**Core Concept**
The oral Ty21a typhoid vaccine is a live attenuated vaccine that provides protection against typhoid fever by stimulating both humoral and cellular immunity. It is administered orally in a specific dosing regimen to ensure adequate immune response and long-term protection.

**Why the Correct Answer is Right**
The Ty21a vaccine is given as a **three-dose series** administered orally, with doses given at 0, 1, and 6 months. This schedule ensures a robust immune response due to the gradual exposure of the immune system to the vaccine strain. The vaccine is most effective when the doses are spaced appropriately to allow for proper antigen presentation and memory B-cell development.

**Why Each Wrong Option is Incorrect**
Option A: 14 doses are not recommended; this is far beyond the standard regimen and contradicts all clinical guidelines.
Option B: Five doses are not part of any official Ty21a schedule; this is an incorrect and unsupported dosage.
Option D: A single dose is insufficient to induce protective immunity and is not recommended in standard protocols.

**Clinical Pearl / High-Yield Fact**
The Ty21a vaccine is most effective when given as **three doses**, and it is particularly recommended in endemic areas or for travelers to high-risk regions. A single dose provides only partial protection and is not sufficient for reliable immunity.

✓ Correct Answer: C. 3