Methylmercury clears the human blood system in 50 days. It can accumulate in human fat, where it won’t do harm to the individual but if there is enough there it will be nasty for anyone who eats him or her.
My generation has the mark of the beast on every shoulder.
I’ve had quite a few vaccinations, since I was in the water quality division of US EPA and they made me trot down to public health and get a large number each visit over several weeks of visits, before I was allowed to do fun stuff like inspect wastewater treatment facilities and waste dumps and toxins sites. I felt like a human pincushion, and had lots of arm pain during the period, but no scarring.
However, I never had the gun. My school district hadn’t adopted it yet when I was in elementary school, and by the time I went to work for the EPA it was already being pulled from use.