No, this wasn't true for smallpox. The smallpox vaccine didn't use the smallpox virus - it used a related virus which isn't very effective at infecting humans a

No, this wasn't true for smallpox. The smallpox vaccine didn't use the smallpox virus - it used a related virus which isn't very effective at infecting humans and doesn't seem to have been able to easily mutate into something that was. While smallpox vaccination isn't risk-free and there is the occasional incident of the vaccine virus spreading to family members of US soldiers and causing serious health problems, it can't turn back into smallpox. The fact that the symptoms are so obvious and reliable probably helps here too.