Fusion-loop-engineered Zika mRNA vaccine elicits convergent B cell immunodominance to uncouple protection from ADE in macaques.

Antibody-dependent enhancement (ADE) remains a major hurdle for flavivirus vaccine development. Engineering the fusion loop (FL) represents a promising solution, yet validations restricted to murine models limit human translatability due to profound species divergence. Here, we provide a primate-level demonstration that FL-engineered Zika virus (ZIKV)-M/E mRNA vaccination protects against ZIKV challenge with negligible dengue virus (DENV) ADE. The vaccine-induced B cells are overwhelmingly ZIKV specific (>97%), dominated by a convergent population using a germline ortholog of human IGHV5-10-1, a gene preferentially recruited during natural human ZIKV infection. Antibodies derived from this population are highly potent and, mechanistically, achieve multi-stage viral entry blockade. Cryo-electron microscopy analysis reveals that these antibodies target a distinct, vulnerable hotspot on ZIKV. Notably, ZIKV specificity is dictated by the epitope residues D67-M68-A69, whereas a homologous DENV N67-linked glycan imposes steric shielding to preclude cross-reactivity. Our findings demonstrate that FL engineering uncouples protective immunity from ADE for safer human flavivirus vaccines.