This is commonly known for people who were vaccinated for smallpox.

Before 1970s, this smallpox vaccine was common.

It used live Vaccinia virus in order to trigger an immune response that would protect people against the dangerous Variola virus that caused smallpox.

Scarring

After the vaccination, blisters forms at the vaccination area, crusted over, and healed in a couple of weeks.

At the end it leaves a round scar.

The doctors used a bifurcated needle dipped into the Vaccinia solution to deliver the vaccine. The individual’s arm was poked few times and each time the needle broke the skin, a tiny amount of the vaccine was inserted, which caused blistering. This is probably why the scars are rather large.