What best describes a live attenuated vaccine?

• A. They contain a version of a living microbe that has been weakened
• B. They are composed of killed organisms
• C. They are made from recombinant DNA only
• D. They are non-replicating vectors

Answer: (A)

Live attenuated vaccines use a living microbe that has been weakened so it can no longer cause disease in healthy people but can still replicate. That controlled replication inside the body presents the immune system with a real, milder infection, which triggers a strong, broad immune response that includes both antibodies and cellular immunity. Because the immune system encounters the pathogen in a way very similar to natural infection, protection tends to be durable and may require fewer doses. Examples include vaccines for measles, mumps, rubella, varicella, and the oral polio vaccine. This differs from vaccines made from killed organisms, which do not replicate and often need more boosters; recombinant DNA–based vaccines, which use only specific gene fragments or proteins; and non-replicating vector vaccines, which deliver genetic material without producing a replicating organism.