**Core Concept**
Measles vaccination is a critical component of childhood immunization schedules, and its administration timing is crucial to prevent measles infection. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recommend a specific age for measles vaccination to ensure its effectiveness.

**Why the Correct Answer is Right**
The measles vaccine is not given before 9 months of age because of the risk of measles infection in infants exposed to the virus through their mother's milk. Infants under 9 months old have incomplete immunity, and maternal antibodies from breast milk may interfere with the vaccine's effectiveness. Additionally, the vaccine's efficacy is lower in infants under 9 months, increasing the risk of vaccine failure.

**Why Each Wrong Option is Incorrect**
**Option B:** 12 months is still too early for measles vaccination, as it does not account for the risk of exposure to measles virus through maternal antibodies.

**Option C:** 15 months is closer to the recommended age, but it is not the minimum age for measles vaccination.

**Option D:** 18 months is not significantly different from 15 months and does not address the primary concern of maternal antibody interference.

**Clinical Pearl / High-Yield Fact**
Infants under 9 months old should be protected from measles exposure through maternal vaccination and careful avoidance of measles-endemic areas.

**✓ Correct Answer: A. 9 months. Measles vaccine is not given before 9 months.**