What characterizes a live attenuated vaccine?

• A. Contains a weakened form of the pathogen that can replicate but cause minimal disease
• B. Inactivated pathogens
• C. Subunit proteins
• D. mRNA vaccine

Answer: (A)

Live attenuated vaccines use a weakened form of the pathogen that can still replicate in the body but causes minimal to no disease. Because the organism can reproduce, it presents a broad set of antigens in a way that closely resembles a natural infection, which stimulates strong and durable immunity, often after just one dose. This replication also helps activate both antibody responses and T-cell–mediated immunity, building lasting immune memory.

The other descriptions don’t fit this concept: inactivated vaccines use pathogens that have been killed and cannot replicate, usually yielding a weaker immune response and often requiring boosters; subunit vaccines contain only specific pieces of the pathogen, not whole organisms; and mRNA vaccines rely on genetic instructions to host cells to make antigen, not on a live, replicating organism.

In short, the defining feature is a live, weakened organism that can replicate and mimic natural infection while causing little or no disease.