Have you ever wondered why some people have a tiny round scar on their upper left arm?
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.

What Happens after the Insertion of the Vaccine?
Right after the vaccine there is a small swelling at the vaccination site which persists for 6-8 hours.

Then, the swelling disappears and the vaccination site looks normal. 6-8 weeks later a swelling appears again which looks like a mosquito bite.

It starts to grow and forms a nodule which breaks open and discharges some fluid and forms an ulcer.

The ulcer heals by forming a scar.

This entire process takes 2-5 weeks.

There are times when this process of ulceration and healing recurs 2-3 times.

After the 70s
In the West, after the 70s, smallpox disappeared; therefore, this vaccine was no longer a practice, unless someone went to a country where the virus was still active. In the 80s, the Variola virus was eradicated from the entire population and there was no longer a need for smallpox vaccines.