Poliomyelitis and the Salk Vaccine: Introduction

On April 12, 1955, the world anxiously awaited the announcement of the results of the largest vaccine field trial in history. The announcement would report on the effectiveness of the polio vaccine developed by Dr. Jonas Salk of the University of Pittsburgh. In 1954, over 1.8 million elementary school children took part in a field trial to test the Salk vaccine. The results of the field trials were rigorously evaluated at the University of Michigan Polio Evaluation Center by Dr. Thomas Francis of the School of Public Health. From the stage of the Rackham Auditorium Francis faced a gathering of distinguished scientists and an anxious crowd of reporters as he read his report.

The Francis report was the culmination of a year-long field trial of the Salk vaccine, unprecedented in its scope and magnitude. Using a double-blind method of statistical analysis, where neither patient nor administering physician knew if the inoculation was the Salk vaccine or a placebo, 444,000 children were given the vaccine and 210,0000 the control substance. In all, approximately 1,800,000 children in 217 areas of the United States, Canada, and Finland were involved in the field trial.

T. Frarcis and B. O'Conner

Thomas Francis and Basil O'Connor
(Thomas Francis Papers, box 65)

As a statistical exercise, the polio field trial was unique in the annals of epidemiological study. Francis and his staff at the University of Michigan produced some 1,800,000 IBM punch cards containing 144,000,000 pieces of information about the test children. On staff at the Polio Evaluation center was a small army of statisticians, epidemiologists, and clerical and support personnel. More than 10 individuals were involved in tabulating the data received from public health officials and doctors participating in the field trials. According to a university press release, more than 300,000 individuals participated in the field trials: 20,000 physicians and public health officers, 40,000 registered nurses, 14,000 school principals, 50,000 teachers, and 200,000 volunteer workers.

photo of tabulating machine

Tabulating results of polio vaccine field tests
(University of Michigan News and Information
Service, Box E-13 (print), A-9 (neg.))

Dr. Francis jealously guarded the findings of the test up until the last moment. Premature disclosure, especially if misreported, could adversely affect the continuing struggle to find a cure for polio. Even Dr. Salk did not know the details of the test results, though he certainly must have had some idea that the vaccine was effective. Swarms of press and TV and radio reporters crowded outside of the Rackham auditorium hoping to grab a copy of the printed report as it arrived. One press report described the scene: "The releases were in boxes on a hand truck. To avoid a crush, public relations men from the university began throwing the releases into the crowd. But still hands grabbed at the boxes. In the next few seconds pandemonium prevailed. Then there was a dash for the couple of dozen typewriters in the press room and for a battery of telephones."1

distributing polio press release

Distributing the Salk vaccine press release and
summary field trials report, Rackham Auditorium,
April 12,1955 (Michigan Bell Telephone Company
Photographs, Box 18.)

"Safe, effective, and potent" were the words that Dr. Francis spoke, announcing to the world that the Salk polio vaccine was 60-90% effective in preventing paralytic polio. The long campaign against this virulent childhood disease, though certainly not over, was about to enter the final stages. Within a generation the scourge of polio would be but a memory to most Americans.

The date selected for the announcement, April 12, 1955, was the tenth anniversary of the death of Franklin D. Roosevelt, perhaps the most famous polio victim, and founder of the National Foundation for Infantile Paralysis, better known as the March of Dimes. Francis was joined on stage by Jonas Salk and Basil O'Connor, president of the National Foundation for Infantile Paralysis. Other polio experts also spoke, but without question the day belonged to Salk and Francis.

Vaccine Summary report

The report the press and the world was waiting for.

The results announced by Francis effectively marked the beginning of the end of polio as the most life-threatening and debilitating public health threat to the children of the United States. Jonas Salk knew full well the value of what Francis had accomplished. Pausing in his presentation to the press, he paid tribute to his former teacher. "For Dr. Francis, whatever is worth doing is worth doing well. He could not do otherwise. His kind of objectivity is rare, even among scientists--and it is the kind that in human terms is called honesty. He is his own severest critic--for which he has both the respect and admiration of his colleagues and students who strive to attain such stature as you have observed during this past year."2

1.     Newspaper article by Dick Shearin "Bedlam Prevails in Press Room Before Salk Polio Report Release" Dayton Daily News, April 13, 1955 (clipping in U-M News and Information Services, Box 25.

