The polio vaccine had a 2+ year experimental phase.
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Salk tested his experimental killed-virus vaccine on himself and his family in 1953, and a year later on 1.6 million children in Canada, Finland and the USA. The results were announced on 12 April 1955, and Salk's inactivated polio vaccine (IPV) was licensed on the same day.
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.
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Covid vaccines were evaluated and authorized for emergency use in under a year.