Actually, the Jenner cowpox vaccine was replaced with a “vaccine” (quotes because the term meant specifically cowpox inoculation at the time) with a smallpox “variolate.” Instead of being vaccinated, people got “variolated.” “Variola” was the name for smallpox. This happened after the microscope and the germ theory, and the discovery that there was a Variola major, which had something like a 90% death rate (at the time, but it was still very high in the 1960s, when the last cases occurred), and a Variola minor, with a death rate of something like 40%. Each conferred immunity for the other, and each was acquired “in the wild” as a respiratory disease. However, as a skin infection, the disease nearly always remained localized as a small rash, but the person who was variolated still ended up with whole-body immunity. It seemed to be better (a higher success rate) than vaccination, and it was hoped would combat the perception that giving people a cow disease would turn them into cows, an actual, genuine fear in the days of vaccination. So people were given a skin infection with V. minor. A few people developed serious skin infections, and some people managed to transfer the infection to their eyes, while a very few people still developed actual V. minor (very rare, though, and it’s always possible they were coincidentally exposed right before variolation).
By the 20th century, an inactivated V. minor “vaccine” was used. By this time, the word “vaccine” meant any inactive, or otherwise introduced (eg, skin vs. respiratory) infectious agent done so with the purpose of stimulating antibodies without causing disease.
It still occasionally caused skin problems, and people with compromised skin, like people with eczema, couldn’t be vaccinated for smallpox.
Probably the most important thing to know about the smallpox vaccine, though, is that is had over 200 proteins. All the vaccinations children receive put together do not have close to 200 proteins, and that is what matters when asking what the immune systems “deals with.” It’s not the number of shots, or the number of diseases, it’s the number of proteins. Back when we (and I was one) got the smallpox vaccine, our infant systems dealt with it just fine. There’s no problem with the number of vaccines kids get now. I wish there were more. I wish there were a strep vaccine. I wish there were a good, safe TB vaccine. I wish there were an Epstein-Barr vaccine (yes, I know there is a post-infection approach, but that’s not what I’m talking about), since this virus has been associated with leukemia. I wish there were an HIV vaccine (unfortunately, if there ever is, it will probably be seasonal, like the flu, because HIV mutates so quickly, and has zoological forms, so it could be reintroduced at some point, albeit, not as easily as the flu).