Vaccines are intended for use in healthy individuals as a disease preventive measure, and are therefore different to conventional drugs, which are aimed at the treatment of an existing condition. Safety is paramount for a vaccine. Vaccine clinical trials must be able to demonstrate both that a vaccine is safe and that it is effective in preventing disease. This means that a higher number of subjects will be required for vaccine clinical trials than for traditional drug trials. For these reasons, before a vaccine is licensed and brought to market it undergoes a long and rigorous period of research followed by many years of testing.