**Question:** A village has a total of 100 under-five children. The coverage with measles vaccine in this age group is 60%. Following the occurrence of a measles case in a child after a visit outside, twenty six children developed Of measles is - measles. The secondary attack rate

A. 26
B. 13
C. 50%
D. 33%

**Core Concept:** The secondary attack rate refers to the proportion of susceptible individuals in a population that become infected after an index case (first case) has occurred. In this scenario, we are calculating the proportion of vaccinated children who become infected after one child with measles gets infected due to exposure outside the vaccinated community.

**Core Concept:** In the given scenario, the measles vaccination coverage is 60%. This means that 60 out of 100 children are immune to measles due to vaccination.

**Why the Correct Answer is Right:** We are given that after a measles case occurs in the village, 26 children develop measles. These 26 children represent the secondary cases (those who developed measles after exposure to the index case). Since the vaccination coverage is 60%, we can calculate the secondary attack rate as follows:

(Number of secondary cases / Total number of susceptible children) * 100%

(26 / 40) * 100% = 65%

**Core Concept:** In this context, "susceptible children" refer to those who have not been vaccinated.

**Why Each Wrong Option is Incorrect:**

A. 26 (incorrectly assumes that all secondary cases contributed to the attack rate, which is not the case)
B. 13 (incorrectly assumes that the number of susceptible children is equal to the number of secondary cases, which is not the case)
C. 50% (incorrectly assumes that the attack rate is equal to the vaccination coverage, which is not the case)
D. 33% (incorrectly assumes that the attack rate is equal to the number of susceptible children, which is not the case)

**Core Concept:** The correct calculation of attack rate requires the number of susceptible children (unvaccinated children) and the number of secondary cases.

**Clinical Pearls:** A higher attack rate indicates a higher likelihood of outbreaks or secondary cases in the unvaccinated population. In this case, the correct calculation of attack rate yields a 65% attack rate, indicating a higher likelihood of measles outbreak in the unvaccinated children in the village.