## **Core Concept**
The secondary attack rate is a measure used in epidemiology to describe the spread of a disease within a defined group, such as a community, household, or other closed population. It is calculated as the number of new cases among contacts within the incubation period following exposure to a primary case, divided by the number of exposed contacts. This metric helps in understanding the contagiousness or transmissibility of a disease.

## **Why the Correct Answer is Right**
To calculate the secondary attack rate, we first need to understand the given scenario: There are 100 children, out of which 28 are immunized against measles, leaving 72 non-immunized children. Two of the non-immunized children acquire measles simultaneously and are considered the primary cases. Subsequently, 14 more children get measles.

The secondary attack rate (SAR) is calculated as the number of new cases (14) divided by the number of exposed contacts (72 - 2 primary cases = 70, since the 2 primary cases are not considered contacts but sources of infection).

So, SAR = 14 / 70 = 0.2 or 20%.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not match our calculated secondary attack rate of 20%.
- **Option B:** This option suggests a calculation that does not align with the provided numbers and the definition of secondary attack rate.
- **Option D:** This option is incorrect as it represents a much higher rate than what is calculated from the given data.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the secondary attack rate is a crucial measure for assessing the effectiveness of public health interventions, such as vaccination, in controlling the spread of infectious diseases. It is particularly useful for diseases like measles, which is highly contagious and can spread rapidly in non-immunized populations.

## **Correct Answer:** B. 20%