system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some vaccines leave scars?

I recently got my BCG vaccine as I work in a lab but when they informed me of the blistering and scaring phase I was a little perplexed. 
What makes the site blister and scar only after several weeks/months? I know it’s to do with your immune system reacting but what is specifically involved / what’s the processes involved and why does sometimes it cause such a prolonged severe reaction at the site of injection? ( How come its only with live vaccines too?)
gpt
For the smallpox vaccination, "leaving a mark" is the sign that the vaccine worked. The vaccine requires multiple punctures giving multiple infection start sites, so it becomes very inflammatory - leaving behind the scar tissue. The TB vaccine is different, in that it is a single injection, but BCG is extremely immunogenic and causes severe local inflammation, which can cause a long-lasting scar.