Risks associated with OPV cessation
Countries should simultaneously stop using oral polio vaccine (OPV) for routine immunization as soon as possible after the interruption of wild poliovirus transmission globally. However, there are a number of risks associated with stopping OPV use that must be minimized and managed.
Emergence of circulating vaccine-derived poliovirus (cVDPV)
Following OPV cessation, there is the immediate risk that a vaccine-derived poliovirus could begin to circulate in a population and trigger an outbreak.
During the first year after OPV cessation, there is an estimated 65–90% chance of a polio outbreak due to cVDPVs occurring somewhere in the world. For any individual country the risk is remote and diminishes rapidly over the 1–2 year period after OPV cessation. Countries with low routine immunization coverage at the time of OPV cessation are expected to be at greatest risk.
OPV cessation must be synchronized across all countries so that the risk of cVDPVs decreases rapidly and uniformly across the world, with no country at risk of importing a cVDPV from another country where OPV is still being used.
Inadvertent re-introduction of poliovirus
A second, longer-term risk to a polio-free world is the inadvertent re-introduction of a poliovirus into a population. The poliovirus could come from:
- a vaccine manufacturing site
- a research facility
- a diagnostic laboratory.
An inadvertent release of poliovirus into a polio-free world could have serious consequences, particularly for countries that stop all polio immunization and therefore would have a highly susceptible population.
The magnitude of this risk is dependent on the extent of poliovirus destruction and the quality of global poliovirus containment measures. These protective measures will focus on both the safety of workers and the minimization of any chance that infectious materials could leave a given facility.