In a village of 100 children, 60% are immunized against measles. One child travelled and returned with measles and infected other 26 children. What is the secondary attack rate –
## Core Concept
The secondary attack rate is a measure used in epidemiology to quantify the spread of a disease within a defined group, such as a family, school, or in this case, a village. It is calculated as the number of new cases among the contacts of the primary case(s) 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 identify the number of exposed contacts and the number of new cases that developed from these contacts. Given that 60% of 100 children are immunized against measles, the number of immunized children is 60, which leaves 40 children unimmunized. When one child returned with measles and infected 26 other children, these 26 infected children are considered new cases. The total number of exposed (susceptible) contacts is the number of unimmunized children, which is 40. The secondary attack rate is then calculated as the number of new cases (26) divided by the number of exposed contacts (40), which equals 0.65 or 65%.
## Why Each Wrong Option is Incorrect
- **Option A:** This option suggests a calculation that does not match the provided scenario. Without a specific percentage or calculation provided in the option, we can infer it's incorrect based on our calculation of 65%.
- **Option B:** Similarly, this option does not align with our calculated secondary attack rate of 65%.
- **Option D:** This option also does not match our calculated rate and thus is incorrect.
## Clinical Pearl / High-Yield Fact
A key point to remember is that the secondary attack rate is a useful measure for understanding the spread of infectious diseases within a closed or defined population. It helps in identifying diseases with high contagiousness and in evaluating the effectiveness of public health interventions. For vaccine-preventable diseases like measles, high vaccination rates can significantly reduce the secondary attack rate by decreasing the number of susceptible individuals.
## Correct Answer: B. 65%