2.     "Introductory Remarks by Jonas E. Salk, April 12, 1955" (U-M News and Information Services, Box 9)





1955 Polio Vaccine Trial Announcement

"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.

Links for more information:

Text from first press release on polio vaccine evaluation results, April 12, 1955:

The University of Michigan Information and News Service
3564 Administration Building, Normandy 3-1511, ext. 2623
April 12, 1955


ANN ARBOR: The vaccine works. It is safe, effective, and potent.

Dr. Thomas Francis, Jr., UM Director of the Poliomyelitis Vaccine Evaluation Center, told an anxious world of parents that the Salk vaccine has been proved to be up to 80-90 percent effective in preventing paralytic polio.

At a meeting of over 500 scientists and physicians and before the penetrating eyes of cameras and powerful spotlights, Dr. Francis spoke on the effectiveness of the Salk vaccine. The meeting was held at the Rackham Auditorium in Ann Arbor under the joint sponsorship of the Rational Foundation for Infantile Paralysis and the University of Michigan.

Dr. Francis declared the vaccine had produced "an extremely successful effect" among bulbar-patients in the areas where vaccine and an inert substance had been tried interchangeably.

Financed by nearly one million dollars worth of dimes which have been donated to the National Foundation, the Francis Report may slow down what has become a double-time march of disease to a snail's pace.

In strong statistical language the historic trial of a vaccine and its subsequent analysis was revealed. Over 113 pages in length, the Report at long last called a halt to speculations and finally re-enforced laboratory findings with concrete field evidence. There can be no doubt now that children can be inoculated successfully against polio.

There can be no doubt that humanity can pull itself up from its own bootstraps and protect its children from the insidious invasion of ultramicroscopic disease.

For one thing what was feared turned out to be unfounded -- the vaccine proved incredibly safe. Reactions were nearly negligible. Only 0.4 percent of the vaccinated children suffered minor reactions. An even smaller percent (0.004-0.006) suffered so-called "major reactions."

And the persistence of protection appears reasonably good. When good antibody responses were obtained from vaccination, the report said "the effect was maintained with but moderate decline after five months."

Distribution of antibody levels among vaccinated persons was much higher than that in the control population from the same areas.

Out of a total population of 1,829,916 children a total of 1013 cases of polio developed during the study period and were reported to the Center.

In placebo control areas, where vaccine was interchanged with an inert substance, 428 out of 749,236 children contracted the disease.

In the observed control areas where only second graders were inoculated, 585 cases out of 1,080,680 children developed.

Percentages in the placebo areas were: 67.5 paralytic, 17.6 non-paralytic, 7.2 doubtful, and 7.6 not polio. Specifically, 33 inoculated children receaving the complete vaccination series became paralyzed in the placebo areas. This is opposed to 115 uninoculated children. Similarly, in the observed areas there were 38 such children who became paralyzed, as opposed to 330 uninoculated children.

There were four deaths among children who received placebo; none among the vaccinated. In observed areas there were 11 fatalities; none among children receiving the vaccine.

Only one child who had been inoculated with the vaccine died of polio, and this death followed a tonsillectomy two days after the second injection of the vaccine in an area where polio was already prevalent.

The Report also stated that in no area did Type II virus prevail. There was, however, prevalence in certain areas of Types I and III.

Marked sociological differences were noted by the U-M's Survey Research Center among the participating and non-participating children in the study. For example, there was a higher proportion of children participating who had been vaccinated against small-pox, diphtheria, and whooping cough than among the non-participants. Significant auxiliary findings were:

  1. The vaccine's effectiveness was more clearly seen when measured against the more severe cases of the disease;
  2. Although data were limited, findings in Canada and Finland support the Report in showing a significant effect of the vaccine among cases from whom virus was isolated
  3. Vaccination protected against family exposure. Only 1 out of 233 inoculated children developed the disease, while 8 out of 244 children receiving placebo contracted the disease from family contact.
  4. In picking the field trial areas, the National Foundation scored a major victory. Although in placebo areas cases were 27 per cent under the 1949-53 average, and 12 per cent less in the observed control areas, it was found that there had been a 26 per cent increase per 100,000 in trial areas as a non-trial areas. This meant that trial areas were appropriately selected for the best testing conditions for the vaccine.

The field trials and the evaluation were made possible by grants totalling $7,500,000 in March of Dimes Funds from the National Foundation for Infantile Paralysis.