This is the fascinating truth behind the scar people have under their left shoulder

I have a vivid memory of a distinct scar on my mother’s arm from when I was a child. It sits high up, near her shoulder, and looks like a ring of small indentations in her skin surrounding a larger central pit.

Don’t ask me why it caught my attention all those years ago; I don’t remember. I just remember that it did. But, as often happens, I forgot about it in the years that followed.

Well—not that I forgot it existed (it’s still in the same place it always was, of course)—but I forgot that at some point, I had been fascinated by what had caused it. Maybe I asked my mother once, and she explained. If so, I’ve forgotten that too.

All of that changed one summer a few years ago when I helped an elderly woman off a train, and I happened to notice that she had the exact same scar, in the same spot as my mother. Needless to say, it piqued my curiosity, but with the train about to leave for my destination, I couldn’t exactly stop and ask her about the scar.

So instead, I called my mother, and she told me—apparently for at least the second or third time (clearly my brain didn’t consider this info important enough to store)—that her scar came courtesy of the famous smallpox vaccine.

Smallpox was a highly contagious viral disease that once plagued humanity. It caused a severe skin rash and high fever, and during the most widespread outbreaks of the 20th century, it killed about 3 in every 10 infected people, according to the CDC. Many other survivors were left with lasting disfigurements.

Thanks to the widespread and successful implementation of the smallpox vaccine, the virus was declared eradicated in the United States in 1952. In fact, by 1972, smallpox vaccinations were no longer part of routine immunizations.

Until the early 1970s, all children were vaccinated against smallpox, and those vaccinations left a very noticeable mark. Think of it as the first kind of vaccine passport, if you will: a scar that told the world you’d been successfully immunized against smallpox.

And yes, you guessed it—that’s exactly the scar my mother has (just like nearly everyone else her age).

Why did the smallpox vaccine leave scars?

The smallpox vaccine caused scarring due to the body’s natural healing process. The vaccine was administered quite differently from most vaccines today: it used a special two-pronged needle to puncture the skin multiple times.

The person administering the vaccine would make several punctures in the skin (instead of the single injection we’re used to with today’s vaccines) in order to deliver the vaccine into the dermis—the layer beneath the epidermis.

After that, the virus in the vaccine would begin to activate, multiply, and cause the formation of round bumps. These bumps would then develop into small fluid-filled blisters, which would eventually burst and leave a scar as they healed over time.

The result? The infamous scar we’ve been talking about in this article.

Are you old enough to have “earned” a smallpox vaccine scar yourself? Tell us in the comments!