A CDC reply (we have to trust the CDC, right?/SARC)
“Does the smallpox vaccine last for life?
Smallpox vaccination can protect you from smallpox for about 3 to 5 years. After that time, its ability to protect you decreases. If you need long-term protection, you may need to get a booster vaccination.”
Well, yeah. Smallpox was “eradicated” except for the supplies they had in deep freeze.
By the way, there have been corpses of people who died of smallpox found biried in permafrost that had active smallpox.