**Core Concept**
The Typhoid Vi polysaccharide vaccine is a type of conjugate vaccine designed to protect against Salmonella Typhi infections, which cause typhoid fever. This vaccine is typically recommended for children in specific age groups based on their immune development and risk factors.

**Why the Correct Answer is Right**
The correct answer is 2 years because the Typhoid Vi polysaccharide vaccine is usually administered to children above the age of 2 years. This is because the immune system of children below 2 years is not mature enough to respond effectively to polysaccharide vaccines. The vaccine is often given to children who have been exposed to typhoid or are traveling to areas where the disease is common. The Vi capsular polysaccharide of Salmonella Typhi is the antigen used in this vaccine, which elicits a T-cell independent immune response.

**Why Each Wrong Option is Incorrect**
**Option A:** 6 months - This age group is too young for the Typhoid Vi polysaccharide vaccine as their immune systems are not mature enough to respond effectively.
**Option B:** 1 year - While some vaccines are given at 1 year, the Typhoid Vi polysaccharide vaccine is typically recommended for children above 2 years.
**Option D:** 1 year 6 months - Similar to option A, children at this age are still too young for the vaccine.

**Clinical Pearl / High-Yield Fact**
The Typhoid Vi polysaccharide vaccine is a conjugate vaccine, meaning it combines the Vi capsular polysaccharide with a carrier protein to enhance immune response, especially in young children. This is crucial for effective protection against typhoid fever.

**✓ Correct Answer: C. 2 years**