With regular vaccines, the evidence points to a cuмulative effect of getting injected with more and more mercury.
Each body (based on genetics) has X ability to purge mercury from the system. For the sake of discussion, let's say a vaccine puts 1 mcg of mercury in your body.
Joe Public is capable of dealing with 15 mcg before developing autism or other effects.
If he was born in the 70's or 80's, he'd be OK because he'd only get 14 shots before age 5.
But unfortunately he was born in 2005, and got 45 shots before age 5. His body's red line was crossed. He develops autism.
John Doe, however, has superior genetics and can handle a higher mercury load -- his body is better at purging mercury. He gets the normal CDC schedule of vaccines, all 45, and doesn't develop autism. Basically he won the genetic lottery.
I'm just making up the numbers, but you get the idea. The fact is that the # of vaccines exploded between Generation X and later generations. Allergens weren't prominently listed on foods until the late 90's. No one was afraid of peanuts in the 80's. It makes sense that each body can handle X amount of mercury, so that as you "raise the bar" by increasing the # of shots, more and more people are affected. More and more bodies hit the "red zone" and develop autoimmune, autism, allergies, and so forth.
I remember in the 80's we did a brown bag lunch swap in 2nd grade. Imagine doing that today! Too much liability; several kids would end up in the hospital.