The smallpox vaccine was discontinued for routine use in the United States in 1972. So, most people in this country younger than 50 years of age have never been vaccinated against smallpox. But what about people older than 50? Does immunity to smallpox last 50 years or longer?
The best study to answer this question was performed in England in the early 1900s. An outbreak of smallpox affecting more than 1,000 people occurred in Liverpool between 1902 and 1903. People infected with smallpox were divided into two groups: those who got smallpox vaccine in infancy and those who did not. The fatality rate for 30- to 49-year-olds was 3.7 percent in the vaccinated group and 54 percent in the unvaccinated group. For those older than 50 years of age, the fatality rate was 5.5 percent in the vaccinated group and 50 percent in the unvaccinated group.
Therefore, smallpox vaccine protected against disease caused by smallpox, even 50 years after vaccination.
There are two situations regarding long-standing vaccine-induced immunity.
There’s the situation where the person is vaccinated, then from time to time actually encounters the pathogen out in the world, which, while not making them sick, keeps the immune system primed and ready. The wild bug acts as a sort of booster via exposure.
Then there’s the situation where the person is vaccinated but then does NOT encounter the pathogen again for decades. There is no “booster” effect. This is the scenario where vaccine-induced immunity starts to fade, and may fade entirely away.
No one has encountered smallpox outside a laboratory since 1978 which Discourse tells me I’ve already linked to in this thread but I do sometimes repeat myself. Janet Parker had been previously vaccinated against smallpox, but died of the disease and there had been less than 50 years between those two events. Prior vaccination decades before is no guarantee of present immunity.