I also understand the effects the shot can have on the immune system of the pups and that can be related to some pups getting parvo still. I just looked at the paper my pup and his littermates had 2 vaccs before being picked up. so those pups that got it had been vacc'd 3 times. but the grown pup that also got it wasn't vacc'd at the same time as them so I wouldn't relate it to the vacc weakening their immune system. I think they just had prolonged immunity and when it "wore off" they contracted it.