Influenza is a topic covered in the 5-Minute Emergency Consult.

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  • Acute, usually self-limited, viral infection
  • Transmission: By dispersion in small-particle aerosols created by sneezing, coughing, and talking
  • Virus is deposited on respiratory tract epithelium and absorbed.
  • Incubation period: 1–4 days (avg. 2)
  • Mean duration in adults: 4 days
  • Seasonal outbreaks most common in February.
  • 2009 novel H1N1 pandemic peaked in fall and early winter of that year. Children and pregnant women had particularly high complication rates.
  • Complications:
    • Primary influenza viral pneumonia
    • Secondary bacterial pneumonia
    • Exacerbations of COPD
    • Otitis and sinusitis in children
    • Reactive airway disease
    • Rare complications: Myositis, myocarditis, pericarditis, Guillain–Barré syndrome, and aseptic meningitis
    • ARDS and multisystem organ failure
  • Key features:
    • Seasonal epidemics are spread by high attack rates in immunologically naive children.
    • Intermittent unpredictable pandemics
    • Mortality results largely from pulmonary complications.

Pediatric Considerations
  • Children exhibit more lower respiratory involvement (croup, bronchitis, bronchiolitis, pneumonitis) and higher temperatures than adults.
  • Children were particularly susceptible to complications of novel H1N1 influenza virus.
  • Myalgias in the calf muscle
  • Febrile convulsions occur in ∼10% of children <5 yr of age with influenza infection.
  • Reye syndrome:
    • Influenza may be a predisposing factor.
    • Rare and severe complication associated with salicylate use (children taking aspirin chronically are recommended to get vaccinated)
    • Acute liver and brain injury


  • Caused usually by 1 of 2 influenza types, A or B, the latter usually less severe.
  • Influenza A subtypes are classified by hemagglutinin antigens H1, H2, or H3 and less importantly by the neuraminidase subtype.
  • Vaccine targets the subtype antigen, which is also the target of natural immunity.
  • Annual epidemics are seasonal:
    • Caused by antigenic drift—new variants from minor changes in surface protein
    • Duration of epidemic <6 wk
  • Pandemics:
    • Unpredictable
    • Caused by antigenic shift—major changes in virus structure
  • Waterfowl reservoir of influenza virus
  • Avian flu has proven difficult to transmit to humans and between humans, but infection is often very severe.
  • The 3 most common strains in 2012 (most vaccinations cover) were influenza B viruses, influenza A (H1N1), and influenza A (H3N2)

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