If tOPV has been so successful in reducing paralysis, why is it being withdrawn?

The trivalent oral polio vaccine (tOPV) has indeed been successful in reducing the incidence of polio worldwide. However, it is being withdrawn for a few reasons:

1. Risk of Vaccine-Derived Poliovirus: While rare, there is a risk that the live, weakened virus in tOPV can mutate and cause vaccine-derived poliovirus (VDPV) in under-immunized populations. This can lead to outbreaks of polio.

2. Shift to bivalent oral polio vaccine (bOPV): The Global Polio Eradication Initiative (GPEI) has recommended a shift from tOPV to bOPV, which protects against two types of poliovirus instead of three. This is because wild poliovirus type 2, which tOPV protects against, has been eradicated worldwide, and continued use of tOPV poses a risk of reintroducing this type of virus.

3. Introduction of Inactivated Polio Vaccine (IPV): IPV, which does not contain live virus, is being introduced into routine immunization schedules to ensure continued protection against all types of poliovirus.

4. Final stages of polio eradication: The withdrawal of tOPV is part of the final stages of the global polio eradication strategy. The goal is to stop all use of oral polio vaccines, which contain live virus, once all types of wild poliovirus have been eradicated.

In summary, while tOPV has been successful, its withdrawal is a strategic move to reduce the risk of VDPV, stop the use of live virus vaccines, and ultimately eradicate polio.