@Sooty7: Well I'm afraid that study is never going to get done, simply because a double-blind trial of non-vaccinated children is completely unethical. You do not necessarily need such a study design to get enough evidence of proven benefit. The weight of evidence across the world is so far in favour of the paradigm of vaccines being associated with a reduction in risk of serious childhood illness that no-one with a human conscience would agree to allow such a study.
Secondly - and let's assume you bribed various people on the ethical committee to agree to consent, plus got a pharmaceutical company to go through 2-3 years' safety & efficacy trials of a vaccine placebo - I wonder how you would persuade parents of either camp (pro- or anti- vaccination) to take part in such a double-blind trial after you tell them they won't know whether their DC would receive vaccine or placebo? Countless parents on this board have rightly or wrongly shown that they're not interested in the public good of protecting populations, but only in avoiding causing perceived damage to their own children.
Who gives consent in this study? Mother? Father? Both? Oh yes, and they'll be followed up for how long? In proposing long-term studies there is a question that is often the death-knell of the application: "What is the estimated number lost to follow-up"?
What clearly end-points of disease would you propose for these poorly-defined conditions ("health", "immune dysfunction", "allergies", "cancer" etc). For each of these you would have to demonstrate biological plausibility.
I'm sorry because the sentiments are very well-meaning, but unfortunately as far as the science goes; these types of studies are way too difficult to be likely to tell us anything. Therefore it is not worth the risk of deliberately not protecting a group of children against childhood disease.