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Across the United States, 623972 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.
https://pmc.ncbi.nlm.nih.gov/articles/PMC1622939/