Why can OPV lead to vaccine-derived outbreaks?

• A. Because it is injected and has slower onset.
• B. Because it is less effective at preventing disease.
• C. Because it doesn't produce antibodies.
• D. Because the vaccine contains a live-attenuated virus that can mutate and circulate in under-immunized populations.

Answer: (D)

The concept tested is that a live-attenuated vaccine can occasionally cause outbreaks when the vaccine strain spreads in a population that isn’t highly immunized. Oral polio vaccine contains a weakened but live virus that replicates in the gut and can be shed in stool. In communities with insufficient immune coverage, this shed virus can continue to circulate among people who are not fully protected. During this circulation, the attenuated virus can accumulate mutations and sometimes revert toward a more neurovirulent form. If this reverted virus remains capable of spreading, it can cause new cases or outbreaks—these are vaccine-derived polio outbreaks. This risk is why OPV, while very effective at boosting intestinal immunity, carries a potential for vaccine-derived transmission in under-immunized populations.

The other statements don’t capture this mechanism. OPV is not injected and doesn’t merely have a slower onset; it’s taken orally and is designed to induce strong mucosal immunity. It does produce antibodies, so the issue isn’t that it fails to generate an immune response. And while it is effective, the specific risk of vaccine-derived outbreaks comes from the live, mutating virus circulating in under-immunized groups.