One of the reasons suggested by some CDC officials (… I think Coleen Boyle for one) for why you cannot do a fair longitudinal cohort study comparing vaccinated versus unvaccinated children is that there would be factors that would skew the results in favor of the unvaccinated children. The main factor mentioned is the likelihood that the unvaccinated kids would come from families in which the parents are more conscientious about maintaining good health habits, such as eating nutritious, unprocessed foods free of chemical additives; getting plenty of sleep and exercise and sunlight; and avoiding prescription drugs and unnecessary medical tests.
In other words, they would be the kind of parents who take an active interest in the natural health of their children and do everything possible to lower the susceptibility of their kids to disease and illness. Ironically, these are the kind of parents (… you know, the really educated and present ones) that the mainstream labels as irresponsible and even whacky for doing their own research, trusting their own instincts, and having the nerve to question the parochial knowledge and wisdom of most healthcare professionals when it comes to vaccination. Now, that’s pretty whacky.