Secondary attack rate measures what?

• A. Incubation period
• B. Probability of transmission among susceptible contacts after exposure
• C. Mortality rate
• D. Length of hospital stay

Answer: (B)

Secondary attack rate captures the probability that a susceptible person who has been exposed to an infectious individual will develop the disease, measured among a defined group of contacts after exposure. It focuses on transmission within close-contact settings, like households or other shared environments, over a period aligned with the disease’s incubation window. To calculate it, count the number of susceptible contacts who become cases after exposure and divide by the total number of susceptible contacts, then multiply by 100 to express as a percentage. For example, if 12 susceptible contacts are exposed and 4 become cases, the secondary attack rate is 33%. This is not about the incubation period (time from exposure to onset), mortality, or length of hospital stay.