I found an interesting study that will let me say something positive about vaccines for a change:
http://pediatrics.aappublications.org/content/109/1/e2.long
The authors looked at all the reported US national reports of child tetanus cases for the years 1992-2000. They found there were 15 cases in the 8 year span. No deaths, but some hospitalizations. 12 of the 15 were un-vaccinated. (I’m assuming all or most cases get reported, which I think is likely. Otherwise you’d need a factor for reporting frequency in the back of the envelope calculations below.)
The first thing their survey tells you is, the tetanus vaccine seems to protect against tetanus. Something like 90% of the population is vaccinated according to the CDC, yet only 20% of the tetanus cases are. That suggests the vaccine is something like 97% effective at protecting against tetanus, which I think makes it probably more effective than any other I know of. If every child were vaccinated the number of cases over the 8 year period might well have dropped from 15 to the 3 vaccinated cases they had or maybe 4 if they got another.
The second thing it tells you is, even if all the vaccinated had been UNvaccinated so they all got it 30 times as much, you might only expect 100 cases nationwide over the 8 year span. (How does that compare to the number of serious complications from the vaccine?) The chance of an unvaccinated child getting tetanus over the 8 year span seems maybe to be something like 1/500,000 or less, figuring that roughly 50 million vaccinated kids generated 3 cases so vaccinated maybe has 3/50 million, unvaccinated maybe 30 times as great or 1/500k.
(The 10% unvaccinated are maybe 5 million who generated roughly 10 cases, which checks the math.)
According to this history https://sites.google.com/site/tetanuswiki/project-definition: there used to be 200 cases per 100K people per year. (Caveat: This may be an overestimate for children, so I’m not quite apples to apples to the above figures which are of children. There maybe should be a fudge factor for that.) A frequency of 1/500. So tetanus frequency has dropped by a factor of maybe roughly 30000, of which maybe a factor 30 was due to the vaccine and 1000 was due to other factors unknown. I don’t see how this drop can possibly be attributed to herd immunity, since tetanus is not passed between humans. So tetanus is another example of a disease that largely vanished where a vaccine seems NOT to have been the major factor in the vanishing.
whyarethingsthisway 
One possible contributor: improvements in wound care.
Second possible contributor: decline in outdoor activities – particularly outdoor work such as barnyards, where there is a higher likelihood of injury, and a higher likelihood of soil-contaminated obstacles that cause injury – but also outdoor play; kids playgrounds and play areas are now practically pre-scrubbed, and certainly cleared of any object that might cause deep wounds. Kids playing unsupervised in the woods, or playing around in a pile of old construction materials is a thing of the past.
I have read the main reason was the demise of the horse. Tetanus is a horse virus, I think, or maybe it was a horse dung thing I forget, and people used to get it specifically from the nails in horse shoes. When the horse went out of fashion, and shoes became mandatory, tetanus went away.
If I were evaluating odds for kids that would run around a lot barefoot in the presence of horses, I would definitely consider them different than 1/500K.
Tetanus is an anaerobic bacteria that lives in damp, warm soil. Manure in the soil provides nutrition for the bacteria, which is probably where you go the ‘horse dung thing’. However, there are places, like California and Florida, where tetanus resides naturally in the soil in higher levels than normal.
The virus does not have anything to do with horses specifically. You might have remembered it as such because horses are incidentally associated with both nails and feces. Most cases of tetanus these days are associated with either iv drug use or traumatic puncture wounds.
Of course, we’ve got a much lower rate tetanus now than in the past: http://www.cdc.gov/vaccines/pubs/surv-manual/chpt16-tetanus.html
I think you’ve got a point that the decline of the agricultural lifestyle likely decreased the incidence of tetanus, but of course, the rise of an industrial world as put people at risk to exposure from other metallic puncture wounds, and other sources of fecal exposure.
Almost all cases these days are due to “Inadequate [tetanus] vaccination and inadequate wound prophylaxis”, as evidenced by the data in this MMWR report concerning the last decades’ worth of tetanus: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm6012a1.htm
Important to stay up on the literature! Can’t just make assumptions about these things and call it fact without making some unbiased observations.
Basically, I’ve said all I’ve read and with cites. I specifically said “I’ve read” without a cite because I haven’t bothered to try to chase it down and didn’t necessarily repose confidence in it. Assuming the history of tetanus website I found is on track, and also the modern survey, though, it does seem the number of cases has fallen by a pretty impressive amount for whatever reason.
It should probably be pointed out that the tetanus shot is for building up antigens to the toxin and not the pathogen that produces it (I had only recently learned this myself).
I’m still against it being so *forced* though — I once was refused hospital treatment on a very busted forearm until I accepted the shot.