A laboratory-engineered, “chimeric” dengue fever vaccine entered late-stage clinical trials in the late 2000s. One possible way of interpreting the arrival of a technology like this is to see it as the end point of a unified global project. Alternatively, it can be understood as a “cosmopolitical event.” Instead of reflecting unity, cosmopolitical events magnify social and technical differences, and they afford space to contemplate alternative forms of life. Drawing on fieldwork with dengue researchers in Puerto Rico, Nicaragua, and Cuba, I argue that the chimera appeared at a liminal moment in dengue science. It prompted researchers to contemplate how the divergent logics of pharmaceutical capital, humanitarianism, and biosecurity shaped their work as well as to imagine how that work might otherwise proceed. I conclude by suggesting that attention to cosmopolitical events puts the anthropology of global health into closer conversation with analyses of other global phenomena.