Eight years after Salk's success, Albert Sabin developed an oral polio vaccine (OPV) using live but weakened ("") virus. Human trials of Sabin's vaccine began in 1957 and it was licensed in 1962. Following the development of oral polio vaccine, a second wave of mass immunizations led to a further decline in the number of cases: by 1961, only 161 cases were recorded in the United States. The last cases of paralytic poliomyelitis caused by endemic transmission of poliovirus in the United States were in 1979, when an outbreak occurred among the Amish in several Midwestern states.