How do vaccine-derived strains emerge, and why are they a concern in polio-free regions?

• A. OPV virus mutates during replication in under-immunized populations, potentially regaining neurovirulence and spreading.
• B. Wild-type PV mutates in fully immunized communities.
• C. OPV cannot mutate in the environment.
• D. Vaccine-derived strains only occur in polio-endemic regions.

Answer: (A)

Vaccine-derived strains emerge because the oral polio vaccine uses a live, attenuated virus. After vaccination, this weakened virus replicates in the child’s gut and can be shed in stool. In communities with low vaccination coverage, that attenuated virus can spread from person to person and gradually accumulate mutations. Sometimes these changes revert the virus toward neurovirulence, restoring its ability to cause paralysis and to spread again. In polio-free regions, pockets of under-immunized people mean the circulating vaccine virus can continue to transmit long enough to mutate, posing a risk to populations that are otherwise protected. This explains why even areas without wild poliovirus can experience outbreaks from vaccine-derived strains if immunity gaps exist.