## **Core Concept**
The World Health Organization (WHO) has a strategy for the elimination of measles, which includes a catch-up vaccination campaign. This campaign targets a specific age group to ensure a significant reduction in measles cases and to help achieve measles elimination. The strategy focuses on vaccinating a broad age range that has not been previously vaccinated or has an uncertain vaccination status.

## **Why the Correct Answer is Right**
The correct answer targets individuals aged 9 months to 14 years. This age range is chosen because it encompasses a wide group of children and adolescents who may not have been vaccinated against measles or may not have received adequate vaccination. By targeting this age group, the strategy aims to rapidly increase measles immunity in the population, thereby interrupting measles transmission. The first dose of measles vaccine is typically given at 9 months of age in many countries, and the second dose, often given as measles, mumps, and rubella (MMR) vaccine, is usually administered later in childhood. The catch-up campaign ensures that those who missed these doses or have uncertain vaccination status are protected.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option suggests targeting children under 5 years. While young children are indeed vulnerable to measles, the catch-up campaign aims to cover a broader age range to ensure herd immunity and to account for individuals who may have missed vaccination at a younger age.
- **Option B:** Targeting only those aged 15-20 years would not be effective as it misses younger children who are also at risk and crucial for achieving herd immunity.
- **Option C:** Focusing on individuals older than 20 years might not be as effective in interrupting measles transmission since the goal is to protect school-aged children and younger populations who are more likely to contribute to ongoing measles transmission.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that measles elimination strategies rely on high vaccination coverage. The catch-up campaign, by targeting a wide age range (usually 9 months to 14 years), helps to quickly boost population immunity against measles. This approach is critical in areas with low vaccination rates or during outbreaks.

## **Correct Answer:** . 9 months to 14 years.