Which setting increases risk of VDPV spread?

• A. Regions with strict border controls.
• B. Under-immunized populations with extended circulation.
• C. Areas with no sewage systems.
• D. Regions with high vaccination coverage.

Answer: (B)

Vaccine-derived poliovirus spreads most when a live attenuated virus from the oral polio vaccine circulates in a population with low immunity for an extended period. In such settings, the weakened virus can be shed and pass from person to person for months or years, allowing it to accumulate mutations. Some of these mutations can restore the virus’s ability to cause disease and spread, turning it into a vaccine-derived poliovirus that behaves like wild poliovirus in susceptible networks. The key factors are ongoing transmission in an under-immunized population and the time for the virus to evolve, which together increase spillover risk and outbreak potential.

Regions with strict border controls limit cross-border transmission, reducing spread. Areas with no sewage systems can have transmission pathways, but without gaps in immunity and sustained circulation the risk of evolving into VDPV remains much lower. Regions with high vaccination coverage break transmission entirely, preventing the virus from circulating long enough to mutate into a VDPV.