To Documents
Experimental Trials for the Salk Polio Vaccine
- Reference: Freedman, Pisani, and Purves, Statistics, Fourth Edition,
W. W. Norton and Company, 2007.
Here are the first two chapters
of the International Edition.
- The first polio epidemic struck the United States in 1916.
- Polio caused paralysis in hundreds of thousands of victims, especially children, for forty years.
- Several polio vaccines had been developed by 1950; the one
Jonas Salk developed was the most promising.
- Why not just give the vaccine to large numbers of children and
see if the incidence of polio drops?
- What issues can you think of that complicate the situation?
- The number of polio cases dropped by half from 1952 to 1953 with no
vaccine.
- Very few of the persons exposed to polio actually get the disease.
- Children from wealthy families are more likely to get polio than
children from poor families. This is because children from poor
families often grew up in non-hygenic surroundings so they developed a
partial immunity to polio.
- Some parents thought that the vaccine was dangerous and would not give
their consent. Such parents were more likely to come from poor
families than from wealthy ones.
- People that are vaccinated may behave differently than those who are not.
- The vaccine might have dangerous side effects that were not noticed in
small scale trials. The vaccine can actually cause polio in some cases.
- Two clinical trials were conducted to test the effectiveness of the
vaccine. One by the National Foundation for Infantile Paralysis (NFIP,
otherwise known as the March of Dimes).
- Another study was conducted by Dr. Thomas Francis of the University of
Michigan.
- Here are the results. The rate is polio cases per 100,000.
| | NFIP Study |
Francis Study |
| | Size | Rate |
Size | Rate |
| Treatment | 225,000 | 25 |
200,000 | 28 |
| Control | 725,000 | 54 |
200,000 | 71 |
| No Consent | 125,000 | 44 |
350,000 | 46 |
- For the NFIP study, grades 1 and 3 were the control group, grade
2 was the treatment group; this study was neither randomized, nor
double blind.
- For the Francis study, all children in grades 1, 2, and 3 were asked
if they wanted to participate in the study. Of those whose parents gave
consent, half were randomly assigned to the treatment group, the other
half to the control group; this study was randomized and double-blind.
- Additional references:
Some Class Discussion Questions and Answers
- What is a double blind study?
- Isn't it unethical to give some patients the placebo?
- Why did the subjects in the No Consent group have a lower incidence of polio than
the subjects in the placebo group?