Which of the following is an example of a live attenuated vaccine?

• A. Diphtheria toxoid vaccine
• B. Influenza inactivated vaccine
• C. MMR vaccine
• D. Hepatitis B vaccine

Answer: (C)

Live attenuated vaccines use weakened versions of pathogens that can still replicate in the body, which helps the immune system recognize and respond in a way very similar to a natural infection. Because they imitate real infection, they often provoke strong, long-lasting immunity with fewer doses. The MMR vaccine contains attenuated measles, mumps, and rubella viruses, so it exposes the immune system to all three antigens in a live, albeit weakened, form. This leads to robust antibody and T-cell responses and durable protection.

In contrast, a diphtheria toxoid vaccine uses an inactivated toxin and does not involve a live organism. An influenza inactivated vaccine uses killed virus, so it cannot replicate. A hepatitis B vaccine is a recombinant protein vaccine, not a live organism.