other than agreeing with rachelsmama that it is a guess…..
It is a really long temporary period. My 16 and 13 year olds had CP (an this was before anyone around here even got the CP vaccine -so no one can argue this is a choice we made). They will be at increased risk of shingles due to less immune-boosting wild cases of it for the rest of their lives. We are talking 60-70 years.
I think 60-70 years of an increase in shingle due to a decrease in the CP rate (shingles is much more dangerous than CP) is an inappropriate choice from a public health POV.
It is a long period, you're right. I have mixed feelings about that.
I'd be interested in some documentation showing shingles is much more dangerous that chicken pox.