**Core Concept**
Live vaccines contain weakened or attenuated forms of a pathogen, which stimulate a mild infection and induce long-term immunity. They are often used to prevent serious or life-threatening diseases. However, administering two live vaccines simultaneously can lead to enhanced reactivation of the attenuated pathogens, potentially causing severe illness.

**Why the Correct Answer is Right**
Administering two live vaccines at the same time can result in an increased risk of adverse effects due to the enhanced reactivation of the attenuated pathogens. This is because live vaccines can replicate in the body, and combining them can lead to an overactive immune response. For example, administering live measles and live mumps vaccines simultaneously can increase the risk of measles and mumps disease. Therefore, live vaccines are typically administered separately, with a minimum interval of 30 days between doses.

**Why Each Wrong Option is Incorrect**
**Option B:** Booster doses are often required for live vaccines, especially in individuals with weakened immune systems, to ensure long-term immunity.
**Option C:** While live vaccines can provide long-term immunity, they do not necessarily guarantee lifelong immunity. Booster doses may be required to maintain immunity over time.
**Option D:** Live vaccines can contain either major or minor antigens, but they typically contain a single type of antigen. The major antigen is usually the primary antigen responsible for inducing immunity.

**Clinical Pearl / High-Yield Fact**
When administering live vaccines, it's essential to follow the recommended schedule and interval between doses to minimize the risk of adverse effects and ensure optimal immunity.

**✓ Correct Answer: A. Two live vaccines cannot be administered simultaneously**