Which one of the following is an Index of communicability of an Infection?
An  important  measure  of communicability is 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 :
Carrier rate
Prevalence rate
Secondary attack rate
Primary attack rate