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Influenza causes up to 4000 deaths per year in the UK. The risk of complications of influenza is high in the following groups of patients:

  • people of all ages with chronic respiratory disease including asthma, chronic heart disease, chronic renal disease, chronic liver disease
  • with conditions and treatments that suppress the immune function
  • diabetes mellitus
  • all those aged 65 years or older
  • those in long stay residential care (1).

Complications include:

  • respiratory
    • acute bronchitis
      • most common lower respiratory tract complication and is seen in around 20% of influenza cases (2)
      • more common in elderly and those with medical conditions
    • influenza related pneumonia – the incidence varies widely (from 2% to 38%), generally seen frequently and with greater severity in patients with pre existing chronic cardiac and respiratory conditions
      • secondary bacterial pneumonia
        • more common
        • typically occurs four to five days after onset of illness
        • implicated pathogens include – Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenza Groups A, C and G beta-haemolytic streptococci
      • primary viral pneumonia
        • uncommon
        • typically become breathless within the first 48 hours of onset of fever
        • mortality in hospitalised patients is high (>40%) despite maximum supportive treatment on intensive care
    • exacerbations of chronic lung diseases such as asthma and COPD
      • up to 28% of exacerbations of COPD are associated with influenza A or B (2)
    • lung abscess, emphysema and invasive pulmonary aspergillosis (2)
  • cardiovascular
    • exacerbation of heart failure
    • ECG abnormalities – common, but most are not associated with cardiac symptoms
    • myocarditis – rare (1)
  • CNS
    • encephalitis/encephalopathy – rare, occurs within first week of illness, more common in children and Japan
    • Guillain-Barre syndrome
  • other complications
    • toxic shock syndrome
    • otitis media (in children) (3)
    • diabetic complications
    • myositis – occurs mostly in children infected with influenza B


  1. British Infection Society; British Thoracic Society; Health Protection Agency. Pandemic flu: clinical management of patients with an influenza-like illness during an influenza pandemic. Provisional guidelines from the British Infection Society, British Thoracic Society, and Health Protection Agency in collaboration with the Department of Health. Thorax. 2007;62 Suppl 1:1-46.
  2. Turner D et al. Systematic review and economic decision modelling for the prevention and treatment of influenza A and B. Health Technol Assess. 2003;7(35):1-170
  3. Immunisation Against Infectious Disease - "The Green Book". Chapter 19. Influenza (October 2020)

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