Which of the following vaccines has maximum efficacy after a single dose –
**Core Concept**
The question is testing the efficacy of single-dose vaccines. Live attenuated vaccines, such as the measles vaccine, are known to provide high immunity after a single dose, whereas inactivated vaccines like tetanus toxoid and DPT require multiple doses to achieve full protection.
**Why the Correct Answer is Right**
The measles vaccine is a live attenuated vaccine that induces a strong immune response after a single dose. This is due to the replication of the weakened virus in the host cells, which stimulates both cellular and humoral immunity. The measles vaccine contains the Edmonston-Zagreb strain, which is highly effective in preventing measles. Studies have shown that a single dose of the measles vaccine provides around 93% immunity against measles.
**Why Each Wrong Option is Incorrect**
**Option A:** Tetanus toxoid is an inactivated vaccine that requires multiple doses (typically 3-4) to achieve full protection against tetanus. It induces humoral immunity by stimulating the production of antibodies against tetanus toxin.
**Option B:** DPT (Diphtheria, Pertussis, Tetanus) is a combination vaccine that also requires multiple doses to achieve full protection. The inactivated diphtheria and tetanus toxoids, along with the pertussis component, induce humoral immunity and provide protection against these diseases.
**Option D:** Typhoid vaccines are typically inactivated or conjugate vaccines that require multiple doses to achieve full protection. The most common typhoid vaccine is the Vi capsular polysaccharide vaccine, which induces humoral immunity by stimulating the production of antibodies against the Vi capsular antigen of Salmonella Typhi.
**Clinical Pearl / High-Yield Fact**
It's essential to note that some vaccines, like the measles vaccine, can provide high immunity after a single dose, whereas others require multiple doses to achieve full protection. Understanding the type of vaccine and its required dosing schedule is crucial for effective vaccination strategies.
**✓ Correct Answer: C. Measles**