The correct response explaining the impact of the creation of the smallpox vaccine on the Americas is: "It led to the complete eradication of the disease."
Although the smallpox vaccine did not lead to immediate eradication in the Americas, it was a crucial factor in the eventual global eradication of smallpox, which was officially declared in 1980. The other options provided either relate to other diseases or misrepresent the impact of the smallpox vaccine.