Babies are given vaccines early on because this is the time when they are most vulnerable to certain diseases. For example, the first recommended vaccination – for hepatitis B – happens at the hospital when babies are born because it’s a disease that can pass from an unknowingly infected mother to her baby during birth. Newborn babies may be immune to some diseases if they have antibodies given to them from their vaccinated mothers, but this immunity only lasts a few months. Also, if there are older children at home, it’s important to protect your baby as soon as possible from the disease exposure a sibling or his/her friends might bring into the home from school, child care, travels, and so on.
The recommended schedule is designed to protect infants and children by providing immunity early in life, before they are exposed to life-threatening diseases. The schedule is designed to work best with a child’s immune system at certain ages and at specific times. There is no research to show that a child would be equally protected against diseases with a very different schedule.