Many adults still carry a small, round scar on their upper arm, a lasting reminder of one of the most significant vaccination efforts in history. For most people, this distinctive mark was left by the smallpox vaccine, which was routinely administered for decades before smallpox was officially declared eradicated around the world.

Unlike most vaccines used today, the smallpox vaccine was not injected into the muscle. Instead, healthcare workers used a special two-pronged needle to make several tiny punctures in the surface of the skin, introducing the vaccinia virus. This mild infection prompted the immune system to build protection against the deadly smallpox virus.

As the body healed, the vaccination site usually formed a blister that later turned into a scab before falling off. Because the reaction occurred within the skin itself, it often left behind a small, circular, slightly indented scar. For many people, this mark became a lifelong sign that they had received the vaccine.

Although a similar scar can sometimes result from the BCG vaccine, which is used to help protect against tuberculosis, the round scar on the upper arm is most commonly linked to the historic smallpox vaccination. Today, it remains a visible reminder of the global campaign that helped eliminate one of humanity’s deadliest diseases.