For many people, it helps to compare the public health perspective on vaccines versus individual family choice by using an example with numbers. Imagine an illness named virus X that infects every child in the first year of life. While most children with virus X stay at home with a moderate fever and rash for seven to ten days, one in a thousand children develops the severe form of virus X, which requires a week of hospitalization. Some of those hospitalized children recover completely, some have significant developmental delays, and 20 percent die. Given that four million children are born in the United States every year, we would expect four thousand children to develop the severe version of virus X annually, of which eight hundred would die.
Now imagine we have a vaccine that prevents virus X and is 90 percent effective, meaning that 90 percent of children will develop antibodies and be protected if exposed to virus X. Of the four million children previously mentioned, 3,600,000 will be protected by the vaccine, and four hundred thousand will still be susceptible. Using the one in one thousand risk of severe disease, we would now only have four hundred children being hospitalized with the severe disease and only eighty children dying.
However, because of herd immunity, after a time, practically nobody gets virus X because it is not around to spread to other people. So in fact, while four hundred thousand children lack the antibodies even after vaccination, most don’t get the disease. In fact, we might have only a hundred cases of virus X in the United States per year. Statistically, this means that we will see the severe disease only once every ten years and a death from the illness only once every fifty years. That is the benefit of herd immunity.
Now, nothing in life or medicine is perfect, including our virus X vaccine. We find that one in fifty thousand children has a severe reaction to the vaccine and is severely developmentally delayed, and 10 percent of those children die. That means that if four million kids are vaccinated each year, eighty children get a severe reaction, and eight children die.
So now you are a parent and have to make a decision about the vaccine for virus X. If the vaccine has just been introduced, you probably have heard of people in your community with the severe disease and probably want to vaccinate to protect against those severe complications.
However, forty years later, families are not aware of the severe version of virus X because it only happens in the whole United States once every ten years. More importantly, because it is so rare, the risk of severe illness from virus X is actually lower than the risk of adverse reaction to the vaccine. From a purely personal point of view, it is safer for your child to skip the vaccine.
On the other hand, from a public health point of view, if enough parents avoid the vaccine, then virus X will return, and ultimately more children will be harmed or will die from the illness than from the vaccine. In a very simplistic way, the public health assessment is that the vaccine, even when causing eighty severe reactions and eight deaths every year, is better than the reemergence of the illness.
Of course this is a simplistic scenario. What if there are more adverse reactions to the vaccine? Or unknown ones? What if the numbers are different? But to be fair, experts in international medicine see children die regularly of vaccine-preventable diseases. Yet even though they are highly publicized, severe vaccine reactions are extremely rare.
You may be surprised to know that virus X is not hypothetical. It actually has a name: measles. Before the vaccine was introduced, every child developed measles, one in a thousand children developed severe complications, and one in five thousand children died. However, because of herd immunity from the vaccine, we’ve had less than a hundred cases of measles in the United States in recent years, and most of those were imported from overseas. The only significant change we made in this scenario is modifying the risk of severe reactions to the vaccine. We chose one in fifty thousand for illustrative purposes. The quoted risk for encephalitis developing after the measles vaccine is one in two million, so the risk of serious side effects is actually much less than shown in our virus X example.