumonia, severe illness):
Antivirals (oseltamivir first-line, IV peramivir if unable to take oral)
Empirical antibiotics if secondary bacterial pneumonia suspected (cover S. pneumoniae, S. aureus including MRSA coverage if severe/necrotizing; e.g., co-amoxiclav or ceftriaxone ± vancomycin/linezolid depending on severity and local MRSA prevalence)
Supportive care: oxygen, respiratory support (up to mechanical ventilation/ECMO in ARDS), hemodynamic support
Prevention:
Annual vaccination; recommended for all ≥6 months, particularly high-risk groups, healthcare workers, pregnant women (protects mother and passively protects infant); inactivated or live-attenuated (nasal, contraindicated in immunosuppressed/pregnancy/severe asthma) formulations; updated annually per circulating strain surveillance
Infection control: droplet precautions, hand hygiene, isolation of hospitalized cases, mask use
Post-exposure chemoprophylaxis (oseltamivir) considered for high-risk unvaccinated contacts during outbreaks (e.g., care home settings)
Admission criteria: hypoxia, respiratory distress, hemodynamic instability, altered mental status, inability to maintain oral intake, significant comorbidity decompensation, or high-risk patient with rapid deterioration.
