My opinion is that it's not just one thing - it's a combination of a massive amount of different pollutants we are exposed to these days. My guess would be heavy metals and that might be why vaccines could potentially also contribute. Read up about how lead is now everywhere thanks to the lead they added to gasoline - so everybody gradually had more and more lead in their system. Add to that the mercury from vaccines and it is perhaps just what pushes some kids over the limit. But it could also be completely unrelated to the vaccines and be some other pesticide or other pollutant or just our bad eating habits, not enough iron or too much iron or some other nutrient. There are so many things it can be and it really doesn't help doubting yourself about giving your kid a vaccine - but that is one of the reasons this is such a difficult thing for parents, you don't want to find out that something you gave your kid was bad for them, and that's why it's so reprehensible that anybody with doubts about giving their kids vaccines are labeled some loony anti-vax flat-earther. It's just a very difficult decision for parents to make, especially knowing how little you can trust pharma companies.