North Americans don't have vaccination scars on their shoulders because the smallpox vaccine is no longer one of the routine vaccines in use in the United States, according to Drugs.com. In the United States, the last known case of smallpox occurred in 1949. Worldwide, the last case occurred in 1977; however, due to threats of terrorism, the United States has stockpiled enough of the vaccine for every resident.
Unlike most other vaccines, the smallpox vaccine contains a live virus, according to the CDC. It causes a sore to form on the arm that requires special care as it heals. The vaccine provides protection for three to five years. Receiving the vaccine after exposure to smallpox reduces the severity the disease. Due to complications and undesirable side effects of the vaccine along with the eradication of the disease, routine smallpox vaccinations ended in 1972 in the United States.
Smallpox is highly contagious and often deadly. Because indigenous Americans had no immunity to the disease, it contributed to the decline of the Aztec empire, according to National Geographic. The smallpox vaccine was responsible for eradication of the disease as the result of a global campaign for vaccination, according to Mayo Clinic; however, scientists kept samples of the smallpox virus for research purposes. These samples have the potential for use as an agent for bioterrorism.