There are at least five vaccines in common use may have ‘illicit’ (per Church teaching) origins:
Rubella, (MMR, MMVax, etc.)
Poliovax
Havrix (hepatitis A)
Imovax Rabies ID (which also inherently taints Immune Globulin)
Varivax (chicken pox and shingles)
Poliovax is probably a good example of ‘why’ (in secular scientific terms). Poliomyelitis vaccine had a relatively high ‘negative reaction rate’. That is, the vaccine itself was fairly dangerous. However, although most here may be too young to remember, polio itself was a lot scarier, and everyone could see the effects first hand.
But a live viral cultivated in cells from a monkey kidney has a lot of problems. For example, the kidneys have been shown to carry SV-40, a form of aids, and the same SV-40 strain shows up in cancer tumors found in children whose parents have received tainted vaccine. This started raising a lot of concerns about new immune deficiency problems from the vaccine, so a safer vaccine was highly desired.
Poliovax uses inactivated versions of the polio vaccine, so direct side effects are much lower. But to accomplish this, very pristine (non damaged) copies of the virus must be used. Human diploid cells provide am optimum growth medium to accomplish this. They also minimize the problems of cross species cultivation.
It isn’t a matter of killing babies as an ingredient in the vaccine, but the human diploid cell type (MRC-5) used was originally cultivated from an aborted fetus. Subsequently, the diploid cells are ‘grown’. For example:
jcs.biologists.org/cgi/reprint/35/1/381.pdf
To be fair, quite a bit of this research predates the Church’s express statements on the matter. And one Wistar Institute researcher involved in the original “WI-38” culture (essential in developing the rubella vaccine) not only raised the bioethical questions, but actively pursued a dialog with the Church looking for guidance.
Everyone has to follow their own concience but, frankly, the only one that really troubles me is Varivax (chicken pox). Chicken pox is not wholly benign, and I can personally attest that the shingles (the same virus coming back for one last fling after lying dormant in the body) are pure agony. But the risk/return for the vaccine still isn’t great. And part of the use of human diploid cells seems like pure convenience, since was then propogated in guinea pig cell cultures.
Again, to be fair, Merck (Varivax is Oka/Merck), has since been somewhat responsive to consumer complaints about fetal tissue use in vaccine development.