Yellow fever remains a formidable public health threat. The World Health Organization currently designates 42 countries across sub-Saharan Africa and the Americas as being at risk of transmission. Clinical severity ranges from asymptomatic infection and mild flu-like illness to severe disease marked by fever, jaundice, hemorrhage, and death, with case-fatality rates for severe illness varying between 20 and 60 percent across populations. Vaccination is the most effective prophylactic strategy, and the YF-17D vaccine, developed in the 1930s, stands among the most effective and safe live-attenuated vaccines ever created. Recent studies have shown that YF-17D activates all arms of the immune system—innate, cellular, and humoral—and that neutralizing antibodies constitute the correlate of protection induced by yellow fever vaccines.

Despite this legacy of success, existing YF-17D vaccines are produced in embryonated chicken eggs, a system with inherent constraints on scalability and flexibility. These limitations pose challenges in meeting rising global demand, particularly in the context of recent outbreaks and the anticipated expansion of yellow fever-endemic regions driven by climate and demographic change. Although serious adverse events such as vaccine-associated viscerotropic and neurotropic diseases remain rare, they underscore the importance of continued efforts to develop next-generation candidates with optimized safety and manufacturing profiles. The vYF vaccine candidate was cloned from YF-VAX and adapted for propagation in Vero cells cultured in serum-free media, aiming to maintain the immuno-protective features of current vaccines while offering enhanced production scalability, reduced dependency on egg-based manufacturing, and potentially improved safety. Previous work showed vYF was less neurovirulent in mice than currently marketed YF-17D vaccines, and safety and immunogenicity profiles in non-human primates were comparable with those of two currently licensed vaccines. A first-in-human phase 1 study demonstrated a similar safety and immunogenicity profile to YF-VAX, and vYF is now in phase 2 clinical development.

The central question of the new study was whether the immune response elicited by vYF matches not only the magnitude but also the breadth of neutralization achieved by the reference vaccine. Because yellow fever virus strains constitute a single serotype with relatively high genetic stability compared with many other RNA viruses—attributed to the high fidelity of its RNA polymerase—a single vaccine strain has historically protected against diverse circulating lineages. Nevertheless, some studies have demonstrated lower neutralization of recent South American isolates, and specific neutralizing epitopes linked with reduced neutralization sensitivity have been mapped on the envelope protein, including residues E67 and E83 in domain I, E71/72 in domain I, E153/155 in domain II, and the epitope from E269 to E272 in the domain I/II hinge. The composite polyclonal response to YF-17D vaccination is therefore capable of protecting against multiple strains that might differ at one but not all epitopes.

To select a minimal panel of viral strains for comprehensive coverage of yellow fever virus genetic diversity, the researchers retrieved 705 full-length polyprotein sequences from GenBank, spanning 34 countries and years from 1927 to 2019. The sequences were aligned using the MAFFT algorithm, and a phylogenetic tree was constructed with PhyML 3.0 and visualized with Dendroscope 3.0. Four major sequence clusters, named A, B, C, and D, were clearly distinguished, and one field isolate was chosen from each branch. The panel comprised a 1981 Peruvian strain from Cusco (cluster C), a 2018 Brazilian strain BeH622205 (cluster D), a 1940 Sudanese strain (cluster B), and a 1965 Guinea-Bissau strain known as Cachatra (cluster A). Two control viruses were included: the live-attenuated YF 17D-204 reference vaccine Stamaril and the parent wild-type Asibi strain, obtained from an infectious clone provided by Professor Stephen Higgs at the World Reference Center for Emerging Viruses and Arboviruses.

The neutralization assays used sera pooled from non-human primates vaccinated in an earlier study. Two vYF pools were prepared, each from seven animals: a high-titer pool sampled at day 60 post-vaccination and a low-titer pool sampled at day 221, with individual titers against Stamaril ranging from roughly 2.37 to 4.14 log PRNT50. A third pool was constituted from five YF-VAX-vaccinated animals sampled at days 27 to 60. Statistical analysis confirmed significant titer differences between the two vYF pools, with fold-changes ranging from 1.9- to 3.6-fold. All assays were performed using the SN50 seroneutralization method on Vero cells, in which heat-inactivated sera were serially diluted four-fold, incubated with standardized virus doses, and applied to pre-seeded Vero cells; after six days of incubation, cells were fixed and immunostained with the antiflavivirus monoclonal antibody 4G2, and titers were calculated by least square regression as the reciprocal dilution neutralizing 50 percent of viral foci. Each combination was tested in triplicate across three independent assay runs conducted on separate days by different operators.

