There are two types of polio vaccines: inactivated poliovirus vaccine (IPV) and oral poliovirus vaccine (OPV). IPV contains a killed (inactivated) form of poliovirus and is given by injection. It cannot cause polio and provides strong protection against paralysis. IPV is used in many countries, including the United States.
OPV contains a weakened (attenuated) live virus and is given by mouth. It is easy to administer and helps stop the spread of the virus by boosting immunity in the intestines. However, in rare cases, the weakened virus can mutate, leading to vaccine-derived poliovirus in under-immunized populations.