The truth about Bernard Greenberg can be found by searching for “thomas w kavanagh” and “bernard greenberg”.
Bottom line: Greenberg (not a medical doctor) never testified before Congress, but at a conference in 1960 (before the Sabin vaccine) he pointed out that the statistics on the Salk vaccine’s effectiveness were not clear, so we would have to wait and see whether the decline continued or was just a statistical artifact, and also we should look for a better vaccine. Both of these things happened. The Sabin vaccine replaced the Salk, and the decline in polio continued dramatically, until polio was completely eradicated, thus proving that it was not due to the redefinition in the mid-’50s.