Columns show studies conducted in in vitro assays, mouse models, non-human primate challenge studies, human epidemiological and clinical studies, and human pregnancy/infant studies. Rows show the sequence of infecting viruses: primary DENV followed by secondary DENV infection with a different serotype, primary DENV infection followed by secondary ZIKV infection, and primary ZIKV infection followed by secondary DENV infection. Green “P” indicates protection, red “E” indicates enhancement, and question marks indicate unknown relationships.