These two concepts are critical in understanding our attitude toward vaccinations. On the one hand, vaccines are not perfect. There is no guarantee that if you receive a vaccine, you will become immune. Some of the rates of protection are over 98 percent, but no one can ever guarantee 100 percent immunity. So it is still possible to contract a disease even after receiving a vaccine. The vaccine makes it less likely that you will get the disease and often leads to a milder illness if you do end up contracting the disease, but you can’t assume 100 percent protection from any given vaccine.
On the other hand, the natural history of the diseases themselves is not guaranteed. Most people who get chickenpox have an annoying rash and fever for seven to ten days. However, some people end up in the hospital because of complications from the chickenpox. They might acquire pneumonia or a skin infection that requires antibiotics. In very rare circumstances, people die of complications from chickenpox. That is incredibly uncommon: there were only forty deaths a year in the United States in the pre-vaccination era, but still, they happened. Even as benign a disease as chickenpox can occasionally be life threatening. Nothing in life is guaranteed.
As for bad outcomes, they are often random and unpredictable. We know that, on average, one child in fourteen thousand will develop a seizure after receiving a DTaP vaccine, but we don’t know which child that will be. You hope it will not be your child, but you don’t know.
And while you do your best to protect your child from dangers, sometimes bad things still happen. We have some parents tell us that they don’t want to vaccinate because they don’t think their baby is at particular risk. They are breast-feeding and are not using any kind of day care. While it is very true that breast-feeding and avoiding day care lowers that baby’s risk of contracting many dangerous diseases, it doesn’t lower the risk to zero. There are still plenty of chances for exposure to certain diseases, from a random exposure to a friend in the grocery store to a relative who might share the disease while visiting to an unknown source in the community. And the protection a baby receives from his or her mother’s breast milk is nowhere near the protection received from a vaccine.
A dramatic example of both of these concepts occurs when meningococcal meningitis strikes a college student. Here we have a healthy young adult, walking around campus with no medical concerns at all, and twenty-four hours later she is in the ICU fighting for her life. The scary thought is that many people carry the bacteria around in their upper respiratory tract without ever knowing they are carriers or even becoming slightly ill. So why does one particular person develop this life-threatening disease? We don’t know. Bad things beyond our control still sometimes happen.