I have no idea TMK. Apparently about 90% (or something like that) have full immunity from the first jabs but they give the booster to all children just to catch the 10% who don't. You can get your child's blood titres tested to find out if he already has immunity and then there is no need to have the booster at all. I didn't give DS the booster although he had the standard MMR when he was 1 yr or so. I don't think MMR had anything to do with his ASD but I wasn't going to give him a booster if he didn't need it. DD has had singles and I will check her titres when booster time comes.