Many adults still have a small, round scar on their upper arm that serves as a reminder of one of history’s most important vaccination campaigns. In most cases, this distinctive mark comes from the smallpox vaccine, which was routinely given for decades before the disease was officially eradicated worldwide.

Unlike most modern vaccines, the smallpox vaccine was not injected deep into the muscle. Instead, it was administered by repeatedly puncturing the top layer of the skin with a special two-pronged needle containing the vaccinia virus. This created a small, controlled infection that trained the immune system to recognize and fight smallpox.

As the body responded, the vaccination site typically developed a blister, followed by a scab that eventually healed. Because the reaction occurred directly in the skin, it often left behind the familiar circular, slightly indented scar that many people still have today. The mark became a visible sign that a person had been vaccinated.

In some cases, a similar scar can also result from the BCG vaccine, which helps protect against tuberculosis and is given just beneath the skin. However, for many older adults, that small round mark on the upper arm is most commonly associated with the historic smallpox vaccination that helped eliminate one of the world’s deadliest diseases.