This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Epidemiology, Subject name: Social & Preventive Medicine
Which of the following is the best tool to measure Communicability of an infection? 
Select and explain. 
A. Prevalence rate, B. Primary attack rate, C. Secondary attack rate, D. Carrier rate
C: Secondary attack rate
Answer: C: Secondary attack rate 
Explanation: 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%.