Secondary attack rate


In epidemiology, the secondary attack rate is the proportion or probability of susceptible contacts who develop infection within a reasonable incubation period after known exposure to a primary case or the same infectious source. It is typically calculated in households, families, dormitories, barracks, or other closed groups, where close contact facilitates person-to-person transmission.

Calculation

Secondary Attack Rate is expressed by the following formula:
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. In practice, it is often approximated as the total population in the household minus the number of primary cases.
SAR can be estimated using many different epidemiologic study designs, models, and methods. While traditionally termed a "rate," SAR is not a true rate, but a proportion.

Use

SAR is a key epidemiologic parameter used to assess contagiousness 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.