**Core Concept**
The Hepatitis A vaccine is a inactivated vaccine that provides long-term immunity against Hepatitis A virus (HAV) infection. The vaccine schedule is designed to ensure optimal protection and to minimize the risk of adverse reactions.

**Why the Correct Answer is Right**
The standard schedule for Hepatitis A vaccine is **2 doses**, administered 6-12 months apart. This allows for the development of adequate antibody levels and provides long-term immunity. The Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) recommend this schedule for individuals at high risk of exposure to HAV.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Hepatitis A vaccine schedule is not typically administered as a single dose.
* **Option B:** This option is incorrect because the Hepatitis A vaccine schedule is not typically administered as a booster dose every 6 months.
* **Option C:** This option is incorrect because the Hepatitis A vaccine schedule is not typically administered as a series of 3 doses, although this may be the case in certain high-risk individuals or in areas with high HAV transmission.

**Clinical Pearl / High-Yield Fact**
It's essential to note that individuals who have a history of liver disease or are immunocompromised may require a different vaccination schedule or additional precautions when receiving the Hepatitis A vaccine.

**Correct Answer: D. Three doses administered in a series of 3.