Implications of immune responses to DENV, JEV, and ZIKV Infections for cross-reactivity and considerations for vaccine evaluation.

Dengue virus (DENV), Japanese encephalitis virus (JEV), and Zika virus (ZIKV) are mosquito-borne flaviviruses of global concern, causing illnesses ranging from mild fever to severe neurological or hemorrhagic disease. Antibodies against these viruses exhibit complex cross-reactivity, influencing both neutralization and antibody-dependent enhancement (ADE). DENV comprises four serotypes (65-70% sequence identity) with multiple genotypes. Secondary heterologous infections increase the risk of dengue with or without warning signs and severe dengue, largely mediated by pre-existing antibodies from prior infection or vaccination. We assessed total antibody levels, neutralizing antibody (NAb) titers, and ADE activity of serum samples from Vietnamese patients with confirmed DENV infection and evaluated potential cross-reactivities with JEV and ZIKV. In primary infections, NAb titers were generally low (<1:10), except for JEV. Secondary infections showed the highest NAb titers against DENV-4, followed by DENV-2, DENV-3, and the lowest against DENV-1. In ADE assays, DENV-2 exhibited the highest enhancement, followed by DENV-1 and DENV-3, whereas DENV-4 showed minimal ADE. ADE was also detected for JEV and ZIKV, demonstrating that cross-reactive antibodies can facilitate infection across flaviviruses. These findings underscore the impact of pre-existing cross-reactive antibodies in endemic regions and highlight the importance of evaluating vaccine-induced cross-reactive immunity.