Secondary attack rate Secondary attack rate (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 given by the formula: The denominator consists of all persons who are exposed to the case. More specifically, the denominator may be restricted only to "susceptible" contacts, if means are available to distinguish the susceptible persons from the Immune. The primary case is excluded from both the numerator and denominator. In the above, Total number of susceptible individuals are 3 Number of children affected from primary case =2 SAR = 2/3x100= 66%(approx) Ref : Park 23rd edition Pgno : 100