ABSTRACT Because of the nature of preventive vaccination programs, the viability of these public health interventions is particularly susceptible to public perceptions. This is because vaccination relies on a concept of ‘herd immunity’, achievement of which requires rational public behavior that can only be obtained through full and accurate communication about risks and benefits. This paper describes how irrational behavior that threatens the effectiveness of vaccination programs – both in crisis and non‐crisis situations – can be tied to public perceptions (...) created by media portrayals of health risks. I concentrate on childhood vaccination as an exemplar of ‘non‐crisis’ preventive vaccination, and on the recent flu vaccine shortage as a ‘crisis’ situation. The paper concludes with an examination of the steps necessary to resolve these threats through better public communication. (shrink)