Vaccinology — In an attenuated (live-attenuated) vaccine, what is true about the vaccine viruses?

Introduction:
Live-attenuated vaccines contain weakened versions of pathogens that retain the ability to replicate, but with markedly reduced virulence. This question checks understanding of how attenuation differs from inactivation and why attenuation elicits durable immunity.

Given Data / Assumptions:

• Attenuated viruses are weakened, not killed.
• Chemical inactivation is associated with inactivated vaccines, not attenuated ones.
• Viruses are much smaller than bacteria and always require a genome to replicate.

Concept / Approach:
Attenuated strains replicate at low levels after vaccination, providing antigenic stimulation that mimics natural infection without causing disease in immunocompetent hosts. This limited replication enhances both humoral and cellular immune responses and often yields long-lasting protection after a single dose.

Step-by-Step Solution:

Verification / Alternative check:
Clinical examples include measles, mumps, rubella, varicella, and oral polio vaccine strains, all of which replicate transiently to induce robust immunity.

Why Other Options Are Wrong:

• No genome: impossible for a replicating virus.
• Larger than bacteria: virology fundamentals contradict this.
• Chemically altered or killed: describes inactivated vaccines, not attenuated vaccines.

Common Pitfalls:
Confusing live-attenuated with inactivated vaccines and assuming replication is absent.

Final Answer:
They continue to replicate in the recipient to a limited extent