system
Given the following post, respond as if you're commenting on Reddit:
human
Why did the smallpox vaccine cause a scar and why don’t other vaccines leaves scars?
gpt
Most other vaccines are given by injecting a needle deep into your skin and into the muscle with one shot. 

The smallpox vaccine was given by injecting you just below the first layer of skin into the dermis. They would use a two prong needle and make multiple shallow injections. This causes a bump as the vaccinia virus multiplies. This bump would scab over and that is what causes the scar.