**Core Concept**
The introduction of Td vaccine (tetanus and diphtheria toxoids) is a significant change from the traditional TT vaccine (tetanus toxoid). This shift aims to provide broader immunity against diphtheria, a potentially life-threatening bacterial infection. The Td vaccine contains a reduced dose of tetanus toxoid and a standard dose of diphtheria toxoid.

**Why the Correct Answer is Right**
The Td vaccine offers improved protection against diphtheria, which is caused by the bacterium Corynebacterium diphtheriae. The diphtheria toxoid in the Td vaccine stimulates the production of antibodies that neutralize the toxin produced by this bacterium, thereby reducing the risk of diphtheria infection. In contrast, the traditional TT vaccine primarily focused on tetanus immunity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the Td vaccine does indeed contain a reduced dose of tetanus toxoid, making it a true statement.
* **Option B:** This option is incorrect because the Td vaccine is indeed a safer alternative to the traditional TT vaccine, especially for individuals with a history of allergic reactions to tetanus toxoid.
* **Option C:** This option is incorrect because the Td vaccine is indeed recommended for individuals who have completed their primary tetanus-diphtheria vaccination series, making it a true statement.

**Clinical Pearl / High-Yield Fact**
The Td vaccine is recommended for booster shots every 10 years to maintain immunity against diphtheria, and it is also recommended for individuals who have not completed their primary tetanus-diphtheria vaccination series.

**Correct Answer: D.**