"WHY ARE FETAL CELLS USED TO MAKE VACCINES? To develop and manufacture some vaccines, scientists working with pharmaceutical companies prefer human cell lines over other cells because: 1. Viruses need cells to grow, and the viruses tend to grow better in cells from humans than animals (because they infect humans); 2. Fetal cells can be used longer than other cell types; and 3. Fetal cells can be maintained at low temperatures, allowing scientists to continuing using cells lines from decades ago."