Why did the total number of polio cases drop off so suddenly

"The Minnesota State Department of Health Laboratories developed the use of a single oral dose of polio vaccine containing the three types, 1, 2 and 3; of the live attenuated Cox strains of polio virus. It is important, for the mass public health immunization programs, that the three types of virus be combined in a single oral dose. More than 100,000 Minnesotans volunteered to receive the Cox oral vaccine, which in this and other carefully designed testing programs, proved the oral vaccine to be safe and efficacious. The use of a single oral dose of the Cox vaccine provided for the uniform immune status of all persons getting the vaccine, which neither the Salk killed virus nor the Sabin live oral vaccine provided in the 1950s

"The Sabin live oral vaccine, after it was licensed, is being produced with all three types of virus in a single dose as recommended. The Cox vaccine and the Sabin vaccine were arbitrated, and the conclusion favored the Sabin strains of polio viruses because the arbitrators felt the Sabin strains were more stable. The Salk vaccine was dropped because of the cost of needle injections and the difficulty in getting vaccinated individuals to return at set intervals for the second and third dose.