Taken from various sources on the internet.
Poliomyelitis, also known as infantile paralysis, is an infectious viral disease that enters through the mouth and is usually spread by contaminated drinking water or food. The virus passes through the stomach and then replicates in the lining of the intestines. Most healthy people infected with virus experience little more than mild fever or diarrhea. In about 1% of infections however, for reasons that are still unclear, the virus spreads to the bloodstream and central nervous system, causing varying degrees of paralysis, and in extreme cases, death.
Strangely enough, severe polio tends to be rare in communities with poor hygiene. The reason is that the virus is abundant in such communities, so babies are likely to be exposed to the virus early, while still protected with antibodies from their mothers. Later (assuming that they survive other diseases associated with poor hygiene), these children develop their own antibodies to the virus. The net effect is that in communities with poor hygiene, most people have a natural immunity.
By the early 1950's several research groups were trying to develop a vaccine. One of the was Jonathan Salk of the University of Pittsburgh. In the method favored by Dr. Salk, the vaccine is prepared from a sample of the
virus that is first killed using heat and chemicals (such as formaldehyde). In
principle, this method results in a vaccine that is very safe, because
naturally, the killed virus in the vaccine is incapable of causing the disease.
The vaccine can also be quite effective, if the killed virus is sufficiently
intact to cause a strong antibody response in the vaccinated person. Thus, the
process of killing the virus can be quite delicate and challenging: . If the
process is insufficient, the vaccine will contain live virus and will itself be
capable of causing the disease. If the process is too severe, the proteins of
the killed virus will be destroyed to such an extent that the vaccine will be
ineffective.
By the early 1950s, Salk had finished the development of a killed-virus vaccine, administered in a series of three inoculations. The vaccine was believed to be safe (prematurely, it turned out). But how do we judge the effectiveness of the new vaccine? How about this method: simply innoculated all the children and see whether the incidence of polio goes down. The main problem with approach is that it seems impossible to say by how much the incidence rate has to drop before we can say that the vaccine works. For this look at the polio dataset. Clearly there is a big variation of the incidences from year to year, and there is an increase starting at about 1940 or so.
Instead it was decided to do a combination of a randomized double-blind experiment and a simple observational study. In the randomized double-blind experiment 401,974 children ages 6-9 were randomly split into two groups. 200,745 received the vaccine, the other 201,229 received a placebo (three injections of sugar-water). Neither the children nor their parents knew what they received. After several month doctors examined the children to see whether they had contracted polio. The doctors did not know either whether a child was in the treatment or in the control group (double-blind)(?).
They found the of the 200,745 vaccinated children 33 (or 0.016%) developed paralytic polio, but of the 201,229 non-vaccinated children 115 (or 0.057%) did. So the "incidence rate" was 0.057/0.016 = 3.6 times higher for non vaccinated children. This was conclusive evidence that the vaccine "works".
Postscript
After the success of the field trials, the Salk vaccine was put into widespread use. Unfortunately, one improperly manufactured batch of the vaccine caused polio in a number of children, and the vaccination program was temporarily halted. By the early 1960s, a different vaccine, the Sabin live-strain virus had been perfected and soon replaced the Salk vaccine.
Salk was at once lionized by the public and the media and criticized by other scientists, who felt that he had rushed the development of the vaccine for the sake of personal glory (and to beat Sabin).
In an invited review article for the Journal of the American Statistical Association in 1955, shortly after the trials, KA Brownlee did not mince words in his scathing criticism of the original observed control experiment, referring to the design as "stupid and futile," and the results as "worthless."
In spite of the problems with the initial design of the statistical experiment, the manufacturing problems, and the excessive media hype, the development of the polio vaccines remains a supreme triumph of the scientific method, a triumph in which statistics played a fundamental role. Because of the vaccines, polio has largely been eradicated in the industrialized world, and countless children and others have been spared untold misery.