I've said this somewhere else too, but....

if we are so cautious when we are pregnant that we won't even ingest HAM or certain CHEESE etc etc, why the hell would you shoot something (ANYTHING) directly into your blood stream?!
Not the same thing at all. Our blood already contains heaps of things, including mercury and other heavy metals...

Ham itself is perfectly safe.
The listeria bacteria it commonly contains is definitely not safe for the unborn babies. That's the point.

Unfortunately there is no vaccine for listeriosis.