IPV = Injected polio vaccine (a killed virus)
OPV = Oral polio vaccine (an attenuated virus)

OPV occasionally reverts and thus produces ACTUAL POLIO. That's bad, obviously. But it also produces sterilizing immunity. IPV cannot cause actual polio as it is a killed virus, but DOES NOT produce sterilizing immunity -- specifically you can still get AND SHED the virus through your intestinal tract. This means that IF there is circulating polio IPV *CAN* potentiate worse strains via adverse selection where OPV cannot.

----------