This is a good question, and the answer has to do with both your child’s immune system and whether the vaccine is live-attenuated or inactivated. Children’s immune systems are not always developed enough to produce a long-term immune response to the initial vaccination, and thus require what is called a booster dose. Another reason that vaccines require multiple doses is if the vaccine is live-attenuated or inactivated. A live-attenuated vaccine contains weakened traces of the living virus. For a healthy immune system, these types of vaccines are an excellent teacher for your immune system to develop the necessary antibodies required to fight the disease. However, since such viruses are active in the vaccine, they cannot be used on people with weakened immune systems. Examples of live vaccines include MMR, chickenpox, and rotavirus. Inactivated or killed vaccines are made by killing the antigen while making the vaccine. Since inactivated vaccines are not as strong as live-attenuated vaccines, they usually require more than one dose in order to properly build up the immune system against the disease or virus. Vaccines that usually require booster doses are MMR, chickenpox, human papillomavirus vaccine (HPV), DTaP, influenza, hepatitis A and B, meningitis, and polio.