In house with 9 susceptible individuals 1 gets infected and other two get infected after two days what is the secondary attack rate ?
**Core Concept**
The secondary attack rate (SAR) is a measure of the frequency of secondary cases (individuals who become infected after exposure to an initial case) in a specified time period and population at risk. It is calculated as the number of secondary cases divided by the total number of susceptible individuals in the population.
**Why the Correct Answer is Right**
To calculate the secondary attack rate, we need to identify the number of secondary cases and the total number of susceptible individuals. In this scenario, one individual was initially infected, and two more became infected after two days. This means that the two new cases are considered secondary cases. The total number of susceptible individuals in the household is 9 (initially all 9 were susceptible, but 3 have now become infected). Therefore, the secondary attack rate is calculated as the number of secondary cases (2) divided by the total number of susceptible individuals (9).
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not take into account the correct number of secondary cases and susceptible individuals.
**Option B:** This option is incorrect because it does not provide a clear calculation for the secondary attack rate.
**Option C:** This option is incorrect because it does not consider the time period between the initial infection and the secondary infections.
**Clinical Pearl / High-Yield Fact**
The secondary attack rate is an important measure of the contagiousness of a disease and can help in assessing the effectiveness of control measures.
**Correct Answer: B. 22.22% (2/9) is not a correct option, so it must be another number)**