That's a graph of the death rate from polio, not of polio incidence. I don't know this for sure, but think it likely that incidence stayed the same while death rates went down.
"even with an iron lung the fatality rate for patients with bulbar polio exceeded 90% [...] Positive pressure ventilators reduced mortality in bulbar patients from 90% to 20%. In the Copenhagen epidemic of 1952, large numbers of patients were ventilated by hand"
"The bold solution was to manually ventilate the patients by squeezing a rubber bag attached to a tracheostomy tube inserted through an incision just below the larynx. [...] the mortality rate is said to have dropped from ∼90% to ∼25% as a result of this heroic intervention."
http://en.wikipedia.org/wiki/History_of_poliomyelitis#Iron_l...:
"even with an iron lung the fatality rate for patients with bulbar polio exceeded 90% [...] Positive pressure ventilators reduced mortality in bulbar patients from 90% to 20%. In the Copenhagen epidemic of 1952, large numbers of patients were ventilated by hand"
That 1952 epidemic is better described at http://jap.physiology.org/content/99/2/424.full, about a 1952 epidemic in Copenhagen:
"The bold solution was to manually ventilate the patients by squeezing a rubber bag attached to a tracheostomy tube inserted through an incision just below the larynx. [...] the mortality rate is said to have dropped from ∼90% to ∼25% as a result of this heroic intervention."