In addition, OPV will no longer be available for routine purchase when the current supply is depleted. Due to limited stocks and expiration dates throughout 1999, implementation of an all IPV schedule will become effective much sooner in many practices. The elimination of OPV for routine use in the U.S. is the latest in a series of changes made possible by the declining incidence of polio in the U.S. and throughout the world. From 1963 until 1997, OPV had been the vaccine of choice, replacing an earlier form of IPV which was first licensed in 1955. The series of changes in the recommended polio schedule has been driven by issues of effective community protection and vaccine safety.
Poliovirus is an enterovirus which replicates in the oropharynx and gastrointestinal tract.
Both IPV and OPV are highly effective at producing immunity to poliovirus and protection from paralytic polio. OPV which replicates in the oropharynx and intestinal tract, however, also protects against intestinal polio infection. Without this extra protection, an individual could become infected with poliovirus and have no symptoms, but spread infection to others who are not protected. The sequential schedule was used to reduce the incidence of VAPP while maintaining a high level of protection from importation. Between 1980 and 1996, an average of 8 to 10 cases of VAPP were reported in the U.S. each year, but currently no cases have been associated with use of the sequential schedule.