Have you have a little, rounded scar on your upper arm from the smallpox shot. A lot of people had this smallpox vaccination prior to the 1970s.
To induce an immune response that would shield humans from the dangerous Variola virus that caused smallpox, live Vaccinia virus was utilized.
Blisters develop at the injection site after receiving a shot. Within a few weeks, the blisters heal and form a crust.
The final end has a circular scar. Each time the needle pierced the skin, a tiny amount of the vaccine was applied, and blisters developed. The scars are so evident because of this.