April 12, 1955

Polio vaccine

Salk vaccine trial results announced and U.S. licenses issued

Thomas Francis — not Salk — told a packed auditorium in Ann Arbor that the vaccine worked. Licenses to manufacture it were issued the same day.

Rackham Auditorium, 12 April 1955

At the University of Michigan's Rackham Auditorium on 12 April 1955, trial director Thomas Francis Jr. announced that Jonas Salk's inactivated polio vaccine was safe, effective and potent. More than 500 scientists and physicians were present.

Francis reported the vaccine as much as 80 to 90 percent effective against paralytic polio, with the underlying results varying by poliovirus type and trial design. The trial's reported minor-reaction rate was 0.4 percent of vaccinated children.

The finding completed the independent evaluation of a field trial involving more than 1.8 million children.

Licensed the same day — but not by the FDA

Federal licenses allowing production of Salk IPV were issued that day, with six pharmaceutical manufacturers licensed in the initial program.

This was not an FDA approval. Authority over U.S. biologics then sat within the Public Health Service and NIH, and regulation of biologics moved to the FDA only in 1972.

Weeks later, the Cutter lots

Within weeks, live virus in some Cutter Laboratories lots caused polio cases. That was improperly inactivated commercial product, not a reversal of the field-trial finding.

The episode prompted recall and stronger federal oversight before vaccination resumed. Injected IPV chiefly protects individuals from paralysis and produces less intestinal immunity than the later oral vaccine, which became the principal tool for interrupting community transmission.

Sources

Researched 23 Aug 2026 6 sources 2 audit passes

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

What the sources leave uncertain

The date of the announcement and licensure is solid. The historically correct term is federal licensure, not modern FDA approval.
Accounts distinguish the advisory committee's initial recommendation for five named producers from the federal program ultimately described as licensing six manufacturers. The distinction should be preserved rather than presented as one blanket approval.