A study was made to determine to what extent polioviruses were causing paralytic and nonparalytic poliomyelitis in the Houston area. The two series of patients studied consisted of 126 and 125 patients hospitalized with the clinical diagnoses of paralytic poliomyelitis and aseptic meningitis, respectively. Only 3 of 100 laboratory-confirmed paralytic poliomyelitis patients had received 3 or more Salk vaccine inoculations, whereas 87 were unvaccinated and 10 had received either 1 or 2 vaccine inoculations. From these and other data the authors conclude that poliomyelitis continues to appear in clinical form because vaccine is not administered to susceptible persons.
Melnick JL, Benyesh-Melnick M, Peña R, Yow M. Effectiveness of Salk VaccineAnalysis of Virologically Confirmed Cases of Paralytic and Nonparalytic Poliomyelitis. JAMA. 1961;175(13):1159–1162. doi:10.1001/jama.1961.03040130043010