What Did Thomas Francis Jr. Report on April 12, 1955?
On April 12, 1955, Thomas Francis Jr., Jonas Salk’s mentor and the director of the polio vaccine field trial, reported that the vaccine was safe, potent, and 90% effective in protecting against paralytic poliomyelitis. The account I am drawing on here is a historical narrative of the Salk and Sabin vaccine programs rather than the trial report or a press release, so it carries the outline of the trial and its verdict rather than a full statistical appendix. The trial itself had been launched in 1954 and sponsored by the National Foundation for Infantile Paralysis. Salk’s vaccine was approved in 1955. It was the first polio vaccine but not the last: Albert Sabin’s oral vaccine, introduced in the United States in the 1960s, replaced it, and the disease was brought under control because of both.
Dr. Kumar’s Take
Paralytic poliomyelitis is a central nervous system disease, and that is why it frightened parents the way it did. A virus that starts in the gut and ends in the anterior horn cells of the spinal cord produces exactly the injury I spend my career trying to prevent by other means: motor neurons destroyed, muscles denervated, function that does not come back. Francis reporting the vaccine safe, potent, and 90% effective against the paralytic form is the part of that sentence I would underline. Preventing infection is useful. Preventing the neurological outcome is the thing that changes lives.
I also take something from who delivered the verdict. Francis had been Salk’s mentor since Salk’s medical school years, and he had every professional reason to want the result to be good. He ran the trial and announced the numbers anyway, in a design that included real controls. The habit of separating the person who makes a thing from the person who judges it is one that medicine has kept because of moments like this one.
What the Research Shows
Salk built the polio vaccine on the same killed-virus principle he and Francis had used for influenza. Working at the University of Michigan on U.S. Army supported research, the two developed a formalin-killed influenza vaccine in 1943, a preparation that could not cause the disease but did induce protective antibodies. Until then, vaccines generally relied on attenuated live viruses. Salk and Francis were among the pioneers of the killed-virus approach.
In 1951 the National Foundation typing program confirmed that there were three types of poliovirus. Salk was by then convinced the killed-virus principle would work for polio and would be safer than a live vaccine, because dead virus cannot accidentally cause the disease in the person inoculated. The practical obstacle was quantity: a killed virus does not replicate in the body after administration, so large amounts are needed. The 1949 discovery by John Enders, Thomas Weller, and Frederick Robbins that poliovirus grows in tissue cultures of non-nerve tissue solved that problem and earned them the Nobel Prize in Physiology or Medicine in 1954. Salk grew all three poliovirus types on cultures of monkey kidney cells, killed them with formaldehyde, and showed the resulting vaccine protected monkeys against paralytic poliomyelitis.
Human testing began in 1952, starting with children who had already been infected. Salk measured antibody levels before vaccination and found them significantly raised afterward. The full field trial followed in 1954, and Francis reported its result on April 12, 1955.
Study Snapshot
Trial Design and Results:
- Sponsor: National Foundation for Infantile Paralysis, now the March of Dimes
- Launched: 1954, as a massive controlled field trial
- Participants: almost two million U.S. children between the ages of six and nine
- Placebo areas: half of these “polio pioneers” received the vaccine, half received a placebo
- Observed areas: children who received no vaccine were carefully observed
- Director: Thomas Francis Jr., Salk’s mentor
- Reported: April 12, 1955, as safe, potent, and 90% effective against paralytic poliomyelitis
Manufacturing:
- Producing vaccine at the scale the trial required needed the pharmaceutical industry
- Eli Lilly and Company, Wyeth Laboratories, and Parke, Davis and Company agreed to make it
Historical Context
In the early 1950s, 25,000 to 50,000 new cases of polio occurred in the United States each year. In the first half of the twentieth century, summer was known as “polio season,” and children were among the most susceptible to paralytic poliomyelitis, also called infantile paralysis.
The epidemiology has an irony in it. Poliovirus is present in human sewage, and infants exposed in the first months of life usually showed only mild symptoms, because maternal antibodies still circulating in their bodies protected them from paralysis. As hygienic conditions improved and fewer newborns met the virus early, paralytic poliomyelitis began appearing in older children and adults who had never acquired that infant protection.
The most famous American victim was Franklin Delano Roosevelt, who contracted the disease in 1921 at the age of 39, one of thousands afflicted that year.
Why This Matters for Modern Medicine
The 1954 trial and the announcement that followed established several principles worth keeping.
