To the Editor The modeling study of DeAngelis et al1 suggested that substitution of 1 dose of whole-cell pertussis vaccine for the first dose of acellular vaccine at age 6 to 8 weeks would result in at least a 91% reduction in the incidence of pertussis. As noted in the accompanying editorial, the validity of these findings relies on both explicit and implicit assumptions within the model structure and its parameters. We go further to say that such a profound effect on pertussis disease and transmission seems inherently unlikely and that implementation barriers are understated.