I think it poo that was about. Some vax's are 'live' and can survive in fecal matter. Say polio was a live vaccine( it's NOT don't worry) your dd's fecal matter could possibly carry some live polio increasing the chances of it being 'spread' to others. Sorta like sharing chicken pox.
Anyone wanna chime in with a more coherent answer?
The old oral polio vaccine has been known to give polio or polio syndrome (I think it is called that) to caregivers handling the diapers after the vaccine. It was shed in the feces for weeks after the vaccine.
I think they discontinued this vaccine in Spring of 1999.