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

• A. Polio
• B. Rabies
• C. Hepatitis B
• D. Yellow Fever

Answer: (D)

Live attenuated vaccines use a weakened form of the pathogen that can still replicate in the person but does not cause disease in healthy individuals. This replication presents a broad and durable immune stimulus, often yielding strong humoral and cellular immunity with fewer doses.

Yellow Fever vaccine is a classic example: it uses a live but weakened yellow fever virus that remains replication-competent long enough to provoke a robust immune response. In contrast, hepatitis B is a recombinant subunit vaccine containing only a viral surface antigen, not a live organism. Rabies vaccines for humans are typically inactivated or recombinant, not live attenuated. Polio vaccines can be either live attenuated (oral polio vaccine) or inactivated, depending on the formulation, so it’s not as clean a single example as yellow fever.