While vaccines do not contain stem or fetal cells, many common vaccines are developed by growing viruses in fetal embryo cells.
Examples include those for treating hepatitis A, rabies and chickenpox. Fetal cells were also used during the testing and development of the Pfizer, Moderna and Johnson & Johnson COVID-19 vaccines.
The fetal cells were initially taken from two fetuses aborted in the early 1960s. However, from the beginning, scientists have duplicated these cells in the lab for use — no cells from aborted fetuses are used.
According to scientists, fetal human cells are used rather than animal cells because viruses "tend to grow better" in human cells. Furthermore, as fetal cells do not divide as much as other types of cells, they can be used for far longer.