The DPT (diphtheria/pertussis/tetanus) vaccine was reformulated because there was some (erroneous) evidence that the P component could trigger brain damage in a very small percentage of the children who got the vax, and it seemed to run in families.
In the end, it was discovered that these people actually had a rare genetic error of sodium metabolism in the brain, and would have become disabled, vaccine or not. The first symptoms are usually seizures, triggered by fevers which the vaccine can indeed cause, and even without the vaccine, the child will start to develop symptoms at the approximate age where DPT is given to babies.
As an aside, I had a friend on another website (the site was shut down) whose son as a toddler was diagnosed with severe autism, coupled with terrible bowel problems, and at that time, the GF diet for autism was all the rage, and they agreed with their doctors that it would be harmless to try it. And guess what? Their son's condition improved so much, the doctors all agreed that most likely, their son has a metabolic disorder that produces autism-like symptoms. (The same diagnosis was given to a family at my old church, although his specific disorder was identified; if you remember the story I've told about the child who was kicked out of Sunday School because he was constantly pinching other children, that's him.)