Secondary attack rate
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In epidemiology, the secondary attack rate (SAR) is the proportion or probability of susceptible contacts who develop infection within a reasonable incubation period after known exposure to a primary (index) case (or patient zero, i.e. the case which introduced the pathogenic organism into the population) or the same infectious source.[1][2][3] It is typically calculated in households, families, dormitories, barracks, or other closed groups (such as attending medical staff), where close contact facilitates person-to-person transmission.[2]
Secondary Attack Rate (SAR) is expressed by the following formula:[4]
where
- = Number of new cases among contacts of primary cases
- = Total number of susceptible contacts
The denominator may be restricted to susceptible persons when these can be determined (e.g., by excluding immune or already-infected individuals). In practice, it is often approximated as the total population in the household (or other group) minus the number of primary cases.[5]
SAR can be estimated using many different epidemiologic study designs, models, and methods.[3] While traditionally termed a "rate," SAR is not a true rate, but a proportion.[1][4]