Which statement about live attenuated vaccines is correct?

• A. They cannot replicate in the host
• B. They require only one dose for lifelong immunity in all cases
• C. They mimic natural infection by replicating to induce immunity
• D. They are safe for all immunocompromised individuals

Answer: (C)

Live attenuated vaccines use weakened forms of a pathogen that can still replicate in the body. That replication makes the immune system see the pathogen in a way that closely resembles a natural infection, so it mounts a strong, broad immune response and develops lasting memory. Because the virus or bacterium is replicating, these vaccines often stimulate both antibody production and cellular immunity, leading to durable protection with fewer doses in many cases.

This replication-based mechanism is why the statement about mimicking natural infection by replicating to induce immunity is the best choice. The other ideas don’t fit: vaccines that cannot replicate miss the defining feature of live attenuated vaccines; immunity that is guaranteed with a single dose for everyone isn’t accurate because protection duration and the need for boosters vary by vaccine and person; and vaccines that are safe for all immunocompromised individuals aren’t correct because replication can pose risks to those with weakened immune systems.