Flu

Definition

Influenza is an acute, contagious respiratory illness caused by influenza viruses, characterised by sudden onset of fever, cough, sore throat, myalgia, and fatigue. It is a self-limiting disease in most healthy individuals but can lead to severe complications in high-risk groups.

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Etiology and Classification

Influenza viruses belong to the Orthomyxoviridae family and are classified into:

• Type A – causes seasonal epidemics and pandemics; infects humans and animals; undergoes antigenic shift and drift

• Type B – causes seasonal outbreaks; infects humans only; undergoes antigenic drift

• Type C – mild respiratory illness, no epidemics

• Type D – primarily affects cattle, not humans

Antigenic variation:

• Antigenic drift: gradual mutations in surface glycoproteins (hemagglutinin [HA] and neuraminidase [NA])

• Antigenic shift: abrupt genetic reassortment, often leading to pandemics

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Transmission and Pathophysiology

• Spread via respiratory droplets (coughing, sneezing), direct contact, and contaminated surfaces

• Incubation: 1–4 days (average 2 days)

• Infectious period: 1 day before symptom onset to 5–7 days after (longer in children and immunocompromised individuals)

• Virus infects respiratory epithelial cells, leading to cell death, inflammation, and impaired mucociliary clearance

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Risk Factors

• Age: young children, elderly

• Chronic conditions: cardiovascular disease, chronic lung disease, diabetes, renal disease

• Immunosuppression

• Pregnancy

• Obesity (BMI ≥40)

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Clinical Presentation

Abrupt onset of symptoms:

• Fever (usually >38°C)

• Chills

• Headache

• Myalgia (especially back and limb pain)

• Fatigue and malaise

• Cough (usually dry)

• Sore throat

• Nasal congestion or rhinorrhoea

Complications:

• Primary influenza pneumonia

• Secondary bacterial pneumonia (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae)

• Exacerbation of chronic medical conditions

• Myocarditis, pericarditis, encephalitis, myositis, Guillain-Barré syndrome

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Diagnosis

Clinical diagnosis during seasonal outbreaks is often sufficient in otherwise healthy patients.
Laboratory confirmation may be used for hospitalised patients, outbreaks, or high-risk groups:

• Rapid influenza diagnostic tests (RIDTs) – detect viral antigens (moderate sensitivity)

• Reverse transcription polymerase chain reaction (RT-PCR) – high sensitivity and specificity

• Viral culture – for epidemiological purposes

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Management

1. Supportive Care (mainstay for most patients)

• Adequate rest and hydration

• Antipyretics/analgesics:

  • Paracetamol (acetaminophen) 500–1000 mg orally every 4–6 hours as needed (max 4 g/day)

  • Ibuprofen 200–400 mg orally every 6–8 hours as needed (max 1200 mg/day OTC, 2400 mg/day prescribed)

• Avoid aspirin in children/adolescents due to risk of Reye’s syndrome

2. Antiviral Therapy (for high-risk patients or severe illness)

Neuraminidase inhibitors – inhibit viral release from infected cells:

• Oseltamivir

  • Adults: 75 mg orally twice daily for 5 days

  • Children: weight-based dosing (e.g., 30–75 mg twice daily)

  • Can be used for prophylaxis: 75 mg once daily for 7–10 days after exposure

• Zanamivir (inhalation powder)

  • Adults and children ≥7 years: 10 mg (two 5 mg inhalations) twice daily for 5 days

  • Avoid in patients with underlying airway disease (e.g., asthma, COPD) due to risk of bronchospasm

• Peramivir (intravenous)

  • Adults: 600 mg IV single dose (over 15–30 min)

  • For hospitalised patients unable to take oral medications

Cap-dependent endonuclease inhibitor – inhibits viral replication:

• Baloxavir marboxil

  • Adults and adolescents ≥12 years: single oral dose based on body weight (40 mg if <80 kg, 80 mg if ≥80 kg)

  • Not recommended in pregnancy or breastfeeding due to limited data

Key points:

• Initiate antiviral therapy within 48 hours of symptom onset for maximum benefit

• In severe or hospitalised cases, treatment may still be beneficial beyond 48 hours

3. Antibiotics

• Not indicated for viral influenza itself

• Reserved for confirmed or strongly suspected secondary bacterial infections

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Prevention

Vaccination – primary preventive measure

• Annual vaccination recommended for all individuals ≥6 months old, especially high-risk groups

• Types:

  • Inactivated influenza vaccine (IIV) – IM injection

  • Live attenuated influenza vaccine (LAIV) – intranasal spray (for healthy, non-pregnant individuals aged 2–49 years)

• Vaccine composition updated annually based on WHO recommendations

Non-pharmacological prevention:

• Frequent handwashing

• Respiratory hygiene (covering coughs/sneezes)

• Avoid close contact with infected individuals

• Use of masks in outbreak settings

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Prognosis

• Most recover within 1–2 weeks

• Severe disease and complications more likely in high-risk populations

• Annual vaccination reduces risk of severe illness, hospitalisation, and death