Description

• 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

• 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

Etiology

• 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