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]