Secondary attack rate

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NEET PG High-Yield: Secondary Attack Rate (SAR)

The Secondary Attack Rate measures the frequency of infection in a well-defined group (like a household) following the introduction of an index case.

Formula:

SAR = \frac{\text{Number of exposed persons who developed the disease within the range of incubation period}}{\text{Total number of exposed susceptible persons}} \times 100

Key Concepts

Factor Detail
Numerator New cases among contacts.
Denominator Total susceptible contacts exposed to the index case.
High-Yield NEET PG Pearls:

  • Utility: Indicates the infectivity of an agent and the effectiveness of control measures (like isolation) within a closed setting.
  • Denominator Logic: It is crucial to exclude those already immune (previous infection or vaccination) from the denominator.
  • Range: A high SAR suggests high communicability (e.g., Measles or Varicella), whereas a low SAR suggests lower infectivity (e.g., Polio).