This article takes an interest in the effects of the institutionalization of communication on policy-makers’ representations of their public. Using the case of the 2009 pandemic flu vaccination campaign, I describe how communication experts from the Service d’information du gouvernement (SIG) participated in legitimizing and promoting representations of the population as potentially irrational. Firstly, I show that, in the standard crisis management scheme, the SIG constitutes the main source of expertise on the public’s behavior. Then, I underline how, during the process of preparation for a pandemic crisis and of defining the principles of “crisis communication,” agents from the SIG insisted on the irrational potential of the population in order to present their expertise as necessary. As a consequence of this strategy, the surveillance of “public opinion” during the pandemic was particularly oriented toward identifying signs of irrational reactions. These reports focused the attention of the actors involved in handling the crisis, to the detriment of other messages transmitted by the SIG. As a result of the lack of legitimacy attributed to their function within the crisis management scheme, experts from the SIG did not succeed in countering this over-irrationalist interpretation of the data they presented. The capacity that experts in public opinion have to legitimize irrationalist representations of the population lies paradoxically, at least in part, in the relative illegitimacy of their expertise.