"Safe, effective, and potent." 
With these words on April 12, 1955, Dr. Thomas Francis Jr., director of the Poliomyelitis Vaccine Evaluation Center at the University of Michigan School of Public Health, announced to the world that the Salk polio vaccine was up to 90% effective in preventing paralytic polio.
Dr. Francis made the announcement to a crowd of scientists and reporters at the University of Michigan's Rackham Auditorium, concluding his two-year national field trials of the poliomyelitis vaccine developed by his former student, Jonas Salk. Francis was chair of the School of Public Health Department of Epidemiology where Salk did postgraduate training. Over 1,800,000 children participated in the field trials, which were unprecedented in magnitude.
The event was held on the tenth anniversary of the passing of President Franklin D. Roosevelt, one of the most well-known victims of polio.
This article is dedicated to keeping  the story alive since it's been over 60 years since polio has been much of a concern to parents in the U.S.
More complete background on the event that day is available from the U-M School of Public Health: https://sph.umich.edu/polio/. Also of interest is a 2005 Emmy-award winning 15-minute video (https://www.youtube.com/watch?v=5BfxpdzwXLA) and a 90-second newsreel from the day: https://www.youtube.com/watch?v=2LlDn_MQDkc. Even more information is available on Wikipedia: https://en.wikipedia.org/wiki/Announcement_of_polio_vaccine_success.