Which statement correctly describes a living vaccine?

Which statement correctly describes a living vaccine?

• A. They contain inactivated pathogens.
• B. They are composed of subunit fragments.
• C. They contain attenuated organisms that can replicate to induce immunity.
• D. They are synthetic peptides only.

Answer: (C)

Live vaccines use weakened, attenuated organisms that can still replicate in the host. This replication presents the immune system with a broad set of antigens in a way that closely mimics natural infection, leading to strong and often long-lasting immunity that engages both antibody (humoral) and T-cell (cell-mediated) responses. Because they replicate, these vaccines can sometimes provide protection with fewer doses and can spread by contact in rare cases, though they are generally safe for individuals with normal immune function. Inactivated vaccines, by contrast, contain pathogens that have been killed and cannot replicate, so they tend to produce a weaker immune response and typically require multiple doses or boosters. Subunit vaccines use only parts of the pathogen, not whole organisms, and synthetic peptide vaccines rely on short peptide sequences; neither involves a live, replicating organism.

Live vaccines use weakened, attenuated organisms that can still replicate in the host. This replication presents the immune system with a broad set of antigens in a way that closely mimics natural infection, leading to strong and often long-lasting immunity that engages both antibody (humoral) and T-cell (cell-mediated) responses. Because they replicate, these vaccines can sometimes provide protection with fewer doses and can spread by contact in rare cases, though they are generally safe for individuals with normal immune function.

Inactivated vaccines, by contrast, contain pathogens that have been killed and cannot replicate, so they tend to produce a weaker immune response and typically require multiple doses or boosters. Subunit vaccines use only parts of the pathogen, not whole organisms, and synthetic peptide vaccines rely on short peptide sequences; neither involves a live, replicating organism.