Real reason so many people have this exact same scar
Suddenly, that childhood curiosity came rushing back.
I couldn’t exactly stop the woman and ask her about the scar, so instead, I called my mother.
That’s when I finally learned the answer: the mysterious mark on her arm was left behind by the smallpox vaccine.
And millions of other people have the same story.
The distinctive, round indentation has recently sparked a wave of curiosity online, with people sharing photos of their own scars and asking why so many people have an almost identical mark.
One viral question asked, “Can anyone explain why everyone has this mark?”
Another person wondered, “Why does this old vaccine leave a permanent circular scar? Can you explain the science?”
The answer goes back to the way the smallpox vaccine was administered.
Unlike most vaccines given today with a single fine needle, the smallpox vaccine was delivered into the skin using a special two-pronged needle. The healthcare worker made several small punctures in the skin, allowing the vaccine to enter the dermis.
The vaccine triggered a localized immune response. Small bumps formed at the injection site, followed by fluid-filled blisters that eventually burst, scabbed over and healed.
That healing process left behind the familiar circular scar.
For many people who received the vaccine, the mark became a permanent reminder of childhood vaccination — something that could remain visible decades later.
And the scar wasn’t necessarily unique to the United States. Depending on where someone grew up and when they were born, the mark could be the result of smallpox vaccination or, in some countries, the BCG vaccine against tuberculosis, which can also leave a distinctive scar.
