Vaccines are not simple. They are complex medications and contain viruses that we hope are all dead. We are extremely careful and attempt to prevent problems but any time you introduce a bunch of new chemicals into the body you always run a risk of problems developing, sometimes long after being administered. Add to that the reality that the average person gets about 30 vaccine doses by the time they're 18 years old, and there's room for unintended problems to creep in. Like I said, vaccines in general work but at what point do the multiplicity of vaccines start causing problems as they interact and as the substances build up in the body? Thus far, we've accepted (or turned a blind eye toward) potential risks because we want to prevent potentially deadly diseases and it's been working. There's always a good reason to be cautious when deciding whether to get a medical treatment or not. I come down neither on the side of everyone should get every vaccine nor on the side of no one should get any vaccines. Some vaccines have had incredibly positive results, smallpox and measles come to mind.