evidence: VDPVs can cause outbreaks in places where vaccine coverage is low. In addition, people with certain immunodeficiency disorders can shed the virus for long periods of time, during which the virus can continue to change and can infect an unvaccinated person. Because OPV has not been used in the United States since 2000 and vaccine coverage with IPV is high, it is unlikely that any VDPV would become widespread in the United States.