"For demonstrating that poliovirus could be reliably grown without using nervous tissue, Enders and his colleagues Thomas Weller and Frederick Robbins were awarded the Nobel Prize in Physiology or Medicine in 1954. Their discovery proved to be the breakthrough needed to develop a polio vaccine. In 1951, Jonas Salk and his colleagues at the University of Pittsburgh found that poliovirus could also be propagated on a large scale in monkey kidney cells. Over time, most vaccine development efforts shifted to cell strains—cultures made up of only a single type of cell. These strains can be derived from tissue cultures, which contain multiple types of cells. While viruses can be grown in tissue cultures, cell strains allow continuous observation and control that may not be possible in cultures containing multiple types of cells. This same transition was made in the development of polio vaccines. A monkey kidney cell strain is used to grow poliovirus for the inactivated polio vaccine made today."