control group. It would need to be a different sort of study (cohort or case-control) to start investigating whether there was an association between MMR and autism." Not (strictly) so. Modern vaccination studies often don't use classical (case-control or cohort) designs. Comparisons between populations perform poorly in studies of routine vaccination effects because almost all children are vaccinated. Only very large (often pan-continental) studies are able to provide the statistical power necessary to detect moderate or small risk increases, and even then the results are often questionable as those who go unvaccinated tend to have immune illnesses or to be from minority groups, which can introduce all manner of biases or confounding. One way around this problem is to use a population consisting only of those who received the vaccine and who had the outcome of interest, and to statistically examine the temporal relationship between the two events. md1364 said, Can anyone please provide an answer to a question that occurs to me regarding the vaccination business. If the MMR and other vaccines are so reliable and safe then what is the problem with parents individually making a decision to have them or not? I have heard it argued that if the uptake is low then it puts others at risk, how can this be, if the vaccine prevents contraction of the diseases then if one child in a class has had it and one has not how can the un-vaccinated child pass it on to the one who has