Oral and injected polio vaccine have been implicated several times. Nothing is certain though. These vaccines may have been contaminated with simian viruses due to the manufacturing process. Experimental vaccines are MUCH older. They were tested in hospitals. Epidemics tended to occur in hospitals. There is no way to be sure there is not a link at this time. Don't take this as accurate, my memory is fubar, but I think an early experimental vaccine was being tested at the site of the 1934 Los Angeles epidemic. Those who got sick, and they did, were awarded massive compensation. Last time I went looking for the online evidence of this, to confirm it, I could not find it. Previously it was available. This was circa 1931.