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 childrem from poor families 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.
|NFIP Study||Francis Study|