While this statement is sometimes used in anti-vaccine literature to show that vaccines don’t work, in truth it shows the benefits of vaccines.
Let’s suppose that there are a thousand children in a school and that 90 percent of them have been vaccinated against chickenpox with a single dose of the vaccine five years ago. This means that nine hundred students have been vaccinated, and one hundred have not been vaccinated. We know about 15 percent of vaccine recipients do not have continuing protection after five years. This means that of the nine hundred vaccinated students, 15 percent, or 135 students, are no longer immune to chickenpox.
Now, let’s expose all one thousand children to chickenpox. In general, 95 percent of children without immunity to chickenpox will develop the disease. For the one hundred unvaccinated students, ninety-five of them will develop chickenpox. For the nine hundred vaccinated students, 95 percent of the
135 students who no longer have protection, or 128 students, will develop the disease.
So the statement is true. The majority of people who develop a disease during an epidemic have already been vaccinated. In our chickenpox example, more vaccinated students (128) developed the disease than unvaccinated students (95). However, this is only because there were so many more vaccinated students to begin with. The percentages are what matter. Only 14 percent (128/900) of the vaccinated students developed chickenpox, compared to 95 percent of the unvaccinated students. The vaccine, even with a waning immunity over five years, significantly protected the vaccinated students from the disease. Moreover, the vaccinated students who did get chickenpox would generally have had a much milder case of the illness than the unvaccinated children.