The results demonstrated that sera from vYF-vaccinated animals neutralized all four wild-type isolates with titers at least 3-fold above the established correlate of protection threshold, defined as detectable neutralizing antibodies at 10 PRNT50 and, by homology, 10 SN50. For two of the four isolates—the Peruvian and Sudanese strains—titers were statistically indistinguishable from those measured against the Asibi parent strain. For the remaining two—the Brazilian and Guinea-Bissau isolates—titers were 0.3 to 0.5 log lower, differences close to the assay variability of plus or minus 0.3 log. The neutralization patterns were remarkably similar between vYF and YF-VAX sera: in both cases, isolates 1 and 3 matched the Asibi reference, while isolates 2 and 4 showed modestly reduced neutralization. Notably, the lower neutralization of the Cachatra strain was consistently observed across all nine replicates when tested with both high-titer vYF sera and pooled YF-VAX sera, and the authors identified no evidence to suggest these results were artifactual.

The researchers also sequenced the viral genomes and analyzed envelope protein residues associated with known neutralizing epitopes, including the neutralization-resistant motifs described by Haslwanter and colleagues—linked to reduced activity of YF-17D vaccine-induced antibodies against South American strains—and the highly potent double-lock supersite recognized by monoclonal antibodies 5A, YD6, and YD73. No obvious correlation emerged between neutralization titers and these epitope motifs. Strains sharing identical residue patterns at key positions exhibited divergent neutralization profiles, while strains with mutations in three of the four documented epitopes were neutralized at levels equivalent to Asibi. The authors conclude that the vaccine-induced neutralizing response is not confined to a limited set of known epitopes and that the broad seroneutralization likely reflects the contribution of additional, yet undefined epitopes targeted by the polyclonal immune response. They also note that factors beyond direct sequence variation—such as virus fitness, the ratio of non-infectious viruses, neutralization cell type, and viral quasi-species—may influence results, and that recent work by Bibby and colleagues demonstrated that a single attenuation-linked residue, E380, modulates virion architecture and neutralization sensitivity, suggesting that electron microscopy polyclonal epitope mapping would be instrumental for documenting structural effects on the polyclonal response.

The study had limitations. The restricted availability of non-human primate serum samples necessitated pooled analysis and limited strain selection, and the temporal sampling strategy—drawing high-titer sera from day 60 and low-titer sera from day 221—may introduce bias, as both the magnitude and breadth of immune responses can vary over time. The authors also observed that titers against Stamaril were lower than in a prior study, an variation attributed mainly to methodological differences in assay type, virus quantity, counting method, and statistical calculation. The clinical impact of the modestly reduced neutralization of certain strains in vaccinees requires further investigation, particularly since some studies have identified cases of vaccine escape in Brazil that may be linked to the circulating strain. Nevertheless, the authors conclude that the neutralizing antibody profiles elicited by vYF in non-human primates were similar to those elicited by the reference vaccine YF-VAX, with titers consistently remaining above the correlate of protection threshold regardless of the diverse strains tested. These findings have significant implications for global yellow fever prevention strategies, highlighting the potential of vYF to provide broad protection comparable to current vaccines while offering a Vero cell-based production platform that could support more rapid scale-up during outbreaks than traditional egg-based manufacturing, potentially improving vaccine availability in endemic regions.

Subject of Research: A study in Frontiers in Virology reports that the next-generation live-attenuated yellow fever vaccine candidate vYF induces neutralizing antibodies in non-human primates that broadly neutralize diverse yellow fever virus strains, with profiles similar to the licensed YF-VAX vaccine.

Article Title: Breadth of neutralization induced by vYF next generation yellow fever vaccine

Article References: Breadth of neutralization induced by vYF next generation yellow fever vaccine. (n.d.). https://doi.org/10.3389/fviro.2026.1833390

Image Credits: AI Generated

DOI: 10.3389/fviro.2026.1833390

Keywords: yellow fever vaccine, vYF, neutralizing antibodies, non-human primates, YF-VAX, Vero cell production, Breadth, neutralization, induced, next, generation, yellow