"One concern that must be considered is the potential for the vaccine virus to revert to a form capable of causing disease. Mutations that can occur when the vaccine virus replicates in the body may lead to a more virulent strain. This is unlikely, as the vaccine virus’s ability to replicate is limited. However, possible mutations are considered when developing an attenuated vaccine. It is worth noting that mutations are somewhat common with the oral polio vaccine (OPV), a live vaccine that is ingested instead of injected. The vaccine virus can mutate into a virulent form and lead to rare cases of paralytic polio. For this reason, OPV is no longer used in the United States, and has been replaced on the Recommended Childhood Immunization Schedule by the inactivated polio vaccine (IPV). Protection from a live, attenuated vaccine typically outlasts the protection provided by a killed or inactivated vaccine."