The emergence of Web 2.0 has led to more and more Web-based resources demonstrating three defining characteristics: user participation, openness and network effects. This paper discusses these characteristics in the context of the online vaccination debate, explores how they structurally alter the way people might interact with vaccination information online, and describes ways in which such characteristics support particular tendencies in human decision making processes. Specifically, user participation supports the influence of narratives and personal accounts, openness shapes expectations for greater levels of detail and movement toward models of informed decision making, and network effects demonstrate the social nature of decision making, the influence of like-minded others and thus, the pitfalls of polarization in the online vaccination debate. Web 2.0 means that concerns about vaccination information online must expand beyond simply the possibility that people might access information of varying quality to incorporate a more comprehensive understanding of how people use current Web functionality, how such usage influences expectations about information sources and decision making processes, and the implications for communication strategies about vaccination.