Melissa: If the 18-month-old is current on his polio immunizations (i.e. has had his 2, 4, 6 and 18 month OPV's), it means he already has considerable immunity. If a small amount of the baby's OPV transferred from his mouth to the toddler's, it would probably only act as a tiny booster dose. If one is not concerned about giving a full dose of OPV directly to a child, then whatever minimal amount could transfer via tandem nursing should be of much less concern. As for the 18-month-old giving MMR to the baby, that is even more unlikely. I don't know of any data, but there are no known cases of transmission of mumps, measles or rubella to others by any route by someone who has received MMR, and there is little reason to suppose that the viruses would be excreted in the child's saliva. Also, one reason that babies are not given MMR until after a year of age is that prior to that maternal antibodies transferred to the baby may interfere with active immunization, so the baby is probably being passively protected, especially from measles. Hope this helps. Alicia. [log in to unmask]