Opponents of vaccination are quick to point out that many cases of measles occur in vaccinated children. They claim this proves that the vaccine doesn't work. In recent outbreaks of measles in Canada and the United States, it is true that over half of the cases occurred in school-age children who had been given measles vaccine. But to say that this means that the vaccine doesn't work is incorrect. This argument is too simplistic, and it uses faulty logic. We know that one dose of measles vaccine is not 100% effective. We know that about 5­10% of children are not protected after a single dose of vaccine. Let's use these facts in an example. In a school with 1,000 students, assume 95% of children get vaccinated. This means that there are 950 vaccinated children and 50 unvaccinated children. The number of children who are still susceptible to measles is 145: 50 unvaccinated children, plus 10% of the 950 vaccinated children who are unprotected after one dose (or 95). One child in the school comes back from a holiday with measles. Measles is so contagious that it quickly spreads in the school. One-half of the 145 susceptible children catch it: half of the 50 unvaccinated children (or 25) get measles and half of the 95 vaccine-failure children (or 47) get measles. Therefore, there are 72 cases of measles. The proportion of cases that occurred in vaccinated children is 47 of the total 72, or 65%! This seems very high, but it does not mean that the vaccine works only 65% of the time. Whenever a vaccine is not 100% effective and most children have been vaccinated, more cases will occur in vaccinated than in unvaccinated children during outbreaks, only because there are more vaccinated than unvaccinated children. The 25 cases of measles in the unvaccinated group came from a total of 50 children; the attack rate is then 50%. The 47 cases among vaccine failures came from a total of 950 children; their attack rate was only 4.9%. From these numbers, we can see that unvaccinated children were 10 times more likely to catch measles! The vaccine was 90% effective. This is the commonly observed figure when only one dose of measles vaccine is given. When two doses are routinely given to children, the vaccine is nearly 100% effective and outbreaks no longer occur among vaccinated children.

This document was published by the Canadian Paediatric society, 1997.

Posted by the Canadian Paediatric Society, July 1997.

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