What is secondary attack rate?
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Here is a thorough breakdown of the Secondary Attack Rate (SAR) based on Park's Textbook of Preventive and Social Medicine:
Secondary Attack Rate (SAR)
Definition
SAR is defined as "the number of exposed persons developing the disease within the range of the incubation period, following exposure to the primary case."
It is an important measure of the communicability of an infectious disease - specifically how readily a disease spreads from an index (primary) case to susceptible contacts within a defined group.
Formula
$$\text{SAR} = \frac{\text{Number of exposed persons developing the disease within the incubation period}}{\text{Total number of exposed / susceptible contacts}} \times 100$$
Important: The primary case is excluded from both the numerator and denominator.
Worked Example
Consider a family of 6:
- 2 parents (immune)
- 4 children (susceptible)
One child develops measles (primary case). Within the incubation period, 2 more children fall ill.
SAR = 2/3 = 66.6%
(The primary case is excluded from both numerator and denominator; the 2 immune parents are excluded from the denominator since we restrict to susceptibles.)
Special Case: Long-Infectious Diseases (e.g., Tuberculosis)
When the primary case is infective over a long period, the standard formula is modified to account for duration:
$$\text{SAR} = \frac{\text{Number of contacts developing TB}}{\text{Number of person-weeks (months/years) of exposure}} \times 100$$
Limitations
- Short-infectious diseases only - SAR works best for diseases with a short communicable period (e.g., measles, chickenpox). For diseases like TB with prolonged infectivity, duration of exposure must be factored in.
- Identifying susceptibles - Identifying who is truly susceptible is only feasible when history is a reliable indicator (e.g., measles, chickenpox). For many diseases (e.g., influenza), this is difficult.
- Subclinical cases - Where numerous subclinical (asymptomatic) infections exist, SAR has limited meaning without laboratory investigations.
Uses
- Measures the communicability of a disease within a closed group (household, dormitory, school)
- Compares attack rates between vaccinated and unvaccinated populations across families
- Helps assess the effectiveness of control measures and vaccines
- Can be applied beyond households to any circumscribed group
SAR vs. Other Measures
| Measure | Scope | Population |
|---|---|---|
| SAR | Close contacts of a known case | Susceptibles in a defined setting |
| Attack Rate | All exposed in an outbreak | General population at risk |
| R0 (Basic Reproduction Number) | Average new infections per case | Fully susceptible population |
- Park's Textbook of Preventive and Social Medicine, pp. 1874-1925
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