I would wager that if you looked back 8 and 13 months ago those NOT vaccinated were infected at the same rate............... or near.
It's kinda like saying if you put 50 cows in a pasture with a bull 2 months ago, and then another 50 one month ago, the first cows are twice as likely to get phugged by the bull............ and nobodys sure what happened for about 9 months.......
But maybe I misread something, but they seem to 'prove' that if you hang around long enough you're going to get it.....