First, let me say this was a example that I studied in college in statistics to show the reason why you need to set up a proper control.
The gist of it is that in the 60s vaccines were still relatively new and not fully accepted by the population. Those who were fine with vaccines, and giving them to their children, at the time happened to better educated, and better educated people tend to be cleanlier. If you have a clean childhood, your immune system does not develop as much as those whom have an unwashed childhood (cant think of a good way to put that, but you know what I mean). So, the students who's parent did not give permission had a better developed immune, more likely to naturally fight off Polio (but certainly not 100%), and giving the vaccine to the child of the better educated group was just evening the playing field.
Therefore, there was no real difference between the groups.
Interesting example, Joe. There does seem to be sound evidence that excessive cleanliness during childhood can lead to harmful conditions such as asthma, various allergies and autoimmune disease.
I also wonder if the children of wealthier and more educated parents would get less exposure to sunlight because they were more often indoors doing their school homework, and were driven back and forth to school, and elsewhere, in a car rather than having to walk to school and elsewhere, exposed to the sun, as the children of poorer parents might have done, as well as playing around more often in both the dirt and the sun.
Vitamin D deficiency is now a major problem world-wide. Vitamin D is essential for a strong immune system and sun exposure is the main source.
The current emphasis on excessive cleanliness, due to Covid-19, might have future consequences for the immune system of today's children.
