**Core Concept**
Measles vaccine is a live attenuated vaccine designed to protect against measles, a highly contagious viral infection caused by the measles virus. The vaccine is typically administered in two doses to children, with the first dose usually given at 9-12 months of age and a second dose at 16-18 months.

**Why the Correct Answer is Right**
Measles vaccine works by inducing a mild infection with the attenuated virus, which triggers an immune response and provides long-term protection against the disease. The vaccine is effective in preventing severe measles and its complications, such as pneumonia and encephalitis. The measles vaccine also has a high efficacy rate, with a reported efficacy of 93-97% after two doses.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because measles vaccine is not known to cause significant side effects, such as anaphylaxis or severe allergic reactions.
**Option B:** This option is incorrect because measles vaccine is not a combination vaccine, although it may be administered in combination with other vaccines, such as MMR (measles, mumps, and rubella) vaccine.
**Option C:** This option is incorrect because measles vaccine is not a killed-virus vaccine; it is a live attenuated vaccine.

**Clinical Pearl / High-Yield Fact**
The measles vaccine is a crucial component of childhood vaccination schedules worldwide, and its administration has led to a significant reduction in measles-related morbidity and mortality. However, vaccination rates have declined in some regions, leading to outbreaks of measles.

**Correct Answer: A.**