Dengue virus infections cause a substantial public health burden in tropical and subtropical regions. A single dengue vaccine has been approved by regulatory authorities in 19 countries, but concerns regarding vaccine safety in people who are dengue naive at the time of immunisation has introduced uncertainty into the vaccine's future. As other dengue vaccines complete or enter large-scale efficacy trials, we argue that foundational work by Sabin, historic epidemiological observations of dengue outbreaks, and prospective cohort studies in Asia and the Americas indicate that modifications must be made to the methods of assessing dengue vaccines. In this Personal View, we review and relate previous data that supports a dynamic role of cross-protective dengue immunity to the goals and challenges of measuring and interpreting dengue vaccine immunogenicity, efficacy, and safety in clinical trials. We suggest that for partly protective vaccines, temporary cross-protective immunity could lead to overestimation of vaccine safety and efficacy in the early years following vaccination. We recommend that assessment of dengue vaccines should span several years, involve active surveillance to clinically characterise incident infections and regular blood draws to define kinetic changes in immunological profiles, and include sample sizes that are large enough to support detailed analyses of vaccine trial subgroups, such as individuals who are dengue naive.