Measure of communicability of a disease
Secondary attack rate : SAR is defined as “the number of exposed persons developing the disease within the range of incubation period, following exposure to primary case. SAR = No of exposed persons developing disease within the range of the incubation period ——(Total number of exposed/susceptible contact) x 100
The important features of SAR are:-

The primary case is excluded from both the numerator and denominator.
Denominator is restricted to susceptible contacts of primary case.
In denominator, person should be susceptible → Immune person (to disease) are not susceptible to develop the disease, So they are excluded.
Disease should develop within the range of incubation period.

SAR is an important measure of communicability. Higher secondary attack rate means, more numbers of susceptible contacts are developing the disease after exposure to primary case.
So, higher the SAR higher is the communicability (infectiousness) of disease.
Case fatality rate
Secondary attack rate
Sullivan index
Incubation period