Two pears on which I have never seen a hint of fireblight are Moonglow and Ayers. I'm not saying they are completely immune, but in many, many years of growing them in a FB prone area, I have never seen them affected. Last year my neighbor had an apple tree close to two of my Ayers trees which had FB so bad I thought it was going to die. Neither of my pears showed anything, and I had a bumper crops of fruit.
I hear the horror stories on Kieffer and FB but have never seen it myself. I have seen new growth on Kieffer affected by FB in really bad years, but have never seen it go into second year wood. I'm not doubting what anyone else says - just relaying my experience.
I do believe that there are a lot of pears out there being sold as Kieffer that are not true Kieffer. Stop and think about it - boo boos happen all the time.
Even one of the best apples to ever come from the PRI Breeding Program almost went obscure because a mix-up was made, and the wrong apple was being sold for a long time.
With humans, stuff happens, and as long as Kieffer has been around (150+ years) - lots of stuff has probably happened.