I have read that a child may have a repeat infection if exposed <18 months of age...
(edited to correct, a child may have a repeat infection if they have an infection <18 months of age... it appears the immune response isn't that efficient at that point in time)
this would also indicate that getting the vaccine <18 months wouldn't be as "effective" either.
"If it takes a lab to make it, it takes a lab to digest it". Kris Carr, Hungry For Change