The credit for actual smallpox vaccination goes to Edward Jenner who noticed that milkmaids who had cowpox did not come down with smallpox so he came up with the idea of 'vaccinating' someone with the cowpox virus - administering a small dose of the cowpox virus to people with the hope that the antibodies would keep them from getting infected with smallpox. That was in 1796. By 1800, over 100,000 people worldwide had been vaccinated against smallpox. The vaccine used in the USA was made by Wyeth and called Dryvax. A small drop of the vaccine was usually placed on a persons upper arm and then inserted just under the skin by several pricks of a needle. A blister formed within 3-4 days and by the end of the third week, the scab of the blister fell off leaving a circular scar. The last outbreak of smallpox in the US was in Texas in 1949. The last worldwide case was in Somalia in 1977. The US officially stopped vaccinating the general public against smallpox in 1972 but continued to vaccinate certain military personnel until 1990. In 2002 with fear of rogue nations using smallpox as a weapon, the US began testing the old vaccine on volunteers to see if it was still potent and in what strength. The US also made a contract with the Acambis company to make a new and safer smallpox vaccine. They also began vaccinating certain health care personnel and military personnel again but it has not been released for the general public. Usually the first smallpox vaccination was given when a child reached age one and since it was required for school entry, at least by age 6. The vaccination was believed to provide immunity for 10 years so some people may have received a booster vaccination later. Most people born up until 1968-71 received a smallpox vaccination, usually given in the upper left (or non-dominant) arm.
BTW - people in certain European countries get vaccinated against Tuberculosis at birth and/or in their teen years with the BCG vaccine. It is also a live vaccine such as the smallpox vaccine. It is administered just under the skin (intradermally) with a needle, usually in the upper arm. A blister forms where the vaccine was given and after 3-4 months, the blister goes away and a scar is left, which sometimes can be very simular to the smallpox vaccination scar. Often they will have more than one scar due to getting re-vaccinated years later.