This is another argument frequently found in anti-vaccine literature, the implication being that this proves that vaccines are not effective. In fact it is true that in an outbreak those who have been vaccinated often outnumber those who have not — even with vaccines such as measles, which we know to be about 98% effective when used as recommended.

This apparent paradox is explained by two factors. First, no vaccine is 100% effective. To make vaccines safer than the disease, the bacteria or virus is killed or weakened (attenuated). For reasons related to the individual, not all vaccinated persons develop immunity. Most routine childhood vaccines are effective for 85% to 95% of recipients. Second, in most high-income countries, the people who have been vaccinated vastly outnumber those who have not.

How these two factors work together to result in outbreaks in which the majority of cases have been vaccinated can be more easily understood by looking at a hypothetical example:

Consider a high school of 1,000 students where none has ever had measles. All but 5 of the students have had 2 doses of measles vaccine, and so are fully immunized. The entire student body is exposed to measles, and every susceptible student becomes infected. The 5 unvaccinated students will be infected, of course. But of the 995 who have been vaccinated, we would expect several not to respond to the vaccine. The efficacy rate for 2 doses of measles vaccine can be as high as 99%. In this class, 7 students do not respond, and they, too, become infected. Therefore 7 of 12, or about 58%, of the cases occur in students who have been fully vaccinated.

As you can see, this doesn’t prove the vaccine didn’t work — only that most of the children in the class had been vaccinated, so those who were vaccinated and did not respond outnumbered those who had not been vaccinated. Looking at it another way, 100% of the children who had not been vaccinated got measles, compared with less than 1% of those who had been vaccinated. Measles vaccine protected most of the class; if nobody in the class had been vaccinated, there would probably have been 1,000 cases of measles.

Looking at risk alone is not enough; you must always look at both risks and benefits. Even one serious adverse effect in a million doses of vaccine cannot be justified if there is no benefit from the vaccination. However, if there were no vaccines, there would be many more cases of disease, and along with them, more serious side effects and more deaths. For example, according to an analysis of the benefit and risk of DTP immunization, if there was no immunization program in the United States, pertussis cases could increase 71-fold and deaths due to pertussis could increase 4-fold. Comparing the risk from disease with the risk from the vaccines can give us an idea of the benefits we get from vaccinating our children.

The fact is that a child is far more likely to be seriously injured by one of these diseases than by any vaccine. While any serious injury or death caused by vaccines is too many, it is also clear that the benefits of vaccination greatly outweigh the slight risk, and that many, many more injuries and deaths would occur without vaccinations. In fact, to have a medical intervention as effective as vaccination in preventing disease and not use it would be unconscionable.