Some vaccines protect against very common diseases. Before the 1950s, there were millions of cases of measles and mumps and chickenpox every year. These were common diseases that swept through a community and parents expected their children to get sick sooner or later. So when vaccines against these diseases were created, we were able to prevent millions of episodes of illness with just one vaccine. That shows a very good return on our investment.
However, sometimes the disease in question is rarer. Haemophilus influenzae type B (HiB) can lead to a devastating illness in children, including meningitis, an infection of the fluid surrounding the brain and spinal cord, and sepsis, a bloodstream infection. The HiB vaccine mentioned earlier is one of the best vaccines we have today. It is very effective with only minimal side effects. Before the vaccine was developed, there were about twenty thousand cases of these dangerous HiB infections in the United States every year. In 2007, there were fewer than one hundred cases.
However, there are approximately four million babies born in the United States every year. If you assume all twenty thousand cases of invasive HiB disease that existed in the prevaccine era occurred in the first year of life (which is not completely accurate), you will see that we are vaccinating four million babies every year to prevent twenty thousand severe illnesses.
In other words, you have to vaccinate two hundred babies to prevent one case of HiB meningitis. In fact, because babies usually receive three doses of the HiB vaccine in the first year of life, you have to give six hundred shots to prevent one case of HiB meningitis. And that is the most cost-effective vaccine we use.
What happens when the illness itself becomes rare because of the mass vaccination campaign? Instead of four million cases of measles, we now usually have fewer than one hundred cases every year in the United States. These rare events are where the game of statistics becomes harder to comprehend. If the disease is rare and the side effects of the vaccine are rare, how should we decide what to do? In this situation, some parents wonder if the risk of side effects from a given vaccine is greater than the risk of getting the disease.
The average human brain has a hard time comprehending these kinds of statistics. When it comes to personal decisions, we might understand a one in two risk (fifty/fifty) or maybe even a one in ten or one in one hundred risk. However, when we get to a one in one thousand or one in ten thousand or one in one million risk, it is harder to comprehend what those numbers mean when it comes to our own personal risk. We usually just lump them into the category of “rare.”
However, there is a big difference between one in one thousand and one in one million. If there are three hundred million people in the United States, a one in one million risk of contracting an infection would mean that three hundred people would develop the disease. However, a one in one thousand risk would mean that three hundred thousand people would develop the disease, a significant difference.
Many of the side effects of vaccines and many of the rare consequences of a given illness occur at a rate of one in one thousand to one in one million. If we actually do the math, we might rationally be able to say that a one in one thousand risk of brain damage from a disease is greater than a one in one million risk of brain damage from a disease. However, if we are simply lumping both risks together as “rare,” we don’t comprehend that one risk is actually a thousand times greater than the other.
Furthermore, our emotions come into play when comparing the tiny risks of a vaccine and the tiny risks of a disease. Some of our patients feel that they would rather take the risk of a natural process, like a disease, instead of an artificial process, like a vaccine. They also tell us they would feel less guilty if their child had a bad outcome from the natural process instead of one where they actively intervened to give their child the vaccine. Even if the numerical risks are the same, emotionally we may view them very differently.