You may have noticed it on a parent, grandparent or older relative: a small, round scar high on the upper arm that looks different from an ordinary cut or scrape.
For many people, that mark is a lasting reminder of a vaccination received decades ago.
One possible explanation is the smallpox vaccine, which commonly left a permanent scar. But the scar alone isn’t enough to identify the vaccine with certainty. BCG vaccination against tuberculosis can also leave a noticeable scar, particularly among people born in countries where BCG has been routinely given to children.