Here’s an interesting graph comparing disease prevalence before vaccines and now:
This is quite a strong correlation, but how do we know that vaccines caused the diseases to become so rare? Did vaccines causes disease incidence for all these diseases to bottom out, or is it something else, say a coincidence, or maybe all diseases are just disappearing because Americans are healthier today?
So more information is needed. The first thing to consider is that all infectious disease hasn’t gone away. The cold is still as common as ever. Kids still get sore throats and ear aches. There are also the ones I don’t think about or haven’t heard of, like RSV, croup, Fifth disease. And looking at adults, clap, HPV, and gonnorhia are at epidemic levels. So infectious disease is still very common, but the worst diseases have become rare–the ones for which general vaccination is practiced .
Another line of evidence that vaccines are what stomped out the targeted diseases is the timing. They didn’t all disappear at once, not even close. What was observed is that each disease dropped off after widespread vaccination became common.
Here’s a study that looked at incidence for several disease in the US over decades: pdf
If you look at page 4, they summarize incidence over time for 8 diseases. At the top they summarize incidence. The colored section of the graph is detailed regional data. The grey vertical bar shows when widespread vaccination was introduced–a different year for each disease. After the vaccine is introduced, the disease incidence goes way down. Note that for smallpox, it was better vaccines replacing ones started in the 1800’s, so no grey bar is shown.
Here’s a simpler graph of measles from the CDC site:
So it isn’t general health or healthier people with immune systems that prevent disease causing a gradual decline in infectious disease. Instead, the incidence of a specific disease drops when the vaccine is introduced.
And here’s one of the new vaccines–for rotavirus. Nearly all kids used to get it: “four of five children in the US had symptomatic rotavirus gastroenteritis, one in seven required a clinic or emergency department (ED) visit, one in 70 was hospitalized”. The vaccine was introduced in 2006 and the disease has already become much less common: CDC Surveillance of Rotavirus
See fig 4 especially.
So what I conclude from these lines of evidence is that the introduction of widespread vaccination for a disease causes it to become much, much less common.