"Two human cell lines (MRC-5 and WI-38) that are used to grow these weakened virus strains have their origins in cells derived from the lung tissue of aborted fetuses (Dan Maher, “On the Use of Certain Vaccines,” unpublished manuscript [1998, NCBC]). Although these human cell lines could have been produced using cells taken from other sources (thus avoiding the moral problem entirely), the fact is that they were not. In many cases, there is no other choice than either to make use of a tainted vaccine or to forgo vaccination altogether. Thus “Meruvax,” a widely used vaccine for rubella (German measles) sold by Merck & Co., Inc., uses the WI38 cell line. The chicken pox vaccine “Varivax,” produced by the same company, uses both MRC-5 and WI-38. SmithKline Beecham offers a vaccine called “Havrix” that has its origins in MRC-5. “Havrix” guards against scarlet fever, rheumatic fever, kidney inflammation, and other hepatitis A infections. Whether immunization with these vaccines is permissible depends upon whether their use involves the Catholic in cooperation with evil. Briefly, formal cooperation arises when an individual shares in the intention or the action of another who does what is wrong. Immoral material cooperation occurs when one who cooperates makes an essential contribution to the circumstances of a wrongdoer’s act. Thus the question about vaccines derived from aborted fetuses concerns whether or not their use involves the Catholic in immoral cooperation with the evil of abortion. The answer, in short, would appear to be “no.” For it seems impossible for an individual to cooperate with an action that is now completed and exists in the past. Clearly, use of a vaccine in the present does not cause the one who is immunized to share in the immoral intention or action of those who carried out the abortion in the past. Neither does such use provide some circumstance essential to the commission of that past act. Thus use of these vaccines would seem permissible."