An attenuated vaccine is one in which:

An attenuated vaccine is one in which:

• A. Microorganisms are killed
• B. Microorganisms are weakened but are still alive
• C. Microorganisms are 100% virulent
• D. No microorganisms are found

Answer: (B)

An attenuated vaccine uses microorganisms that are weakened but still alive. Because the pathogen is weakened rather than killed, it can replicate in the host just enough to present a broad set of antigens and stimulate the immune system more naturally. This tends to produce a strong, long-lasting immune response with memory, often after fewer doses. Inactivated vaccines, by contrast, contain organisms that have been killed and cannot replicate, which usually leads to a weaker immune response requiring boosters. A vaccine that uses fully virulent organisms would risk causing disease, which is unsafe. And a preparation with no microorganisms at all wouldn’t train the immune system.

An attenuated vaccine uses microorganisms that are weakened but still alive. Because the pathogen is weakened rather than killed, it can replicate in the host just enough to present a broad set of antigens and stimulate the immune system more naturally. This tends to produce a strong, long-lasting immune response with memory, often after fewer doses. Inactivated vaccines, by contrast, contain organisms that have been killed and cannot replicate, which usually leads to a weaker immune response requiring boosters. A vaccine that uses fully virulent organisms would risk causing disease, which is unsafe. And a preparation with no microorganisms at all wouldn’t train the immune system.