A couple has 4 children, all unvaccinated for Measles. A child got Measles on 5 August 2015. Two other children developed Measles by 15 August 2015. Secondary attack rate is:
**Core Concept**
The secondary attack rate (SAR) is a measure of the frequency of infection among contacts exposed to a primary case in a population. It is calculated as the number of secondary cases among exposed contacts divided by the total number of exposed contacts, usually expressed as a percentage.
**Why the Correct Answer is Right**
To calculate the SAR, we need to identify the total number of exposed contacts and the number of secondary cases. In this scenario, the total number of exposed contacts is the children who were exposed to the primary case, which is 3 (since the child who got measles is not counted as an exposed contact). The number of secondary cases is 2. The SAR is calculated as (number of secondary cases / total number of exposed contacts) x 100 = (2/3) x 100 = 66.67%. This means that the secondary attack rate for measles in this family is approximately 66.67%.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not provide a clear calculation or percentage value for the SAR.
**Option B:** This option is likely incorrect because it does not take into account the total number of exposed contacts.
**Option C:** This option is likely incorrect because it does not provide a clear calculation or percentage value for the SAR.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the secondary attack rate can vary depending on the population, the infectious agent, and the level of exposure. In general, a higher SAR indicates a more contagious disease.
**Correct Answer: C. 66.67%**