Reasons human cells are used in vaccine research and development:

• To develop a vaccine that can be produced on a large scale, researchers must grow viruses or bacteria in large quantities and with great consistency.
• Bacteria can be grown in a laboratory, but viruses need living cells to infect so they can make copies of themselves.
• Viruses tend to grow better in cells from humans than those of other animals.
• Human cells can be maintained for long periods of time at low temperatures, enabling scientists to use the same cell lines from many decades ago.