**Core Concept**
The Vi polysaccharide vaccine of typhoid is a type of conjugate vaccine designed to provide immunity against Salmonella Typhi, the causative agent of typhoid fever. This vaccine is often recommended for individuals traveling to areas where typhoid is endemic.

**Why the Correct Answer is Right**
The Vi polysaccharide vaccine is typically administered to individuals aged 2 years and above, and not at birth. This is because the immune system of newborns is still developing, and they may not respond effectively to polysaccharide-based vaccines. Additionally, the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) do not recommend routine vaccination of infants at birth.

**Why Each Wrong Option is Incorrect**
* **Option A:** A single dose of the Vi polysaccharide vaccine is not sufficient to provide long-term immunity, and booster doses are usually recommended every 2-3 years.
* **Option B:** The recommended schedule for revaccination with the Vi polysaccharide vaccine is every 2-3 years, not at 3 years.
* **Option D:** The Vi polysaccharide vaccine is administered intramuscularly, not subcutaneously.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the Vi polysaccharide vaccine is not effective in individuals with a weakened immune system, such as those with HIV/AIDS or undergoing chemotherapy.

**✓ Correct Answer: C. Given at birth**