Why will countries need to switch from tOPV to bOPV?
There are three types of wild poliovirus (WPV) - type 1, 2 and 3 - each of which is targeted by a different component of the trivalent oral polio vaccine (tOPV).
Live attenuated vaccines are very effective against the wild virus, but in very rare cases can lead to paralysis. There are two ways this can occur:
- Vaccine Associated Paralytic Poliomyelitis (VAPP): At a global level for every birth cohort of 1 million children in OPV-only using countries, there are 2-4 cases of VAPP. This translates to an estimated 250 — 500 VAPP cases globally per year. Of these, about 40% are caused by OPV's type 2 component. In the Region of the Americas, the VAPP risk is 1 case per 7.68 million doses administered.
- Circulating Vaccine Derived Poliovirus (cVDPV) outbreaks: these rare outbreaks occur when a vaccine-related virus is passed from person-to-person, mutating over time and acquiring wild virus transmissibility and neurovirulence characteristics. Almost all cVDPV outbreaks in recent years have been caused by a type 2 vaccine-derived virus.
Although wild poliovirus type 2 appears to have been eradicated globally in 1999, vaccine-related type 2 viruses continue to cause the majority of cVDPV outbreaks and many VAPP cases. Therefore, OPV type 2 now carries more risk than benefit and undermines global polio eradication efforts. Thus, tOPV will be replaced with bivalent OPV (bOPV), which will continue to target the remaining polio types 1 and 3. Once these types are eradicated, bOPV will also be withdrawn.