A single residue in domain II of envelope protein of yellow fever virus is critical for neutralization sensitivity.

The live-attenuated yellow fever (YF) vaccine strain 17D (referred to as YF-17D) is the only commercially available vaccine against YF. Recent yellow fever virus (YFV) isolates of South American genotype I (SAI) showed reduced sensitivity to sera from individuals vaccinated with YF-17D. However, the underlying mechanisms remain inadequately elucidated. Here we conducted a screening of a panel of amino acid changes prevalent in domain II of the envelope protein (EDII) of SAI YFV isolates. Each mutation was introduced into the infectious clone of YF-17D utilizing standard reverse genetics. Subsequent neutralization assays demonstrated that the A83E mutant was more sensitive to sera from YF-17D-vaccinated mice and individuals than YF-17D did. Additionally, the A83E mutant displayed neurovirulence and viscerotropic characteristics comparable to those of the parental YF-17D in murine models. Notably, administration of a single dose of the A83E mutant resulted in an enhanced neutralization response against this variant compared to YF-17D and provided complete protection against lethal YFV challenge in mice. Collectively, these findings not only identify a crucial amino acid residue that influences the neutralization sensitivity of YFV but also provide a promising candidate for the development of an updated YF vaccine. IMPORTANCE: The YF-17D vaccine has been used to prevent YF disease. However, recent strains belonging to the SAI displayed reduced sensitivity to the antibodies produced by vaccination, raising concerns about potential future outbreaks. To identify potential amino acid residues responsible for the decreased neutralizing activity of YF-17D-vaccinated sera, we conducted a screening and generated recombinant viruses with amino acid changes specific to the SAI in EDII using the YF-17D genome as a genetic backbone. We found the A83E mutation played a key role in reducing neutralizing sensitivity to YF-17D-vaccinated mouse sera. Importantly, the A83E mutant maintained a comparable attenuation phenotype to YF-17D but elicited enhanced neutralization activity and conferred protection in mice. Together, we identify a key amino acid residue responsible for the neutralization escape of SAI YFV isolates. We propose that this substitution could act as a target for developing an updated YF-17D vaccine.