If you have a small, round scar on your upper arm, you’re not alone. This distinctive mark is a relic from the smallpox vaccine, a reminder of a time before the 1970s when vaccinations for smallpox were routine.

The purpose of the smallpox vaccine was to create an immune response against the Variola virus, the cause of smallpox. The vaccine used a live Vaccinia virus, closely related to smallpox, which triggered the immune system to build protection.

The vaccination process was quite unique. After receiving the shot, small blisters would appear at the injection site, which would crust over in the following weeks. The repeated needle pricks delivered small doses of the vaccine, each causing a tiny blister that eventually left a scar.

Immediately after the shot, the injection area would swell slightly, a reaction that typically subsided within 6 to 8 hours. Once this swelling faded, the injection site looked normal for a while. However, around 6 to 8 weeks later, a small bump—much like a mosquito bite—would reappear.

This bump would enlarge, forming a nodule that eventually broke open, oozed fluid, and created an ulcer. As the ulcer healed, a scar developed. This healing process took about two to five weeks and could repeat two or three times, creating scars that have persisted for decades.

By the early 1970s, as smallpox was nearly eradicated in the Western world, routine smallpox vaccinations were discontinued. Today, these scars serve as reminders of a disease that has been largely eradicated, thanks to widespread vaccination efforts.