Vaccine is a biological preparation which provides a immunity and helps in prevention of diseases. Though it can be considered as a important factor to children because the children are more prone to disease, vaccines are widely considered one of the greatest achievements in public health having had a dramatic impact on the prevalence of several infectious diseases including smallpox, poliomyelitis, and measles. However it has been prepared it should have its own safety and efficacy till the entire shelf life of the product. Clinical trials have become critical for evaluating new vaccines as well as other prevention and treatment strategies in combating human diseases. Today the randomized, controlled trial is the gold standard for providing scientific evidence regarding the efficacy of a candidate vaccine. In this book the immune status against measles vaccine was evaluated in healthy infants under one years of age who received a single dose of the measles vaccine (Schwarz strain) and healthy adults who received a booster dose of the measles vaccine (Schwarz strain).