Independent Analysis: Francis, rather than Salk, directed the trial and reported its findings, separating the inventor from the evaluator.
Rigorous Study Design: The trial used placebo-controlled areas in some parts of the country and observed unvaccinated children in others, giving more than one way to read the result.
Manufacturing as Part of the Science: A trial of that size could not run without industrial production. Eli Lilly, Wyeth, and Parke, Davis had to make vaccine at scale before the question could even be asked.
Public Health Impact: Salk’s approved vaccine and then Sabin’s oral vaccine brought polio under control. In 1994 the World Health Organization declared naturally occurring poliovirus eradicated from the Western Hemisphere after repeated mass immunization campaigns with the Sabin vaccine in Central and South America.
Safety, Limits, and Caveats
The killed versus live debate did not end in 1955. Sabin believed only a living virus could guarantee extended immunity, and he pursued an attenuated oral vaccine that would be easier to give than an injection. He tested three mutant strains on himself and his family, on research associates, and on prisoners from the nearby Chillicothe penitentiary.
Sabin’s trial ran abroad because Salk’s vaccine was already in use. Unable to obtain support for a large controlled U.S. trial, Sabin persuaded the Soviet health ministry in 1957 to conduct field studies. After the Soviet trial succeeded in 1960, the U.S. Public Health Service approved his vaccine in 1961.
The live vaccine carries a real, small risk. Sabin defended his vaccine throughout his life and refused evidence that it could cause paralytic poliomyelitis. Salk held that killed virus produced equivalent protection in individuals and communities with no risk of causing paralysis. The risk from live virus does exist, though it is slight. After the 1994 declaration, the only cases of paralytic poliomyelitis in the West were the few caused by the live vaccine itself.
Policy followed the evidence. In 1999 a federal advisory panel recommended that the United States return to Salk’s vaccine because it cannot accidentally cause polio. After a decade of additional evidence, that recommendation was reconfirmed in 2009.
Practical Takeaways
- Understand the magnitude: almost two million children aged six to nine took part in the 1954 field trial
- Appreciate rigorous design: placebo areas and observed control areas gave two independent readings
- Note the endpoint that mattered: the 90% figure refers to protection against paralytic poliomyelitis
- Value independent evaluation: Francis directed the trial and reported the result rather than Salk
- Remember that control took two vaccines: Salk’s killed-virus injection first, Sabin’s oral vaccine after
- Respect the residual risk: live-virus vaccine can cause paralysis rarely, which is why U.S. policy returned to the killed-virus vaccine
Related Studies and Research
- The 1954 Salk Vaccine Trial: 1.8 Million Children as Polio Pioneers
- The March of Dimes: How FDR’s Campaign Funded the Polio Vaccine
- 1952: The Year Polio Struck 57,879 Americans - A Record-Breaking Epidemic
- What Is Poliomyelitis? The Virus That Once Terrorized America
- Episode 28: Iron Lungs, Fear, and a Miracle: How Polio Was Stopped
FAQs
Who evaluated Salk’s vaccine, and what was the connection to Salk?
Thomas Francis Jr. directed the field trial and reported its results. He had been Salk’s mentor for many years, beginning near the end of Salk’s medical education and continuing through six years of influenza research at the University of Michigan.
How effective was the vaccine according to the April 12, 1955 report?
Francis reported that it was safe, potent, and 90% effective in protecting against paralytic poliomyelitis.
What was the difference between the placebo areas and the observed areas?
In some parts of the country, half the participating children received the vaccine and half received a placebo. In other parts, children who received no vaccine at all were carefully observed for comparison.
Why did Salk favor a killed virus?
He had already used the killed-virus principle to make an influenza vaccine, and he believed a vaccine containing only dead virus could not accidentally cause polio in the person inoculated.
If Salk’s vaccine worked, why did Sabin’s replace it?
Sabin believed only a living virus would confer long-lasting immunity, and his oral vaccine, given in drops or on a sugar cube, was easier to administer. It was approved in the United States in 1961 and replaced Salk’s injectable vaccine in many parts of the world.
Bottom Line
On April 12, 1955, Thomas Francis Jr. reported that Salk’s vaccine was safe, potent, and 90% effective against paralytic poliomyelitis, on the strength of a 1954 field trial that enrolled almost two million American children between six and nine years old. The vaccine was approved that year. Control of the disease came from the pair of vaccines that followed, Salk’s killed-virus injection and then Sabin’s oral live-virus vaccine, and the Western Hemisphere was not declared free of naturally occurring poliovirus until 1994.
