If you have a small circular mark on your upper arm, you're not alone. Millions of people around the world still carry the mark decades after receiving childhood vaccinations, and many have wondered why some vaccines left permanent marks, while modern vaccines usually don't.
The answer is not related to the needle, but to the vaccine itself. Older vaccines, especially the smallpox vaccine, worked by creating a local reaction in the skin, which, as it healed, formed the characteristic circular mark that many adults still have today.
In some countries, a similar mark on the arm can also come from the BCG vaccine against tuberculosis, which also causes a local reaction on the skin and can leave a small permanent mark.
