According to Dr. Bernard Greenberg, head of the Department of Biostatistics of the University of North Carolina School of Public Health:
In order to qualify for classification as paralytic poliomyelitis, the patient had to exhibit paralytic symptoms for at least 60 days after the onset of the disease. Prior to 1954, the patient had to exhibit paralytic symptoms for only 24 hours. Laboratory confirmation and the presence of residual paralysis were not required. After 1954, residual paralysis was determined 10 to 20 days and again 50 to 70 days after the onset of the disease. This change in definition meant that in 1955 we started reporting a new disease, namely, paralytic poliomyelitis with a longer lasting paralysis.
Under the new definition of polio, thousands of cases which would have previously been counted as polio would no longer be counted as polio. The change in the definition laid the groundwork for creating the impression that the Salk vaccine was effective.