Protective efficacy of a whole cell pertussis vaccine.

A trial of the efficacy of a plain whole cell pertussis vaccine was conducted in Sweden. In this non-blinded trial 525 infants aged 2 months who were born on days with an even number received three doses of vaccine one month apart and 615 infants of the same age who were born on days with an odd number were enrolled as controls. During the 18 months of follow up there were 55 cases of pertussis. The attack rate was 1.5% (8/525) among the vaccinated children and 7.6% (47/615) among the unvaccinated children (p less than 0.001). The estimated efficacy of the vaccine was 80% (95% confidence interval 58 to 90). The estimated efficacy of pertussis vaccine was similar to that observed in British trials over 30 years ago.