the dramatic decrease in polio infection statistics after the introduction of the vaccine has a lot to do with the change in the criteria for diagnosing polio.
Prior to 1954, a physician's diagnosis of paralytic polio was confirmed if paralysis was found in 2 examinations done at least 24 hours apart.
In 1955, the criteria was changed so that there had to be paralysis found 10 to 20 days after onset of illness and again 50 to 70 days after onset for there to be a diagnosis of paralytic polio. So what happened to the paralytic diseases that did not last that long? Two new disease diagnoses now exist to distinguish them from paralytic polio: Coxsackie virus infections and aseptic meningitis, which were likely lumped in with it before.
One can see from this that there would have been a sharp decrease in polio cases even without the introduction of a vaccine.
Ming Dinh BScOT, DCH