Salk Polio Vaccine Trials

Experimental Trials for the Salk Polio Vaccine

Reference: Freedman, Pisani, and Purves, Statistics, Fourth Edition, W. H. Norton, 2007.
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.

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:
- Jonas Salk Legacy Exhibit at Pitt Public Health
- "A calculated risk": the Salk polio vaccine field trials of 1954

What is a double blind study?
Ans: it is a study where neither the patient nor the the patient's doctor knows whether the patient is receiving the placebo or the vaccine. Only the clinical trial administrator knows who is getting the vaccine and who is getting the placebo. This is important because patients can get better due to psychological effects if they know that they are getting the vaccine and they can get worse if they know they are getting the placebo. The physician or nurse can treat teh patient differently depending on whether they are getting the vaccine or placebo.
Isn't it unethical to give some patients the placebo?
Ans: at the time that the clinical trial is conducted, it is not clear whether the vaccine really works. In modern clinical trials, as soon as it is determined that the vaccine really works, the subjects in the control group are also given the vaccine.
Why did the subjects in the No Consent group have a lower incidence of polio than the subjects in the placebo group?
Ans: It is possible that better educated subjects would have a better appreciation of the importance of vaccines for diseases, so they would be more likely to volunteer to be in the study. The less educated subjects might be less likely to volunteer for the study, so they would go into the no consent group. However, less educated subjects may live in less hygenic conditions, so they would have built up a natural resistance to polio.