The Salk polio vaccine was declared safe on April 12, 1955

Jonas Salk’s polio vaccine was declared “safe, effective, and potent” on April 12, 1955, when epidemiologist Thomas Francis Jr. announced the results of the 1954 field trial at the University of Michigan’s Rackham Auditorium in Ann Arbor. He reported it 60–70% effective against type 1 poliovirus, the most common strain, and at least 90% effective against types 2 and 3.

How the trial earned trust

Francis agreed to run the trial only on three conditions: a placebo group at least as large as the vaccinated group, a double-blind design, and no interference from its funder, the National Foundation for Infantile Paralysis. Nearly 2 million children, the “Polio Pioneers,” took part, supported by some 150,000 volunteers, 15,000 schools and 44 state health departments. Francis policed details down to the packaging of the vaccine and the safety instructions sent to parents. When he announced the result, celebrations broke out across the country.

What happened next

The celebrations were barely over when vaccine from Cutter Laboratories, a California firm, turned out to contain live virus because its inactivation step had failed. Of the more than 200,000 children in five western and midwestern states who received it, investigators later linked 40,000 cases of polio, 200 children left with some paralysis and 10 deaths.

Vaccination slowed while manufacturers retooled to meet revised production standards. President Eisenhower asked for patience, reminding Americans that Francis had found the vaccine “60 to 90 percent—not 100 percent—effective.”

The program recovered. Salk declined to patent the vaccine, asking “Could you patent the sun?”, and US polio fell from an average of about 45,000 cases a year before the vaccine to 910 by 1962.