Which of the following is the best tool to measure Communicability of an infection?

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. SAR= No. of exposed persons developing the disease within the range of the incubation period x 100 --------------------------------------------------------------------------------------------------------------------- Total no. of susceptible contacts The primary case is excluded from both the numerator and the denominator. It measures communicability/ infectivity of an infection. SAR of Measles = > 90%; Mumps = > 86%; Chicken pox = >90%.