On Apr. 26, 1954, the largest controlled Polio vaccine field trial in the history of medicine got under way. Across the United States, 623,972 schoolchildren were injected with vaccine or placebo, and more than a million others participated as “observed” controls.

The results, announced in 1955, showed good statistical evidence that Jonas Salk’s killed virus preparation was 80-90% effective in preventing paralytic poliomyelitis.