In fact, the chickenpox vaccine was not available when many of us were children, which is why we did not get the shot. However, chickenpox can actually be a painful, serious disease. In many cases, children experience a mild case of chickenpox, but other children may have blisters that become infected. Others may develop pneumonia. There is no way to tell in advance the severity of the symptoms your child will experience. And, if you don’t get vaccinated during childhood and are exposed as an adult, you are more likely to experience severe symptoms.
Before vaccine was available, about 50 children died every year from chickenpox, and about 1 in 500 children who got chickenpox was hospitalized.
Source: CDC: Infant Immunizations FAQ