Few things in medicine work 100% of the time, but vaccines are one of the most effective weapons we have against disease — they work in 85% to 99% of cases. They greatly reduce your child's risk of serious illness (particularly when more and more people use them) and give diseases fewer chances to take hold in a population. It can be difficult to watch kids get a shot, but the short-term pain is nothing compared with suffering through a potentially deadly bout of diphtheria, pertussis, or measles.