• 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 d (average 2)
  • Mean duration in healthy adults: 4 d
  • 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 more often exhibit 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


  • Usually caused 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

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