Misplaced loss of confidence in measles vaccination: an investigation in a primary school.

Doubts were expressed about the effectiveness of measles vaccination during a measles outbreak in a Bristol primary school. Investigation of this outbreak showed that the vaccine uptake rate was 89%, vaccine effectiveness was 84% and that 57% of children developing measles had a history of previous measles vaccination. This apparent paradox arises because, for any vaccine that is not 100% effective, as the vaccine uptake rate rises so the percentage of cases previously vaccinated also rises. An understanding of this relationship may avoid unjustified loss of confidence in vaccine effectiveness as vaccine uptake increases.