In epidemiology, thesecondary attack rate (SAR) is the proportion or probability ofsusceptible contacts who developinfection within a reasonableincubation period after known exposure to aprimary (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
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]
SAR is a key epidemiologic parameter used to assess contagiousness (person-to-person spread) of an infectious agent, differentiate transmission in households or closed settings from community transmission, evaluate control measures for limiting spread, and model transmission dynamics, accounting for correlation among contacts exposed to the same source.[1][2]