Children in the United States, Canada and Finland participated in the trials, which used for the first time the now-standard double-blind method, whereby neither the patient nor attending doctor knew if the inoculation was the vaccine or a placebo.
Polio, known officially as poliomyelitis, is an infectious disease that has existed since ancient times and is caused by a virus. It occurs most commonly in children and can result in paralysis. The disease reached epidemic proportions throughout the first half of the 20th century. During the 1940s and 1950s, polio was associated with the iron lung, a large metal tank designed to help polio victims suffering from respiratory paralysis breathe.
On April 12, 1955, researchers announced the vaccine was safe and effective and it quickly became a standard part of childhood immunizations in America. In the ensuing decades, polio vaccines would all but wipe out the highly contagious disease in the Western Hemisphere.
According to history.com