Polio was declining before immunization was introduced. Graph on page 4 http://www.thinktwice.com/Polio.pdf
Polio also surged after DTP (following links) and I believe there were some changing definitions going on too (thinktwice article).
Or that's what some people believe to be a possibility. 
http://www.pnc.com.au/~cafmr/online/vaccine/polio.html
Intramuscular Injections within 30 Days of Immunization with Oral Poliovirus Vaccine — A Risk Factor for Vaccine-Associated Paralytic Poliomyelitis
N Engl J Med 1995; 332:500-506February 23, 1995






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