The Salk polio vaccine was declared safe and effective

On April 12, 1955, in Ann Arbor, Michigan, Dr. Thomas Francis Jr. of the University of Michigan announced results that the world had been waiting for: Jonas Salk’s polio vaccine was “safe, effective, and potent.” The verdict capped the largest medical field trial in history, involving nearly 2 million children dubbed the “Polio Pioneers.”

Polio had terrified American families for decades, paralyzing or killing thousands of children each summer and filling wards with iron lungs. The announcement, broadcast coast to coast within minutes, set off spontaneous celebrations—church bells, car horns, and tears of relief.

The trial found the vaccine roughly 90% effective against paralytic polio.

The date was chosen to fall on the 10th anniversary of President Franklin Roosevelt’s death; Roosevelt, himself paralyzed, had founded the march of dimes that funded the research. Mass vaccination began almost immediately, and U.S. polio cases plummeted within years.