MMR more effective if delayed?

[anewmama]

Dr Sears states that the efficacy of MMR works better if the shot is delayed, ie. to 4 or so. Does anyone know what the basis (which studies) is for this claim? I am aware of a study showing not enough immunity after one dose and hence why they moved to two MMR.

http://www.sciencedirect.com/science...37871f1f0310ba

2–4 years after receiving a first dose of MMR vaccine at age 12–18 months, it was found that a large proportion of pre-school children had measles (19.5%) and mumps (23.4%) IgG antibody below the putative level of protection.

but then this..

The proportion measles antibody negative (but not rubella or mumps) was significantly higher in children vaccinated at 12 months of age than at 13–17 months.

[roxyrox]

I have read studies that showed MMR was more effective at 15 months as opposed to 12 months or younger, but never anything about delaying to 4yo? I would be interested too if anyone has a link.