Have this round scar on your arm? here's why millions have the exact same mark

If you grew up in a certain generation, there's a good chance you have one too: a small, round scar on the upper part of your arm, often close to the shoulder.

For many people, it's been there since childhood. It doesn't hurt, rarely changes, and is easy to get used to over the years until you almost forget it's there.

But every now and then, someone notices it and asks the same question:

Why do so many people have a scar that looks almost exactly the same?

The Scar His Mother Had Carried for Decades

The article's author remembers noticing an unusual scar on his mother's arm when he was a child.

It sat high on her arm, near the shoulder, and looked like a small ring of indentations surrounding a larger dip in the center.

For some reason, it fascinated him. He can't remember whether he asked his mother what had caused it. Perhaps she explained it, but if she did, the answer wasn't important enough to stick in his childhood memory.

Years later, he spotted an almost identical scar again – this time on a stranger.

Then He Saw the Same Mark on a Stranger

Years later, he helped an older woman get off a train and noticed a scar on her upper arm.

It looked almost exactly like the one his mother had: the same round shape, the same location, and the same distinctive appearance.

His childhood curiosity suddenly returned. He didn't want to stop a stranger and ask why she had a scar on her arm, so he did something much simpler: he called his mother.

This time, he got his answer.

The Scar Was Caused by a Smallpox Vaccination

His mother explained that the mark had been left by a smallpox vaccine she received as a child. Millions of people around the world have similar scars for the same reason.

In recent years, photos of these scars have resurfaced on social media. People have posted pictures of their arms, wondering how so many people – sometimes from entirely different countries – could have almost identical marks.

One viral question summed it up: "Can someone explain why everyone has this mark?" Others wanted to know why an old vaccine could leave a permanent scar that remained visible decades later.

It All Comes Down to How the Vaccine Was Given

The smallpox vaccine was administered differently from most vaccines people receive today.

Instead of a single injection with a thin needle, healthcare workers used a special two-pronged instrument called a bifurcated needle.

They dipped the needle into the vaccine and made a series of quick, shallow punctures in the skin. This allowed the vaccine to enter the skin and trigger a local immune response – the reaction that eventually produced the familiar scar.

First Came a Small Bump

After vaccination, the injection site began to change. Small bumps appeared where the skin had been punctured, followed by a blister that could fill with fluid.

The blister eventually opened, formed a scab, and healed. Once the scab fell off, a round scar often remained.

Some people were left with a relatively small, barely noticeable mark, while others developed a deeper or more prominent scar. Because millions of people received the vaccine using a similar technique, the resulting scars often looked remarkably alike.

That is why the little mark can remain visible on someone's arm decades after vaccination.

But Not Every Similar Scar Came From a Smallpox Vaccine

There's another important detail: depending on where and when someone was born, a similar scar on the upper arm may have been caused by the BCG vaccine against tuberculosis.

BCG has been routinely administered for years in many countries where tuberculosis was common. It can leave a small, round scar as the vaccination site heals.

The World Health Organization notes that a local bump may develop several weeks after an intradermal BCG vaccination, followed by a small sore that heals and leaves a scar.

This means appearance alone cannot always tell you whether a scar came from a smallpox vaccination or BCG. A person's birthplace, age, and vaccination history can help identify the likely cause.

Smallpox Was Once One of the World's Most Feared Diseases

Today, smallpox may sound like something from a history book. But for thousands of years, it was one of humanity's deadliest and most feared infectious diseases.

Caused by the variola virus, it produced symptoms including high fever, severe weakness, and a characteristic rash that developed into fluid-filled and eventually pus-filled lesions. Survivors were often left with deep, permanent scars, particularly on their faces.

The disease was also extremely deadly. According to the World Health Organization, the more common and severe form of smallpox had a fatality rate of around 30%.

For earlier generations, the vaccine that left a little scar on the arm offered protection against a potentially fatal disease.

Then the World Achieved Something Once Thought Almost Impossible

In 1967, the World Health Organization launched an intensified smallpox eradication campaign. Countries around the world carried out mass vaccination programs, identified cases, and monitored people who had been in contact with infected individuals.

The last known naturally occurring case of smallpox was recorded in Somalia in 1977. After years of international effort, the WHO officially declared the disease eradicated in 1980.

It was an extraordinary achievement: smallpox became the first human disease to be eradicated worldwide through a coordinated public health effort, and remains one of the most significant examples of what such cooperation can accomplish.

That's Why Younger Generations Usually Don't Have the Scar

As smallpox disappeared, routine vaccination against it was no longer necessary. In the United States, routine smallpox vaccination ended in the early 1970s, once the disease no longer posed an ordinary public threat. Other countries gradually discontinued routine vaccination as eradication approached.

As a result, people born in later generations are much less likely to have a scar from a smallpox vaccine. However, those born in countries where BCG vaccination against tuberculosis was routine may still have a similar mark.

A Tiny Scar That Carries a Piece of History

For many people, this scar has been there for as long as they can remember. They may have seen it in the mirror thousands of times, never asked their parents about it, or simply assumed everyone had one.

But behind that little circle lies a much bigger story. For many, it connects them to an era when a deadly disease frightened families around the world and a vaccine administered through a series of tiny punctures became a routine part of childhood.

The disease is gone. Mass vaccination against it has ended. Yet millions of people still carry a small mark that serves as a reminder of that time.

So if you have a small, round scar on your upper arm, you may be carrying a reminder of one of the greatest vaccination campaigns in history – against smallpox, or, in many countries, against tuberculosis through the BCG vaccine.