"Unlike bacteria, viruses do not replicate on their own. To make viral vaccines, large numbers of viruses must be grown in cell cultures specific to each virus. Some licensed viral vaccines (i.e., some formulations of hepatitis A, poliovirus, rabies, rubella, and varicellazoster viruses or combination vaccines containing such component viruses) are produced by growing viruses that infect humans in WI-38 or MRC-5 cell cultures."