The<unk>  is the practice of researching, developing, and producing<unk> s through growing viruses in cultured (laboratory-grown) cells that were originally derived from human<unk><unk> . Since the cell strains in<unk>  originate from abortions, there has been opposition to the practice and the resulting<unk> s on religious and moral grounds.<unk>  experts and manufacturers emphasize that<unk> s do not contain any of the original<unk><unk>  or cells, nor cells from lines originally derived from<unk>  materials; that the abortions occurred decades ago; and that the cell lines continue to replicate on their own, with no new<unk><unk> <unk> d after the originating abortion decades in the past. Traces of human DNA fragments do remain in many such<unk> s, but only because the<unk>  materials are purified from cell debris. The Catholic Church has encouraged its members to<unk>  alternative<unk> s, produced without human cell lines, if possible. However, the Vatican has clarified that "all<unk> tions recognized as clinically safe and effective can be<unk> d in good conscience, with the certain knowledge that the<unk>  of such<unk> s does not constitute formal cooperation with the abortion".
Use of fetal tissue in vaccine development
Timeline of human vaccines
Susceptibility and severity of infections in pregnancy
Vaccine contamination with SV40
Specific-pathogen-free
Animal vaccination
Vaccines for Children Program
Viral strategies for immune response evasion